TOMOYO Linux Cross Reference
Linux/Documentation/admin-guide/cgroup-v2.rst

   .. _cgroup-v2:
   
   ================
   Control Group v2
   ================
   
   :Date: October, 2015
   :Author: Tejun Heo <tj@kernel.org>
   
  This is the authoritative documentation on the design, interface and
  conventions of cgroup v2.  It describes all userland-visible aspects
  of cgroup including core and specific controller behaviors.  All
  future changes must be reflected in this document.  Documentation for
  v1 is available under :ref:`Documentation/admin-guide/cgroup-v1/index.rst <cgroup-v1>`.
  
  .. CONTENTS
  
     1. Introduction
       1-1. Terminology
       1-2. What is cgroup?
     2. Basic Operations
       2-1. Mounting
       2-2. Organizing Processes and Threads
         2-2-1. Processes
         2-2-2. Threads
       2-3. [Un]populated Notification
       2-4. Controlling Controllers
         2-4-1. Enabling and Disabling
         2-4-2. Top-down Constraint
         2-4-3. No Internal Process Constraint
       2-5. Delegation
         2-5-1. Model of Delegation
         2-5-2. Delegation Containment
       2-6. Guidelines
         2-6-1. Organize Once and Control
         2-6-2. Avoid Name Collisions
     3. Resource Distribution Models
       3-1. Weights
       3-2. Limits
       3-3. Protections
       3-4. Allocations
     4. Interface Files
       4-1. Format
       4-2. Conventions
       4-3. Core Interface Files
     5. Controllers
       5-1. CPU
         5-1-1. CPU Interface Files
       5-2. Memory
         5-2-1. Memory Interface Files
         5-2-2. Usage Guidelines
         5-2-3. Memory Ownership
       5-3. IO
         5-3-1. IO Interface Files
         5-3-2. Writeback
         5-3-3. IO Latency
           5-3-3-1. How IO Latency Throttling Works
           5-3-3-2. IO Latency Interface Files
         5-3-4. IO Priority
       5-4. PID
         5-4-1. PID Interface Files
       5-5. Cpuset
         5.5-1. Cpuset Interface Files
       5-6. Device
       5-7. RDMA
         5-7-1. RDMA Interface Files
       5-8. HugeTLB
         5.8-1. HugeTLB Interface Files
       5-9. Misc
         5.9-1 Miscellaneous cgroup Interface Files
         5.9-2 Migration and Ownership
       5-10. Others
         5-10-1. perf_event
       5-N. Non-normative information
         5-N-1. CPU controller root cgroup process behaviour
         5-N-2. IO controller root cgroup process behaviour
     6. Namespace
       6-1. Basics
       6-2. The Root and Views
       6-3. Migration and setns(2)
       6-4. Interaction with Other Namespaces
     P. Information on Kernel Programming
       P-1. Filesystem Support for Writeback
     D. Deprecated v1 Core Features
     R. Issues with v1 and Rationales for v2
       R-1. Multiple Hierarchies
       R-2. Thread Granularity
       R-3. Competition Between Inner Nodes and Threads
       R-4. Other Interface Issues
       R-5. Controller Issues and Remedies
         R-5-1. Memory
  
  
  Introduction
  ============
  
  Terminology
  -----------
  
 "cgroup" stands for "control group" and is never capitalized.  The
 singular form is used to designate the whole feature and also as a
 qualifier as in "cgroup controllers".  When explicitly referring to
 multiple individual control groups, the plural form "cgroups" is used.
 
 
 What is cgroup?
 ---------------
 
 cgroup is a mechanism to organize processes hierarchically and
 distribute system resources along the hierarchy in a controlled and
 configurable manner.
 
 cgroup is largely composed of two parts - the core and controllers.
 cgroup core is primarily responsible for hierarchically organizing
 processes.  A cgroup controller is usually responsible for
 distributing a specific type of system resource along the hierarchy
 although there are utility controllers which serve purposes other than
 resource distribution.
 
 cgroups form a tree structure and every process in the system belongs
 to one and only one cgroup.  All threads of a process belong to the
 same cgroup.  On creation, all processes are put in the cgroup that
 the parent process belongs to at the time.  A process can be migrated
 to another cgroup.  Migration of a process doesn't affect already
 existing descendant processes.
 
 Following certain structural constraints, controllers may be enabled or
 disabled selectively on a cgroup.  All controller behaviors are
 hierarchical - if a controller is enabled on a cgroup, it affects all
 processes which belong to the cgroups consisting the inclusive
 sub-hierarchy of the cgroup.  When a controller is enabled on a nested
 cgroup, it always restricts the resource distribution further.  The
 restrictions set closer to the root in the hierarchy can not be
 overridden from further away.
 
 
 Basic Operations
 ================
 
 Mounting
 --------
 
 Unlike v1, cgroup v2 has only single hierarchy.  The cgroup v2
 hierarchy can be mounted with the following mount command::
 
   # mount -t cgroup2 none $MOUNT_POINT
 
 cgroup2 filesystem has the magic number 0x63677270 ("cgrp").  All
 controllers which support v2 and are not bound to a v1 hierarchy are
 automatically bound to the v2 hierarchy and show up at the root.
 Controllers which are not in active use in the v2 hierarchy can be
 bound to other hierarchies.  This allows mixing v2 hierarchy with the
 legacy v1 multiple hierarchies in a fully backward compatible way.
 
 A controller can be moved across hierarchies only after the controller
 is no longer referenced in its current hierarchy.  Because per-cgroup
 controller states are destroyed asynchronously and controllers may
 have lingering references, a controller may not show up immediately on
 the v2 hierarchy after the final umount of the previous hierarchy.
 Similarly, a controller should be fully disabled to be moved out of
 the unified hierarchy and it may take some time for the disabled
 controller to become available for other hierarchies; furthermore, due
 to inter-controller dependencies, other controllers may need to be
 disabled too.
 
 While useful for development and manual configurations, moving
 controllers dynamically between the v2 and other hierarchies is
 strongly discouraged for production use.  It is recommended to decide
 the hierarchies and controller associations before starting using the
 controllers after system boot.
 
 During transition to v2, system management software might still
 automount the v1 cgroup filesystem and so hijack all controllers
 during boot, before manual intervention is possible. To make testing
 and experimenting easier, the kernel parameter cgroup_no_v1= allows
 disabling controllers in v1 and make them always available in v2.
 
 cgroup v2 currently supports the following mount options.
 
   nsdelegate
         Consider cgroup namespaces as delegation boundaries.  This
         option is system wide and can only be set on mount or modified
         through remount from the init namespace.  The mount option is
         ignored on non-init namespace mounts.  Please refer to the
         Delegation section for details.
 
   favordynmods
         Reduce the latencies of dynamic cgroup modifications such as
         task migrations and controller on/offs at the cost of making
         hot path operations such as forks and exits more expensive.
         The static usage pattern of creating a cgroup, enabling
         controllers, and then seeding it with CLONE_INTO_CGROUP is
         not affected by this option.
 
   memory_localevents
         Only populate memory.events with data for the current cgroup,
         and not any subtrees. This is legacy behaviour, the default
         behaviour without this option is to include subtree counts.
         This option is system wide and can only be set on mount or
         modified through remount from the init namespace. The mount
         option is ignored on non-init namespace mounts.
 
   memory_recursiveprot
         Recursively apply memory.min and memory.low protection to
         entire subtrees, without requiring explicit downward
         propagation into leaf cgroups.  This allows protecting entire
         subtrees from one another, while retaining free competition
         within those subtrees.  This should have been the default
         behavior but is a mount-option to avoid regressing setups
         relying on the original semantics (e.g. specifying bogusly
         high 'bypass' protection values at higher tree levels).
 
   memory_hugetlb_accounting
         Count HugeTLB memory usage towards the cgroup's overall
         memory usage for the memory controller (for the purpose of
         statistics reporting and memory protetion). This is a new
         behavior that could regress existing setups, so it must be
         explicitly opted in with this mount option.
 
         A few caveats to keep in mind:
 
         * There is no HugeTLB pool management involved in the memory
           controller. The pre-allocated pool does not belong to anyone.
           Specifically, when a new HugeTLB folio is allocated to
           the pool, it is not accounted for from the perspective of the
           memory controller. It is only charged to a cgroup when it is
           actually used (for e.g at page fault time). Host memory
           overcommit management has to consider this when configuring
           hard limits. In general, HugeTLB pool management should be
           done via other mechanisms (such as the HugeTLB controller).
         * Failure to charge a HugeTLB folio to the memory controller
           results in SIGBUS. This could happen even if the HugeTLB pool
           still has pages available (but the cgroup limit is hit and
           reclaim attempt fails).
         * Charging HugeTLB memory towards the memory controller affects
           memory protection and reclaim dynamics. Any userspace tuning
           (of low, min limits for e.g) needs to take this into account.
         * HugeTLB pages utilized while this option is not selected
           will not be tracked by the memory controller (even if cgroup
           v2 is remounted later on).
 
   pids_localevents
         The option restores v1-like behavior of pids.events:max, that is only
         local (inside cgroup proper) fork failures are counted. Without this
         option pids.events.max represents any pids.max enforcemnt across
         cgroup's subtree.
 
 
 
 Organizing Processes and Threads
 --------------------------------
 
 Processes
 ~~~~~~~~~
 
 Initially, only the root cgroup exists to which all processes belong.
 A child cgroup can be created by creating a sub-directory::
 
   # mkdir $CGROUP_NAME
 
 A given cgroup may have multiple child cgroups forming a tree
 structure.  Each cgroup has a read-writable interface file
 "cgroup.procs".  When read, it lists the PIDs of all processes which
 belong to the cgroup one-per-line.  The PIDs are not ordered and the
 same PID may show up more than once if the process got moved to
 another cgroup and then back or the PID got recycled while reading.
 
 A process can be migrated into a cgroup by writing its PID to the
 target cgroup's "cgroup.procs" file.  Only one process can be migrated
 on a single write(2) call.  If a process is composed of multiple
 threads, writing the PID of any thread migrates all threads of the
 process.
 
 When a process forks a child process, the new process is born into the
 cgroup that the forking process belongs to at the time of the
 operation.  After exit, a process stays associated with the cgroup
 that it belonged to at the time of exit until it's reaped; however, a
 zombie process does not appear in "cgroup.procs" and thus can't be
 moved to another cgroup.
 
 A cgroup which doesn't have any children or live processes can be
 destroyed by removing the directory.  Note that a cgroup which doesn't
 have any children and is associated only with zombie processes is
 considered empty and can be removed::
 
   # rmdir $CGROUP_NAME
 
 "/proc/$PID/cgroup" lists a process's cgroup membership.  If legacy
 cgroup is in use in the system, this file may contain multiple lines,
 one for each hierarchy.  The entry for cgroup v2 is always in the
 format "0::$PATH"::
 
   # cat /proc/842/cgroup
   ...
   0::/test-cgroup/test-cgroup-nested
 
 If the process becomes a zombie and the cgroup it was associated with
 is removed subsequently, " (deleted)" is appended to the path::
 
   # cat /proc/842/cgroup
   ...
   0::/test-cgroup/test-cgroup-nested (deleted)
 
 
 Threads
 ~~~~~~~
 
 cgroup v2 supports thread granularity for a subset of controllers to
 support use cases requiring hierarchical resource distribution across
 the threads of a group of processes.  By default, all threads of a
 process belong to the same cgroup, which also serves as the resource
 domain to host resource consumptions which are not specific to a
 process or thread.  The thread mode allows threads to be spread across
 a subtree while still maintaining the common resource domain for them.
 
 Controllers which support thread mode are called threaded controllers.
 The ones which don't are called domain controllers.
 
 Marking a cgroup threaded makes it join the resource domain of its
 parent as a threaded cgroup.  The parent may be another threaded
 cgroup whose resource domain is further up in the hierarchy.  The root
 of a threaded subtree, that is, the nearest ancestor which is not
 threaded, is called threaded domain or thread root interchangeably and
 serves as the resource domain for the entire subtree.
 
 Inside a threaded subtree, threads of a process can be put in
 different cgroups and are not subject to the no internal process
 constraint - threaded controllers can be enabled on non-leaf cgroups
 whether they have threads in them or not.
 
 As the threaded domain cgroup hosts all the domain resource
 consumptions of the subtree, it is considered to have internal
 resource consumptions whether there are processes in it or not and
 can't have populated child cgroups which aren't threaded.  Because the
 root cgroup is not subject to no internal process constraint, it can
 serve both as a threaded domain and a parent to domain cgroups.
 
 The current operation mode or type of the cgroup is shown in the
 "cgroup.type" file which indicates whether the cgroup is a normal
 domain, a domain which is serving as the domain of a threaded subtree,
 or a threaded cgroup.
 
 On creation, a cgroup is always a domain cgroup and can be made
 threaded by writing "threaded" to the "cgroup.type" file.  The
 operation is single direction::
 
   # echo threaded > cgroup.type
 
 Once threaded, the cgroup can't be made a domain again.  To enable the
 thread mode, the following conditions must be met.
 
 - As the cgroup will join the parent's resource domain.  The parent
   must either be a valid (threaded) domain or a threaded cgroup.
 
 - When the parent is an unthreaded domain, it must not have any domain
   controllers enabled or populated domain children.  The root is
   exempt from this requirement.
 
 Topology-wise, a cgroup can be in an invalid state.  Please consider
 the following topology::
 
   A (threaded domain) - B (threaded) - C (domain, just created)
 
 C is created as a domain but isn't connected to a parent which can
 host child domains.  C can't be used until it is turned into a
 threaded cgroup.  "cgroup.type" file will report "domain (invalid)" in
 these cases.  Operations which fail due to invalid topology use
 EOPNOTSUPP as the errno.
 
 A domain cgroup is turned into a threaded domain when one of its child
 cgroup becomes threaded or threaded controllers are enabled in the
 "cgroup.subtree_control" file while there are processes in the cgroup.
 A threaded domain reverts to a normal domain when the conditions
 clear.
 
 When read, "cgroup.threads" contains the list of the thread IDs of all
 threads in the cgroup.  Except that the operations are per-thread
 instead of per-process, "cgroup.threads" has the same format and
 behaves the same way as "cgroup.procs".  While "cgroup.threads" can be
 written to in any cgroup, as it can only move threads inside the same
 threaded domain, its operations are confined inside each threaded
 subtree.
 
 The threaded domain cgroup serves as the resource domain for the whole
 subtree, and, while the threads can be scattered across the subtree,
 all the processes are considered to be in the threaded domain cgroup.
 "cgroup.procs" in a threaded domain cgroup contains the PIDs of all
 processes in the subtree and is not readable in the subtree proper.
 However, "cgroup.procs" can be written to from anywhere in the subtree
 to migrate all threads of the matching process to the cgroup.
 
 Only threaded controllers can be enabled in a threaded subtree.  When
 a threaded controller is enabled inside a threaded subtree, it only
 accounts for and controls resource consumptions associated with the
 threads in the cgroup and its descendants.  All consumptions which
 aren't tied to a specific thread belong to the threaded domain cgroup.
 
 Because a threaded subtree is exempt from no internal process
 constraint, a threaded controller must be able to handle competition
 between threads in a non-leaf cgroup and its child cgroups.  Each
 threaded controller defines how such competitions are handled.
 
 Currently, the following controllers are threaded and can be enabled
 in a threaded cgroup::
 
 - cpu
 - cpuset
 - perf_event
 - pids
 
 [Un]populated Notification
 --------------------------
 
 Each non-root cgroup has a "cgroup.events" file which contains
 "populated" field indicating whether the cgroup's sub-hierarchy has
 live processes in it.  Its value is 0 if there is no live process in
 the cgroup and its descendants; otherwise, 1.  poll and [id]notify
 events are triggered when the value changes.  This can be used, for
 example, to start a clean-up operation after all processes of a given
 sub-hierarchy have exited.  The populated state updates and
 notifications are recursive.  Consider the following sub-hierarchy
 where the numbers in the parentheses represent the numbers of processes
 in each cgroup::
 
   A(4) - B(0) - C(1)
               \ D(0)
 
 A, B and C's "populated" fields would be 1 while D's 0.  After the one
 process in C exits, B and C's "populated" fields would flip to "0" and
 file modified events will be generated on the "cgroup.events" files of
 both cgroups.
 
 
 Controlling Controllers
 -----------------------
 
 Enabling and Disabling
 ~~~~~~~~~~~~~~~~~~~~~~
 
 Each cgroup has a "cgroup.controllers" file which lists all
 controllers available for the cgroup to enable::
 
   # cat cgroup.controllers
   cpu io memory
 
 No controller is enabled by default.  Controllers can be enabled and
 disabled by writing to the "cgroup.subtree_control" file::
 
   # echo "+cpu +memory -io" > cgroup.subtree_control
 
 Only controllers which are listed in "cgroup.controllers" can be
 enabled.  When multiple operations are specified as above, either they
 all succeed or fail.  If multiple operations on the same controller
 are specified, the last one is effective.
 
 Enabling a controller in a cgroup indicates that the distribution of
 the target resource across its immediate children will be controlled.
 Consider the following sub-hierarchy.  The enabled controllers are
 listed in parentheses::
 
   A(cpu,memory) - B(memory) - C()
                             \ D()
 
 As A has "cpu" and "memory" enabled, A will control the distribution
 of CPU cycles and memory to its children, in this case, B.  As B has
 "memory" enabled but not "CPU", C and D will compete freely on CPU
 cycles but their division of memory available to B will be controlled.
 
 As a controller regulates the distribution of the target resource to
 the cgroup's children, enabling it creates the controller's interface
 files in the child cgroups.  In the above example, enabling "cpu" on B
 would create the "cpu." prefixed controller interface files in C and
 D.  Likewise, disabling "memory" from B would remove the "memory."
 prefixed controller interface files from C and D.  This means that the
 controller interface files - anything which doesn't start with
 "cgroup." are owned by the parent rather than the cgroup itself.
 
 
 Top-down Constraint
 ~~~~~~~~~~~~~~~~~~~
 
 Resources are distributed top-down and a cgroup can further distribute
 a resource only if the resource has been distributed to it from the
 parent.  This means that all non-root "cgroup.subtree_control" files
 can only contain controllers which are enabled in the parent's
 "cgroup.subtree_control" file.  A controller can be enabled only if
 the parent has the controller enabled and a controller can't be
 disabled if one or more children have it enabled.
 
 
 No Internal Process Constraint
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Non-root cgroups can distribute domain resources to their children
 only when they don't have any processes of their own.  In other words,
 only domain cgroups which don't contain any processes can have domain
 controllers enabled in their "cgroup.subtree_control" files.
 
 This guarantees that, when a domain controller is looking at the part
 of the hierarchy which has it enabled, processes are always only on
 the leaves.  This rules out situations where child cgroups compete
 against internal processes of the parent.
 
 The root cgroup is exempt from this restriction.  Root contains
 processes and anonymous resource consumption which can't be associated
 with any other cgroups and requires special treatment from most
 controllers.  How resource consumption in the root cgroup is governed
 is up to each controller (for more information on this topic please
 refer to the Non-normative information section in the Controllers
 chapter).
 
 Note that the restriction doesn't get in the way if there is no
 enabled controller in the cgroup's "cgroup.subtree_control".  This is
 important as otherwise it wouldn't be possible to create children of a
 populated cgroup.  To control resource distribution of a cgroup, the
 cgroup must create children and transfer all its processes to the
 children before enabling controllers in its "cgroup.subtree_control"
 file.
 
 
 Delegation
 ----------
 
 Model of Delegation
 ~~~~~~~~~~~~~~~~~~~
 
 A cgroup can be delegated in two ways.  First, to a less privileged
 user by granting write access of the directory and its "cgroup.procs",
 "cgroup.threads" and "cgroup.subtree_control" files to the user.
 Second, if the "nsdelegate" mount option is set, automatically to a
 cgroup namespace on namespace creation.
 
 Because the resource control interface files in a given directory
 control the distribution of the parent's resources, the delegatee
 shouldn't be allowed to write to them.  For the first method, this is
 achieved by not granting access to these files.  For the second, the
 kernel rejects writes to all files other than "cgroup.procs" and
 "cgroup.subtree_control" on a namespace root from inside the
 namespace.
 
 The end results are equivalent for both delegation types.  Once
 delegated, the user can build sub-hierarchy under the directory,
 organize processes inside it as it sees fit and further distribute the
 resources it received from the parent.  The limits and other settings
 of all resource controllers are hierarchical and regardless of what
 happens in the delegated sub-hierarchy, nothing can escape the
 resource restrictions imposed by the parent.
 
 Currently, cgroup doesn't impose any restrictions on the number of
 cgroups in or nesting depth of a delegated sub-hierarchy; however,
 this may be limited explicitly in the future.
 
 
 Delegation Containment
 ~~~~~~~~~~~~~~~~~~~~~~
 
 A delegated sub-hierarchy is contained in the sense that processes
 can't be moved into or out of the sub-hierarchy by the delegatee.
 
 For delegations to a less privileged user, this is achieved by
 requiring the following conditions for a process with a non-root euid
 to migrate a target process into a cgroup by writing its PID to the
 "cgroup.procs" file.
 
 - The writer must have write access to the "cgroup.procs" file.
 
 - The writer must have write access to the "cgroup.procs" file of the
   common ancestor of the source and destination cgroups.
 
 The above two constraints ensure that while a delegatee may migrate
 processes around freely in the delegated sub-hierarchy it can't pull
 in from or push out to outside the sub-hierarchy.
 
 For an example, let's assume cgroups C0 and C1 have been delegated to
 user U0 who created C00, C01 under C0 and C10 under C1 as follows and
 all processes under C0 and C1 belong to U0::
 
   ~~~~~~~~~~~~~ - C0 - C00
   ~ cgroup    ~      \ C01
   ~ hierarchy ~
   ~~~~~~~~~~~~~ - C1 - C10
 
 Let's also say U0 wants to write the PID of a process which is
 currently in C10 into "C00/cgroup.procs".  U0 has write access to the
 file; however, the common ancestor of the source cgroup C10 and the
 destination cgroup C00 is above the points of delegation and U0 would
 not have write access to its "cgroup.procs" files and thus the write
 will be denied with -EACCES.
 
 For delegations to namespaces, containment is achieved by requiring
 that both the source and destination cgroups are reachable from the
 namespace of the process which is attempting the migration.  If either
 is not reachable, the migration is rejected with -ENOENT.
 
 
 Guidelines
 ----------
 
 Organize Once and Control
 ~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Migrating a process across cgroups is a relatively expensive operation
 and stateful resources such as memory are not moved together with the
 process.  This is an explicit design decision as there often exist
 inherent trade-offs between migration and various hot paths in terms
 of synchronization cost.
 
 As such, migrating processes across cgroups frequently as a means to
 apply different resource restrictions is discouraged.  A workload
 should be assigned to a cgroup according to the system's logical and
 resource structure once on start-up.  Dynamic adjustments to resource
 distribution can be made by changing controller configuration through
 the interface files.
 
 
 Avoid Name Collisions
 ~~~~~~~~~~~~~~~~~~~~~
 
 Interface files for a cgroup and its children cgroups occupy the same
 directory and it is possible to create children cgroups which collide
 with interface files.
 
 All cgroup core interface files are prefixed with "cgroup." and each
 controller's interface files are prefixed with the controller name and
 a dot.  A controller's name is composed of lower case alphabets and
 '_'s but never begins with an '_' so it can be used as the prefix
 character for collision avoidance.  Also, interface file names won't
 start or end with terms which are often used in categorizing workloads
 such as job, service, slice, unit or workload.
 
 cgroup doesn't do anything to prevent name collisions and it's the
 user's responsibility to avoid them.
 
 
 Resource Distribution Models
 ============================
 
 cgroup controllers implement several resource distribution schemes
 depending on the resource type and expected use cases.  This section
 describes major schemes in use along with their expected behaviors.
 
 
 Weights
 -------
 
 A parent's resource is distributed by adding up the weights of all
 active children and giving each the fraction matching the ratio of its
 weight against the sum.  As only children which can make use of the
 resource at the moment participate in the distribution, this is
 work-conserving.  Due to the dynamic nature, this model is usually
 used for stateless resources.
 
 All weights are in the range [1, 10000] with the default at 100.  This
 allows symmetric multiplicative biases in both directions at fine
 enough granularity while staying in the intuitive range.
 
 As long as the weight is in range, all configuration combinations are
 valid and there is no reason to reject configuration changes or
 process migrations.
 
 "cpu.weight" proportionally distributes CPU cycles to active children
 and is an example of this type.
 
 
 .. _cgroupv2-limits-distributor:
 
 Limits
 ------
 
 A child can only consume up to the configured amount of the resource.
 Limits can be over-committed - the sum of the limits of children can
 exceed the amount of resource available to the parent.
 
 Limits are in the range [0, max] and defaults to "max", which is noop.
 
 As limits can be over-committed, all configuration combinations are
 valid and there is no reason to reject configuration changes or
 process migrations.
 
 "io.max" limits the maximum BPS and/or IOPS that a cgroup can consume
 on an IO device and is an example of this type.
 
 .. _cgroupv2-protections-distributor:
 
 Protections
 -----------
 
 A cgroup is protected up to the configured amount of the resource
 as long as the usages of all its ancestors are under their
 protected levels.  Protections can be hard guarantees or best effort
 soft boundaries.  Protections can also be over-committed in which case
 only up to the amount available to the parent is protected among
 children.
 
 Protections are in the range [0, max] and defaults to 0, which is
 noop.
 
 As protections can be over-committed, all configuration combinations
 are valid and there is no reason to reject configuration changes or
 process migrations.
 
 "memory.low" implements best-effort memory protection and is an
 example of this type.
 
 
 Allocations
 -----------
 
 A cgroup is exclusively allocated a certain amount of a finite
 resource.  Allocations can't be over-committed - the sum of the
 allocations of children can not exceed the amount of resource
 available to the parent.
 
 Allocations are in the range [0, max] and defaults to 0, which is no
 resource.
 
 As allocations can't be over-committed, some configuration
 combinations are invalid and should be rejected.  Also, if the
 resource is mandatory for execution of processes, process migrations
 may be rejected.
 
 "cpu.rt.max" hard-allocates realtime slices and is an example of this
 type.
 
 
 Interface Files
 ===============
 
 Format
 ------
 
 All interface files should be in one of the following formats whenever
 possible::
 
   New-line separated values
   (when only one value can be written at once)
 
         VAL0\n
         VAL1\n
         ...
 
   Space separated values
   (when read-only or multiple values can be written at once)
 
         VAL0 VAL1 ...\n
 
   Flat keyed
 
         KEY0 VAL0\n
         KEY1 VAL1\n
         ...
 
   Nested keyed
 
         KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
         KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
         ...
 
 For a writable file, the format for writing should generally match
 reading; however, controllers may allow omitting later fields or
 implement restricted shortcuts for most common use cases.
 
 For both flat and nested keyed files, only the values for a single key
 can be written at a time.  For nested keyed files, the sub key pairs
 may be specified in any order and not all pairs have to be specified.
 
 
 Conventions
 -----------
 
 - Settings for a single feature should be contained in a single file.
 
 - The root cgroup should be exempt from resource control and thus
   shouldn't have resource control interface files.
 
 - The default time unit is microseconds.  If a different unit is ever
   used, an explicit unit suffix must be present.
 
 - A parts-per quantity should use a percentage decimal with at least
   two digit fractional part - e.g. 13.40.
 
 - If a controller implements weight based resource distribution, its
   interface file should be named "weight" and have the range [1,
   10000] with 100 as the default.  The values are chosen to allow
   enough and symmetric bias in both directions while keeping it
   intuitive (the default is 100%).
 
 - If a controller implements an absolute resource guarantee and/or
   limit, the interface files should be named "min" and "max"
   respectively.  If a controller implements best effort resource
   guarantee and/or limit, the interface files should be named "low"
   and "high" respectively.
 
   In the above four control files, the special token "max" should be
   used to represent upward infinity for both reading and writing.
 
 - If a setting has a configurable default value and keyed specific
   overrides, the default entry should be keyed with "default" and
   appear as the first entry in the file.
 
   The default value can be updated by writing either "default $VAL" or
   "$VAL".
 
   When writing to update a specific override, "default" can be used as
   the value to indicate removal of the override.  Override entries
   with "default" as the value must not appear when read.
 
   For example, a setting which is keyed by major:minor device numbers
   with integer values may look like the following::
 
     # cat cgroup-example-interface-file
     default 150
     8:0 300
 
   The default value can be updated by::
 
     # echo 125 > cgroup-example-interface-file
 
   or::
 
     # echo "default 125" > cgroup-example-interface-file
 
   An override can be set by::
 
     # echo "8:16 170" > cgroup-example-interface-file
 
   and cleared by::
 
     # echo "8:0 default" > cgroup-example-interface-file
     # cat cgroup-example-interface-file
     default 125
     8:16 170
 
 - For events which are not very high frequency, an interface file
   "events" should be created which lists event key value pairs.
   Whenever a notifiable event happens, file modified event should be
   generated on the file.
 
 
 Core Interface Files
 --------------------
 
 All cgroup core files are prefixed with "cgroup."
 
   cgroup.type
         A read-write single value file which exists on non-root
         cgroups.
 
         When read, it indicates the current type of the cgroup, which
         can be one of the following values.
 
         - "domain" : A normal valid domain cgroup.
 
         - "domain threaded" : A threaded domain cgroup which is
           serving as the root of a threaded subtree.
 
         - "domain invalid" : A cgroup which is in an invalid state.
           It can't be populated or have controllers enabled.  It may
           be allowed to become a threaded cgroup.
 
         - "threaded" : A threaded cgroup which is a member of a
           threaded subtree.
 
         A cgroup can be turned into a threaded cgroup by writing
         "threaded" to this file.
 
   cgroup.procs
         A read-write new-line separated values file which exists on
         all cgroups.
 
         When read, it lists the PIDs of all processes which belong to
         the cgroup one-per-line.  The PIDs are not ordered and the
         same PID may show up more than once if the process got moved
         to another cgroup and then back or the PID got recycled while
         reading.
 
         A PID can be written to migrate the process associated with
         the PID to the cgroup.  The writer should match all of the
         following conditions.
 
         - It must have write access to the "cgroup.procs" file.
 
         - It must have write access to the "cgroup.procs" file of the
           common ancestor of the source and destination cgroups.
 
         When delegating a sub-hierarchy, write access to this file
         should be granted along with the containing directory.
 
         In a threaded cgroup, reading this file fails with EOPNOTSUPP
         as all the processes belong to the thread root.  Writing is
         supported and moves every thread of the process to the cgroup.
 
   cgroup.threads
         A read-write new-line separated values file which exists on
         all cgroups.
 
         When read, it lists the TIDs of all threads which belong to
         the cgroup one-per-line.  The TIDs are not ordered and the
         same TID may show up more than once if the thread got moved to
         another cgroup and then back or the TID got recycled while
         reading.
 
         A TID can be written to migrate the thread associated with the
         TID to the cgroup.  The writer should match all of the
         following conditions.
 
         - It must have write access to the "cgroup.threads" file.
 
         - The cgroup that the thread is currently in must be in the
           same resource domain as the destination cgroup.
 
         - It must have write access to the "cgroup.procs" file of the
           common ancestor of the source and destination cgroups.
 
         When delegating a sub-hierarchy, write access to this file
         should be granted along with the containing directory.
 
   cgroup.controllers
         A read-only space separated values file which exists on all
         cgroups.
 
         It shows space separated list of all controllers available to
         the cgroup.  The controllers are not ordered.
 
   cgroup.subtree_control
         A read-write space separated values file which exists on all
         cgroups.  Starts out empty.
 
         When read, it shows space separated list of the controllers
         which are enabled to control resource distribution from the
         cgroup to its children.
 
         Space separated list of controllers prefixed with '+' or '-'
         can be written to enable or disable controllers.  A controller
         name prefixed with '+' enables the controller and '-'
         disables.  If a controller appears more than once on the list,
         the last one is effective.  When multiple enable and disable
         operations are specified, either all succeed or all fail.
 
   cgroup.events
         A read-only flat-keyed file which exists on non-root cgroups.
         The following entries are defined.  Unless specified
         otherwise, a value change in this file generates a file
         modified event.
 
           populated
                 1 if the cgroup or its descendants contains any live
                 processes; otherwise, 0.
           frozen
                 1 if the cgroup is frozen; otherwise, 0.
 
   cgroup.max.descendants
         A read-write single value files.  The default is "max".
 
         Maximum allowed number of descent cgroups.
         If the actual number of descendants is equal or larger,
         an attempt to create a new cgroup in the hierarchy will fail.
 
   cgroup.max.depth
         A read-write single value files.  The default is "max".
 
         Maximum allowed descent depth below the current cgroup.
         If the actual descent depth is equal or larger,
         an attempt to create a new child cgroup will fail.
 
   cgroup.stat
         A read-only flat-keyed file with the following entries:
 
           nr_descendants
                 Total number of visible descendant cgroups.
 
           nr_dying_descendants
                 Total number of dying descendant cgroups. A cgroup becomes
                 dying after being deleted by a user. The cgroup will remain
                 in dying state for some time undefined time (which can depend
                 on system load) before being completely destroyed.
 
                 A process can't enter a dying cgroup under any circumstances,
                 a dying cgroup can't revive.
 
                 A dying cgroup can consume system resources not exceeding
                 limits, which were active at the moment of cgroup deletion.
 
   cgroup.freeze
         A read-write single value file which exists on non-root cgroups.
         Allowed values are "0" and "1". The default is "0".
 
         Writing "1" to the file causes freezing of the cgroup and all
         descendant cgroups. This means that all belonging processes will
         be stopped and will not run until the cgroup will be explicitly
         unfrozen. Freezing of the cgroup may take some time; when this action
         is completed, the "frozen" value in the cgroup.events control file
         will be updated to "1" and the corresponding notification will be
         issued.
 
         A cgroup can be frozen either by its own settings, or by settings
         of any ancestor cgroups. If any of ancestor cgroups is frozen, the
         cgroup will remain frozen.
 
         Processes in the frozen cgroup can be killed by a fatal signal.
         They also can enter and leave a frozen cgroup: either by an explicit
         move by a user, or if freezing of the cgroup races with fork().
         If a process is moved to a frozen cgroup, it stops. If a process is
         moved out of a frozen cgroup, it becomes running.
 
         Frozen status of a cgroup doesn't affect any cgroup tree operations:
         it's possible to delete a frozen (and empty) cgroup, as well as
         create new sub-cgroups.
 
   cgroup.kill
         A write-only single value file which exists in non-root cgroups.
         The only allowed value is "1".
 
         Writing "1" to the file causes the cgroup and all descendant cgroups to
         be killed. This means that all processes located in the affected cgroup
         tree will be killed via SIGKILL.
 
         Killing a cgroup tree will deal with concurrent forks appropriately and
         is protected against migrations.
 
         In a threaded cgroup, writing this file fails with EOPNOTSUPP as
         killing cgroups is a process directed operation, i.e. it affects
         the whole thread-group.
 
   cgroup.pressure
         A read-write single value file that allowed values are "0" and "1".
         The default is "1".
 
         Writing "0" to the file will disable the cgroup PSI accounting.
         Writing "1" to the file will re-enable the cgroup PSI accounting.
 
         This control attribute is not hierarchical, so disable or enable PSI
         accounting in a cgroup does not affect PSI accounting in descendants
         and doesn't need pass enablement via ancestors from root.
 
         The reason this control attribute exists is that PSI accounts stalls for
         each cgroup separately and aggregates it at each level of the hierarchy.
         This may cause non-negligible overhead for some workloads when under
         deep level of the hierarchy, in which case this control attribute can
         be used to disable PSI accounting in the non-leaf cgroups.
 
   irq.pressure
         A read-write nested-keyed file.
 
         Shows pressure stall information for IRQ/SOFTIRQ. See
         :ref:`Documentation/accounting/psi.rst <psi>` for details.
 
 Controllers
 ===========
 
 .. _cgroup-v2-cpu:
 
 CPU
 ---
 
 The "cpu" controllers regulates distribution of CPU cycles.  This
 controller implements weight and absolute bandwidth limit models for
 normal scheduling policy and absolute bandwidth allocation model for
 realtime scheduling policy.
 
 In all the above models, cycles distribution is defined only on a temporal
 base and it does not account for the frequency at which tasks are executed.
 The (optional) utilization clamping support allows to hint the schedutil
 cpufreq governor about the minimum desired frequency which should always be
 provided by a CPU, as well as the maximum desired frequency, which should not
 be exceeded by a CPU.
 
 WARNING: cgroup2 doesn't yet support control of realtime processes. For
 a kernel built with the CONFIG_RT_GROUP_SCHED option enabled for group
 scheduling of realtime processes, the cpu controller can only be enabled
 when all RT processes are in the root cgroup.  This limitation does
 not apply if CONFIG_RT_GROUP_SCHED is disabled.  Be aware that system
 management software may already have placed RT processes into nonroot
 cgroups during the system boot process, and these processes may need
 to be moved to the root cgroup before the cpu controller can be enabled
 with a CONFIG_RT_GROUP_SCHED enabled kernel.
 
 
 CPU Interface Files
 ~~~~~~~~~~~~~~~~~~~
 
 All time durations are in microseconds.
 
   cpu.stat
         A read-only flat-keyed file.
         This file exists whether the controller is enabled or not.
 
         It always reports the following three stats:
 
         - usage_usec
         - user_usec
         - system_usec
 
         and the following five when the controller is enabled:
 
         - nr_periods
         - nr_throttled
         - throttled_usec
         - nr_bursts
         - burst_usec
 
   cpu.weight
         A read-write single value file which exists on non-root
         cgroups.  The default is "100".
 
         For non idle groups (cpu.idle = 0), the weight is in the
         range [1, 10000].
 
         If the cgroup has been configured to be SCHED_IDLE (cpu.idle = 1),
         then the weight will show as a 0.
 
   cpu.weight.nice
         A read-write single value file which exists on non-root
         cgroups.  The default is "0".
 
         The nice value is in the range [-20, 19].
 
         This interface file is an alternative interface for
         "cpu.weight" and allows reading and setting weight using the
         same values used by nice(2).  Because the range is smaller and
         granularity is coarser for the nice values, the read value is
         the closest approximation of the current weight.
 
   cpu.max
         A read-write two value file which exists on non-root cgroups.
         The default is "max 100000".
 
         The maximum bandwidth limit.  It's in the following format::
 
           $MAX $PERIOD
 
         which indicates that the group may consume up to $MAX in each
         $PERIOD duration.  "max" for $MAX indicates no limit.  If only
         one number is written, $MAX is updated.
 
   cpu.max.burst
         A read-write single value file which exists on non-root
         cgroups.  The default is "0".
 
         The burst in the range [0, $MAX].
 
   cpu.pressure
         A read-write nested-keyed file.
 
         Shows pressure stall information for CPU. See
         :ref:`Documentation/accounting/psi.rst <psi>` for details.
 
   cpu.uclamp.min
         A read-write single value file which exists on non-root cgroups.
         The default is "0", i.e. no utilization boosting.
 
         The requested minimum utilization (protection) as a percentage
         rational number, e.g. 12.34 for 12.34%.
 
         This interface allows reading and setting minimum utilization clamp
         values similar to the sched_setattr(2). This minimum utilization
         value is used to clamp the task specific minimum utilization clamp.
 
         The requested minimum utilization (protection) is always capped by
         the current value for the maximum utilization (limit), i.e.
         `cpu.uclamp.max`.
 
   cpu.uclamp.max
         A read-write single value file which exists on non-root cgroups.
         The default is "max". i.e. no utilization capping
 
         The requested maximum utilization (limit) as a percentage rational
         number, e.g. 98.76 for 98.76%.
 
         This interface allows reading and setting maximum utilization clamp
         values similar to the sched_setattr(2). This maximum utilization
         value is used to clamp the task specific maximum utilization clamp.
 
   cpu.idle
         A read-write single value file which exists on non-root cgroups.
         The default is 0.
 
         This is the cgroup analog of the per-task SCHED_IDLE sched policy.
         Setting this value to a 1 will make the scheduling policy of the
         cgroup SCHED_IDLE. The threads inside the cgroup will retain their
         own relative priorities, but the cgroup itself will be treated as
         very low priority relative to its peers.
 
 
 
 Memory
 ------
 
 The "memory" controller regulates distribution of memory.  Memory is
 stateful and implements both limit and protection models.  Due to the
 intertwining between memory usage and reclaim pressure and the
 stateful nature of memory, the distribution model is relatively
 complex.
 
 While not completely water-tight, all major memory usages by a given
 cgroup are tracked so that the total memory consumption can be
 accounted and controlled to a reasonable extent.  Currently, the
 following types of memory usages are tracked.
 
 - Userland memory - page cache and anonymous memory.
 
 - Kernel data structures such as dentries and inodes.
 
 - TCP socket buffers.
 
 The above list may expand in the future for better coverage.
 
 
 Memory Interface Files
 ~~~~~~~~~~~~~~~~~~~~~~
 
 All memory amounts are in bytes.  If a value which is not aligned to
 PAGE_SIZE is written, the value may be rounded up to the closest
 PAGE_SIZE multiple when read back.
 
   memory.current
         A read-only single value file which exists on non-root
         cgroups.
 
         The total amount of memory currently being used by the cgroup
         and its descendants.
 
   memory.min
         A read-write single value file which exists on non-root
         cgroups.  The default is "0".
 
         Hard memory protection.  If the memory usage of a cgroup
         is within its effective min boundary, the cgroup's memory
         won't be reclaimed under any conditions. If there is no
         unprotected reclaimable memory available, OOM killer
         is invoked. Above the effective min boundary (or
         effective low boundary if it is higher), pages are reclaimed
         proportionally to the overage, reducing reclaim pressure for
         smaller overages.
 
         Effective min boundary is limited by memory.min values of
         all ancestor cgroups. If there is memory.min overcommitment
         (child cgroup or cgroups are requiring more protected memory
         than parent will allow), then each child cgroup will get
         the part of parent's protection proportional to its
         actual memory usage below memory.min.
 
         Putting more memory than generally available under this
         protection is discouraged and may lead to constant OOMs.
 
         If a memory cgroup is not populated with processes,
         its memory.min is ignored.
 
   memory.low
         A read-write single value file which exists on non-root
         cgroups.  The default is "0".
 
         Best-effort memory protection.  If the memory usage of a
         cgroup is within its effective low boundary, the cgroup's
         memory won't be reclaimed unless there is no reclaimable
         memory available in unprotected cgroups.
         Above the effective low boundary (or 
         effective min boundary if it is higher), pages are reclaimed
         proportionally to the overage, reducing reclaim pressure for
         smaller overages.
 
         Effective low boundary is limited by memory.low values of
         all ancestor cgroups. If there is memory.low overcommitment
         (child cgroup or cgroups are requiring more protected memory
         than parent will allow), then each child cgroup will get
         the part of parent's protection proportional to its
         actual memory usage below memory.low.
 
         Putting more memory than generally available under this
         protection is discouraged.
 
   memory.high
         A read-write single value file which exists on non-root
         cgroups.  The default is "max".
 
         Memory usage throttle limit.  If a cgroup's usage goes
         over the high boundary, the processes of the cgroup are
         throttled and put under heavy reclaim pressure.
 
         Going over the high limit never invokes the OOM killer and
         under extreme conditions the limit may be breached. The high
         limit should be used in scenarios where an external process
         monitors the limited cgroup to alleviate heavy reclaim
         pressure.
 
   memory.max
         A read-write single value file which exists on non-root
         cgroups.  The default is "max".
 
         Memory usage hard limit.  This is the main mechanism to limit
         memory usage of a cgroup.  If a cgroup's memory usage reaches
         this limit and can't be reduced, the OOM killer is invoked in
         the cgroup. Under certain circumstances, the usage may go
         over the limit temporarily.
 
         In default configuration regular 0-order allocations always
         succeed unless OOM killer chooses current task as a victim.
 
         Some kinds of allocations don't invoke the OOM killer.
         Caller could retry them differently, return into userspace
         as -ENOMEM or silently ignore in cases like disk readahead.
 
   memory.reclaim
         A write-only nested-keyed file which exists for all cgroups.
 
         This is a simple interface to trigger memory reclaim in the
         target cgroup.
 
         Example::
 
           echo "1G" > memory.reclaim
 
         Please note that the kernel can over or under reclaim from
         the target cgroup. If less bytes are reclaimed than the
         specified amount, -EAGAIN is returned.
 
         Please note that the proactive reclaim (triggered by this
         interface) is not meant to indicate memory pressure on the
         memory cgroup. Therefore socket memory balancing triggered by
         the memory reclaim normally is not exercised in this case.
         This means that the networking layer will not adapt based on
         reclaim induced by memory.reclaim.
 
 The following nested keys are defined.
 
           ==========            ================================
           swappiness            Swappiness value to reclaim with
           ==========            ================================
 
         Specifying a swappiness value instructs the kernel to perform
         the reclaim with that swappiness value. Note that this has the
         same semantics as vm.swappiness applied to memcg reclaim with
         all the existing limitations and potential future extensions.
 
   memory.peak
         A read-only single value file which exists on non-root
         cgroups.
 
         The max memory usage recorded for the cgroup and its
         descendants since the creation of the cgroup.
 
   memory.oom.group
         A read-write single value file which exists on non-root
         cgroups.  The default value is "0".
 
         Determines whether the cgroup should be treated as
         an indivisible workload by the OOM killer. If set,
         all tasks belonging to the cgroup or to its descendants
         (if the memory cgroup is not a leaf cgroup) are killed
         together or not at all. This can be used to avoid
         partial kills to guarantee workload integrity.
 
         Tasks with the OOM protection (oom_score_adj set to -1000)
         are treated as an exception and are never killed.
 
         If the OOM killer is invoked in a cgroup, it's not going
         to kill any tasks outside of this cgroup, regardless
         memory.oom.group values of ancestor cgroups.
 
   memory.events
         A read-only flat-keyed file which exists on non-root cgroups.
         The following entries are defined.  Unless specified
         otherwise, a value change in this file generates a file
         modified event.
 
         Note that all fields in this file are hierarchical and the
         file modified event can be generated due to an event down the
         hierarchy. For the local events at the cgroup level see
         memory.events.local.
 
           low
                 The number of times the cgroup is reclaimed due to
                 high memory pressure even though its usage is under
                 the low boundary.  This usually indicates that the low
                 boundary is over-committed.
 
           high
                 The number of times processes of the cgroup are
                 throttled and routed to perform direct memory reclaim
                 because the high memory boundary was exceeded.  For a
                 cgroup whose memory usage is capped by the high limit
                 rather than global memory pressure, this event's
                 occurrences are expected.
 
           max
                 The number of times the cgroup's memory usage was
                 about to go over the max boundary.  If direct reclaim
                 fails to bring it down, the cgroup goes to OOM state.
 
           oom
                 The number of time the cgroup's memory usage was
                 reached the limit and allocation was about to fail.
 
                 This event is not raised if the OOM killer is not
                 considered as an option, e.g. for failed high-order
                 allocations or if caller asked to not retry attempts.
 
           oom_kill
                 The number of processes belonging to this cgroup
                 killed by any kind of OOM killer.
 
           oom_group_kill
                 The number of times a group OOM has occurred.
 
   memory.events.local
         Similar to memory.events but the fields in the file are local
         to the cgroup i.e. not hierarchical. The file modified event
         generated on this file reflects only the local events.
 
   memory.stat
         A read-only flat-keyed file which exists on non-root cgroups.
 
         This breaks down the cgroup's memory footprint into different
         types of memory, type-specific details, and other information
         on the state and past events of the memory management system.
 
         All memory amounts are in bytes.
 
         The entries are ordered to be human readable, and new entries
         can show up in the middle. Don't rely on items remaining in a
         fixed position; use the keys to look up specific values!
 
         If the entry has no per-node counter (or not show in the
         memory.numa_stat). We use 'npn' (non-per-node) as the tag
         to indicate that it will not show in the memory.numa_stat.
 
           anon
                 Amount of memory used in anonymous mappings such as
                 brk(), sbrk(), and mmap(MAP_ANONYMOUS)
 
           file
                 Amount of memory used to cache filesystem data,
                 including tmpfs and shared memory.
 
           kernel (npn)
                 Amount of total kernel memory, including
                 (kernel_stack, pagetables, percpu, vmalloc, slab) in
                 addition to other kernel memory use cases.
 
           kernel_stack
                 Amount of memory allocated to kernel stacks.
 
           pagetables
                 Amount of memory allocated for page tables.
 
           sec_pagetables
                 Amount of memory allocated for secondary page tables,
                 this currently includes KVM mmu allocations on x86
                 and arm64 and IOMMU page tables.
 
           percpu (npn)
                 Amount of memory used for storing per-cpu kernel
                 data structures.
 
           sock (npn)
                 Amount of memory used in network transmission buffers
 
           vmalloc (npn)
                 Amount of memory used for vmap backed memory.
 
           shmem
                 Amount of cached filesystem data that is swap-backed,
                 such as tmpfs, shm segments, shared anonymous mmap()s
 
           zswap
                 Amount of memory consumed by the zswap compression backend.
 
           zswapped
                 Amount of application memory swapped out to zswap.
 
           file_mapped
                 Amount of cached filesystem data mapped with mmap()
 
           file_dirty
                 Amount of cached filesystem data that was modified but
                 not yet written back to disk
 
           file_writeback
                 Amount of cached filesystem data that was modified and
                 is currently being written back to disk
 
           swapcached
                 Amount of swap cached in memory. The swapcache is accounted
                 against both memory and swap usage.
 
           anon_thp
                 Amount of memory used in anonymous mappings backed by
                 transparent hugepages
 
           file_thp
                 Amount of cached filesystem data backed by transparent
                 hugepages
 
           shmem_thp
                 Amount of shm, tmpfs, shared anonymous mmap()s backed by
                 transparent hugepages
 
           inactive_anon, active_anon, inactive_file, active_file, unevictable
                 Amount of memory, swap-backed and filesystem-backed,
                 on the internal memory management lists used by the
                 page reclaim algorithm.
 
                 As these represent internal list state (eg. shmem pages are on anon
                 memory management lists), inactive_foo + active_foo may not be equal to
                 the value for the foo counter, since the foo counter is type-based, not
                 list-based.
 
           slab_reclaimable
                 Part of "slab" that might be reclaimed, such as
                 dentries and inodes.
 
           slab_unreclaimable
                 Part of "slab" that cannot be reclaimed on memory
                 pressure.
 
           slab (npn)
                 Amount of memory used for storing in-kernel data
                 structures.
 
           workingset_refault_anon
                 Number of refaults of previously evicted anonymous pages.
 
           workingset_refault_file
                 Number of refaults of previously evicted file pages.
 
           workingset_activate_anon
                 Number of refaulted anonymous pages that were immediately
                 activated.
 
           workingset_activate_file
                 Number of refaulted file pages that were immediately activated.
 
           workingset_restore_anon
                 Number of restored anonymous pages which have been detected as
                 an active workingset before they got reclaimed.
 
           workingset_restore_file
                 Number of restored file pages which have been detected as an
                 active workingset before they got reclaimed.
 
           workingset_nodereclaim
                 Number of times a shadow node has been reclaimed
 
           pgscan (npn)
                 Amount of scanned pages (in an inactive LRU list)
 
           pgsteal (npn)
                 Amount of reclaimed pages
 
           pgscan_kswapd (npn)
                 Amount of scanned pages by kswapd (in an inactive LRU list)
 
           pgscan_direct (npn)
                 Amount of scanned pages directly  (in an inactive LRU list)
 
           pgscan_khugepaged (npn)
                 Amount of scanned pages by khugepaged  (in an inactive LRU list)
 
           pgsteal_kswapd (npn)
                 Amount of reclaimed pages by kswapd
 
           pgsteal_direct (npn)
                 Amount of reclaimed pages directly
 
           pgsteal_khugepaged (npn)
                 Amount of reclaimed pages by khugepaged
 
           pgfault (npn)
                 Total number of page faults incurred
 
           pgmajfault (npn)
                 Number of major page faults incurred
 
           pgrefill (npn)
                 Amount of scanned pages (in an active LRU list)
 
           pgactivate (npn)
                 Amount of pages moved to the active LRU list
 
           pgdeactivate (npn)
                 Amount of pages moved to the inactive LRU list
 
           pglazyfree (npn)
                 Amount of pages postponed to be freed under memory pressure
 
           pglazyfreed (npn)
                 Amount of reclaimed lazyfree pages
 
           zswpin
                 Number of pages moved in to memory from zswap.
 
           zswpout
                 Number of pages moved out of memory to zswap.
 
           zswpwb
                 Number of pages written from zswap to swap.
 
           thp_fault_alloc (npn)
                 Number of transparent hugepages which were allocated to satisfy
                 a page fault. This counter is not present when CONFIG_TRANSPARENT_HUGEPAGE
                 is not set.
 
           thp_collapse_alloc (npn)
                 Number of transparent hugepages which were allocated to allow
                 collapsing an existing range of pages. This counter is not
                 present when CONFIG_TRANSPARENT_HUGEPAGE is not set.
 
           thp_swpout (npn)
                 Number of transparent hugepages which are swapout in one piece
                 without splitting.
 
           thp_swpout_fallback (npn)
                 Number of transparent hugepages which were split before swapout.
                 Usually because failed to allocate some continuous swap space
                 for the huge page.
 
   memory.numa_stat
         A read-only nested-keyed file which exists on non-root cgroups.
 
         This breaks down the cgroup's memory footprint into different
         types of memory, type-specific details, and other information
         per node on the state of the memory management system.
 
         This is useful for providing visibility into the NUMA locality
         information within an memcg since the pages are allowed to be
         allocated from any physical node. One of the use case is evaluating
         application performance by combining this information with the
         application's CPU allocation.
 
         All memory amounts are in bytes.
 
         The output format of memory.numa_stat is::
 
           type N0=<bytes in node 0> N1=<bytes in node 1> ...
 
         The entries are ordered to be human readable, and new entries
         can show up in the middle. Don't rely on items remaining in a
         fixed position; use the keys to look up specific values!
 
         The entries can refer to the memory.stat.
 
   memory.swap.current
         A read-only single value file which exists on non-root
         cgroups.
 
         The total amount of swap currently being used by the cgroup
         and its descendants.
 
   memory.swap.high
         A read-write single value file which exists on non-root
         cgroups.  The default is "max".
 
         Swap usage throttle limit.  If a cgroup's swap usage exceeds
         this limit, all its further allocations will be throttled to
         allow userspace to implement custom out-of-memory procedures.
 
         This limit marks a point of no return for the cgroup. It is NOT
         designed to manage the amount of swapping a workload does
         during regular operation. Compare to memory.swap.max, which
         prohibits swapping past a set amount, but lets the cgroup
         continue unimpeded as long as other memory can be reclaimed.
 
         Healthy workloads are not expected to reach this limit.
 
   memory.swap.peak
         A read-only single value file which exists on non-root
         cgroups.
 
         The max swap usage recorded for the cgroup and its
         descendants since the creation of the cgroup.
 
   memory.swap.max
         A read-write single value file which exists on non-root
         cgroups.  The default is "max".
 
         Swap usage hard limit.  If a cgroup's swap usage reaches this
         limit, anonymous memory of the cgroup will not be swapped out.
 
   memory.swap.events
         A read-only flat-keyed file which exists on non-root cgroups.
         The following entries are defined.  Unless specified
         otherwise, a value change in this file generates a file
         modified event.
 
           high
                 The number of times the cgroup's swap usage was over
                 the high threshold.
 
           max
                 The number of times the cgroup's swap usage was about
                 to go over the max boundary and swap allocation
                 failed.
 
           fail
                 The number of times swap allocation failed either
                 because of running out of swap system-wide or max
                 limit.
 
         When reduced under the current usage, the existing swap
         entries are reclaimed gradually and the swap usage may stay
         higher than the limit for an extended period of time.  This
         reduces the impact on the workload and memory management.
 
   memory.zswap.current
         A read-only single value file which exists on non-root
         cgroups.
 
         The total amount of memory consumed by the zswap compression
         backend.
 
   memory.zswap.max
         A read-write single value file which exists on non-root
         cgroups.  The default is "max".
 
         Zswap usage hard limit. If a cgroup's zswap pool reaches this
         limit, it will refuse to take any more stores before existing
         entries fault back in or are written out to disk.
 
   memory.zswap.writeback
         A read-write single value file. The default value is "1".
         Note that this setting is hierarchical, i.e. the writeback would be
         implicitly disabled for child cgroups if the upper hierarchy
         does so.
 
         When this is set to 0, all swapping attempts to swapping devices
         are disabled. This included both zswap writebacks, and swapping due
         to zswap store failures. If the zswap store failures are recurring
         (for e.g if the pages are incompressible), users can observe
         reclaim inefficiency after disabling writeback (because the same
         pages might be rejected again and again).
 
         Note that this is subtly different from setting memory.swap.max to
         0, as it still allows for pages to be written to the zswap pool.
 
   memory.pressure
         A read-only nested-keyed file.
 
         Shows pressure stall information for memory. See
         :ref:`Documentation/accounting/psi.rst <psi>` for details.
 
 
 Usage Guidelines
 ~~~~~~~~~~~~~~~~
 
 "memory.high" is the main mechanism to control memory usage.
 Over-committing on high limit (sum of high limits > available memory)
 and letting global memory pressure to distribute memory according to
 usage is a viable strategy.
 
 Because breach of the high limit doesn't trigger the OOM killer but
 throttles the offending cgroup, a management agent has ample
 opportunities to monitor and take appropriate actions such as granting
 more memory or terminating the workload.
 
 Determining whether a cgroup has enough memory is not trivial as
 memory usage doesn't indicate whether the workload can benefit from
 more memory.  For example, a workload which writes data received from
 network to a file can use all available memory but can also operate as
 performant with a small amount of memory.  A measure of memory
 pressure - how much the workload is being impacted due to lack of
 memory - is necessary to determine whether a workload needs more
 memory; unfortunately, memory pressure monitoring mechanism isn't
 implemented yet.
 
 
 Memory Ownership
 ~~~~~~~~~~~~~~~~
 
 A memory area is charged to the cgroup which instantiated it and stays
 charged to the cgroup until the area is released.  Migrating a process
 to a different cgroup doesn't move the memory usages that it
 instantiated while in the previous cgroup to the new cgroup.
 
 A memory area may be used by processes belonging to different cgroups.
 To which cgroup the area will be charged is in-deterministic; however,
 over time, the memory area is likely to end up in a cgroup which has
 enough memory allowance to avoid high reclaim pressure.
 
 If a cgroup sweeps a considerable amount of memory which is expected
 to be accessed repeatedly by other cgroups, it may make sense to use
 POSIX_FADV_DONTNEED to relinquish the ownership of memory areas
 belonging to the affected files to ensure correct memory ownership.
 
 
 IO
 --
 
 The "io" controller regulates the distribution of IO resources.  This
 controller implements both weight based and absolute bandwidth or IOPS
 limit distribution; however, weight based distribution is available
 only if cfq-iosched is in use and neither scheme is available for
 blk-mq devices.
 
 
 IO Interface Files
 ~~~~~~~~~~~~~~~~~~
 
   io.stat
         A read-only nested-keyed file.
 
         Lines are keyed by $MAJ:$MIN device numbers and not ordered.
         The following nested keys are defined.
 
           ======        =====================
           rbytes        Bytes read
           wbytes        Bytes written
           rios          Number of read IOs
           wios          Number of write IOs
           dbytes        Bytes discarded
           dios          Number of discard IOs
           ======        =====================
 
         An example read output follows::
 
           8:16 rbytes=1459200 wbytes=314773504 rios=192 wios=353 dbytes=0 dios=0
           8:0 rbytes=90430464 wbytes=299008000 rios=8950 wios=1252 dbytes=50331648 dios=3021
 
   io.cost.qos
         A read-write nested-keyed file which exists only on the root
         cgroup.
 
         This file configures the Quality of Service of the IO cost
         model based controller (CONFIG_BLK_CGROUP_IOCOST) which
         currently implements "io.weight" proportional control.  Lines
         are keyed by $MAJ:$MIN device numbers and not ordered.  The
         line for a given device is populated on the first write for
         the device on "io.cost.qos" or "io.cost.model".  The following
         nested keys are defined.
 
           ======        =====================================
           enable        Weight-based control enable
           ctrl          "auto" or "user"
           rpct          Read latency percentile    [0, 100]
           rlat          Read latency threshold
           wpct          Write latency percentile   [0, 100]
           wlat          Write latency threshold
           min           Minimum scaling percentage [1, 10000]
           max           Maximum scaling percentage [1, 10000]
           ======        =====================================
 
         The controller is disabled by default and can be enabled by
         setting "enable" to 1.  "rpct" and "wpct" parameters default
         to zero and the controller uses internal device saturation
         state to adjust the overall IO rate between "min" and "max".
 
         When a better control quality is needed, latency QoS
         parameters can be configured.  For example::
 
           8:16 enable=1 ctrl=auto rpct=95.00 rlat=75000 wpct=95.00 wlat=150000 min=50.00 max=150.0
 
         shows that on sdb, the controller is enabled, will consider
         the device saturated if the 95th percentile of read completion
         latencies is above 75ms or write 150ms, and adjust the overall
         IO issue rate between 50% and 150% accordingly.
 
         The lower the saturation point, the better the latency QoS at
         the cost of aggregate bandwidth.  The narrower the allowed
         adjustment range between "min" and "max", the more conformant
         to the cost model the IO behavior.  Note that the IO issue
         base rate may be far off from 100% and setting "min" and "max"
         blindly can lead to a significant loss of device capacity or
         control quality.  "min" and "max" are useful for regulating
         devices which show wide temporary behavior changes - e.g. a
         ssd which accepts writes at the line speed for a while and
         then completely stalls for multiple seconds.
 
         When "ctrl" is "auto", the parameters are controlled by the
         kernel and may change automatically.  Setting "ctrl" to "user"
         or setting any of the percentile and latency parameters puts
         it into "user" mode and disables the automatic changes.  The
         automatic mode can be restored by setting "ctrl" to "auto".
 
   io.cost.model
         A read-write nested-keyed file which exists only on the root
         cgroup.
 
         This file configures the cost model of the IO cost model based
         controller (CONFIG_BLK_CGROUP_IOCOST) which currently
         implements "io.weight" proportional control.  Lines are keyed
         by $MAJ:$MIN device numbers and not ordered.  The line for a
         given device is populated on the first write for the device on
         "io.cost.qos" or "io.cost.model".  The following nested keys
         are defined.
 
           =====         ================================
           ctrl          "auto" or "user"
           model         The cost model in use - "linear"
           =====         ================================
 
         When "ctrl" is "auto", the kernel may change all parameters
         dynamically.  When "ctrl" is set to "user" or any other
         parameters are written to, "ctrl" become "user" and the
         automatic changes are disabled.
 
         When "model" is "linear", the following model parameters are
         defined.
 
           ============= ========================================
           [r|w]bps      The maximum sequential IO throughput
           [r|w]seqiops  The maximum 4k sequential IOs per second
           [r|w]randiops The maximum 4k random IOs per second
           ============= ========================================
 
         From the above, the builtin linear model determines the base
         costs of a sequential and random IO and the cost coefficient
         for the IO size.  While simple, this model can cover most
         common device classes acceptably.
 
         The IO cost model isn't expected to be accurate in absolute
         sense and is scaled to the device behavior dynamically.
 
         If needed, tools/cgroup/iocost_coef_gen.py can be used to
         generate device-specific coefficients.
 
   io.weight
         A read-write flat-keyed file which exists on non-root cgroups.
         The default is "default 100".
 
         The first line is the default weight applied to devices
         without specific override.  The rest are overrides keyed by
         $MAJ:$MIN device numbers and not ordered.  The weights are in
         the range [1, 10000] and specifies the relative amount IO time
         the cgroup can use in relation to its siblings.
 
         The default weight can be updated by writing either "default
         $WEIGHT" or simply "$WEIGHT".  Overrides can be set by writing
         "$MAJ:$MIN $WEIGHT" and unset by writing "$MAJ:$MIN default".
 
         An example read output follows::
 
           default 100
           8:16 200
           8:0 50
 
   io.max
         A read-write nested-keyed file which exists on non-root
         cgroups.
 
         BPS and IOPS based IO limit.  Lines are keyed by $MAJ:$MIN
         device numbers and not ordered.  The following nested keys are
         defined.
 
           =====         ==================================
           rbps          Max read bytes per second
           wbps          Max write bytes per second
           riops         Max read IO operations per second
           wiops         Max write IO operations per second
           =====         ==================================
 
         When writing, any number of nested key-value pairs can be
         specified in any order.  "max" can be specified as the value
         to remove a specific limit.  If the same key is specified
         multiple times, the outcome is undefined.
 
         BPS and IOPS are measured in each IO direction and IOs are
         delayed if limit is reached.  Temporary bursts are allowed.
 
         Setting read limit at 2M BPS and write at 120 IOPS for 8:16::
 
           echo "8:16 rbps=2097152 wiops=120" > io.max
 
         Reading returns the following::
 
           8:16 rbps=2097152 wbps=max riops=max wiops=120
 
         Write IOPS limit can be removed by writing the following::
 
           echo "8:16 wiops=max" > io.max
 
         Reading now returns the following::
 
           8:16 rbps=2097152 wbps=max riops=max wiops=max
 
   io.pressure
         A read-only nested-keyed file.
 
         Shows pressure stall information for IO. See
         :ref:`Documentation/accounting/psi.rst <psi>` for details.
 
 
 Writeback
 ~~~~~~~~~
 
 Page cache is dirtied through buffered writes and shared mmaps and
 written asynchronously to the backing filesystem by the writeback
 mechanism.  Writeback sits between the memory and IO domains and
 regulates the proportion of dirty memory by balancing dirtying and
 write IOs.
 
 The io controller, in conjunction with the memory controller,
 implements control of page cache writeback IOs.  The memory controller
 defines the memory domain that dirty memory ratio is calculated and
 maintained for and the io controller defines the io domain which
 writes out dirty pages for the memory domain.  Both system-wide and
 per-cgroup dirty memory states are examined and the more restrictive
 of the two is enforced.
 
 cgroup writeback requires explicit support from the underlying
 filesystem.  Currently, cgroup writeback is implemented on ext2, ext4,
 btrfs, f2fs, and xfs.  On other filesystems, all writeback IOs are 
 attributed to the root cgroup.
 
 There are inherent differences in memory and writeback management
 which affects how cgroup ownership is tracked.  Memory is tracked per
 page while writeback per inode.  For the purpose of writeback, an
 inode is assigned to a cgroup and all IO requests to write dirty pages
 from the inode are attributed to that cgroup.
 
 As cgroup ownership for memory is tracked per page, there can be pages
 which are associated with different cgroups than the one the inode is
 associated with.  These are called foreign pages.  The writeback
 constantly keeps track of foreign pages and, if a particular foreign
 cgroup becomes the majority over a certain period of time, switches
 the ownership of the inode to that cgroup.
 
 While this model is enough for most use cases where a given inode is
 mostly dirtied by a single cgroup even when the main writing cgroup
 changes over time, use cases where multiple cgroups write to a single
 inode simultaneously are not supported well.  In such circumstances, a
 significant portion of IOs are likely to be attributed incorrectly.
 As memory controller assigns page ownership on the first use and
 doesn't update it until the page is released, even if writeback
 strictly follows page ownership, multiple cgroups dirtying overlapping
 areas wouldn't work as expected.  It's recommended to avoid such usage
 patterns.
 
 The sysctl knobs which affect writeback behavior are applied to cgroup
 writeback as follows.
 
   vm.dirty_background_ratio, vm.dirty_ratio
         These ratios apply the same to cgroup writeback with the
         amount of available memory capped by limits imposed by the
         memory controller and system-wide clean memory.
 
   vm.dirty_background_bytes, vm.dirty_bytes
         For cgroup writeback, this is calculated into ratio against
         total available memory and applied the same way as
         vm.dirty[_background]_ratio.
 
 
 IO Latency
 ~~~~~~~~~~
 
 This is a cgroup v2 controller for IO workload protection.  You provide a group
 with a latency target, and if the average latency exceeds that target the
 controller will throttle any peers that have a lower latency target than the
 protected workload.
 
 The limits are only applied at the peer level in the hierarchy.  This means that
 in the diagram below, only groups A, B, and C will influence each other, and
 groups D and F will influence each other.  Group G will influence nobody::
 
                         [root]
                 /          |            \
                 A          B            C
                /  \        |
               D    F       G
 
 
 So the ideal way to configure this is to set io.latency in groups A, B, and C.
 Generally you do not want to set a value lower than the latency your device
 supports.  Experiment to find the value that works best for your workload.
 Start at higher than the expected latency for your device and watch the
 avg_lat value in io.stat for your workload group to get an idea of the
 latency you see during normal operation.  Use the avg_lat value as a basis for
 your real setting, setting at 10-15% higher than the value in io.stat.
 
 How IO Latency Throttling Works
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 io.latency is work conserving; so as long as everybody is meeting their latency
 target the controller doesn't do anything.  Once a group starts missing its
 target it begins throttling any peer group that has a higher target than itself.
 This throttling takes 2 forms:
 
 - Queue depth throttling.  This is the number of outstanding IO's a group is
   allowed to have.  We will clamp down relatively quickly, starting at no limit
   and going all the way down to 1 IO at a time.
 
 - Artificial delay induction.  There are certain types of IO that cannot be
   throttled without possibly adversely affecting higher priority groups.  This
   includes swapping and metadata IO.  These types of IO are allowed to occur
   normally, however they are "charged" to the originating group.  If the
   originating group is being throttled you will see the use_delay and delay
   fields in io.stat increase.  The delay value is how many microseconds that are
   being added to any process that runs in this group.  Because this number can
   grow quite large if there is a lot of swapping or metadata IO occurring we
   limit the individual delay events to 1 second at a time.
 
 Once the victimized group starts meeting its latency target again it will start
 unthrottling any peer groups that were throttled previously.  If the victimized
 group simply stops doing IO the global counter will unthrottle appropriately.
 
 IO Latency Interface Files
 ~~~~~~~~~~~~~~~~~~~~~~~~~~
 
   io.latency
         This takes a similar format as the other controllers.
 
                 "MAJOR:MINOR target=<target time in microseconds>"
 
   io.stat
         If the controller is enabled you will see extra stats in io.stat in
         addition to the normal ones.
 
           depth
                 This is the current queue depth for the group.
 
           avg_lat
                 This is an exponential moving average with a decay rate of 1/exp
                 bound by the sampling interval.  The decay rate interval can be
                 calculated by multiplying the win value in io.stat by the
                 corresponding number of samples based on the win value.
 
           win
                 The sampling window size in milliseconds.  This is the minimum
                 duration of time between evaluation events.  Windows only elapse
                 with IO activity.  Idle periods extend the most recent window.
 
 IO Priority
 ~~~~~~~~~~~
 
 A single attribute controls the behavior of the I/O priority cgroup policy,
 namely the io.prio.class attribute. The following values are accepted for
 that attribute:
 
   no-change
         Do not modify the I/O priority class.
 
   promote-to-rt
         For requests that have a non-RT I/O priority class, change it into RT.
         Also change the priority level of these requests to 4. Do not modify
         the I/O priority of requests that have priority class RT.
 
   restrict-to-be
         For requests that do not have an I/O priority class or that have I/O
         priority class RT, change it into BE. Also change the priority level
         of these requests to 0. Do not modify the I/O priority class of
         requests that have priority class IDLE.
 
   idle
         Change the I/O priority class of all requests into IDLE, the lowest
         I/O priority class.
 
   none-to-rt
         Deprecated. Just an alias for promote-to-rt.
 
 The following numerical values are associated with the I/O priority policies:
 
 +----------------+---+
 | no-change      | 0 |
 +----------------+---+
 | promote-to-rt  | 1 |
 +----------------+---+
 | restrict-to-be | 2 |
 +----------------+---+
 | idle           | 3 |
 +----------------+---+
 
 The numerical value that corresponds to each I/O priority class is as follows:
 
 +-------------------------------+---+
 | IOPRIO_CLASS_NONE             | 0 |
 +-------------------------------+---+
 | IOPRIO_CLASS_RT (real-time)   | 1 |
 +-------------------------------+---+
 | IOPRIO_CLASS_BE (best effort) | 2 |
 +-------------------------------+---+
 | IOPRIO_CLASS_IDLE             | 3 |
 +-------------------------------+---+
 
 The algorithm to set the I/O priority class for a request is as follows:
 
 - If I/O priority class policy is promote-to-rt, change the request I/O
   priority class to IOPRIO_CLASS_RT and change the request I/O priority
   level to 4.
 - If I/O priority class policy is not promote-to-rt, translate the I/O priority
   class policy into a number, then change the request I/O priority class
   into the maximum of the I/O priority class policy number and the numerical
   I/O priority class.
 
 PID
 ---
 
 The process number controller is used to allow a cgroup to stop any
 new tasks from being fork()'d or clone()'d after a specified limit is
 reached.
 
 The number of tasks in a cgroup can be exhausted in ways which other
 controllers cannot prevent, thus warranting its own controller.  For
 example, a fork bomb is likely to exhaust the number of tasks before
 hitting memory restrictions.
 
 Note that PIDs used in this controller refer to TIDs, process IDs as
 used by the kernel.
 
 
 PID Interface Files
 ~~~~~~~~~~~~~~~~~~~
 
   pids.max
         A read-write single value file which exists on non-root
         cgroups.  The default is "max".
 
         Hard limit of number of processes.
 
   pids.current
         A read-only single value file which exists on non-root cgroups.
 
         The number of processes currently in the cgroup and its
         descendants.
 
   pids.peak
         A read-only single value file which exists on non-root cgroups.
 
         The maximum value that the number of processes in the cgroup and its
         descendants has ever reached.
 
   pids.events
         A read-only flat-keyed file which exists on non-root cgroups. Unless
         specified otherwise, a value change in this file generates a file
         modified event. The following entries are defined.
 
           max
                 The number of times the cgroup's total number of processes hit the pids.max
                 limit (see also pids_localevents).
 
   pids.events.local
         Similar to pids.events but the fields in the file are local
         to the cgroup i.e. not hierarchical. The file modified event
         generated on this file reflects only the local events.
 
 Organisational operations are not blocked by cgroup policies, so it is
 possible to have pids.current > pids.max.  This can be done by either
 setting the limit to be smaller than pids.current, or attaching enough
 processes to the cgroup such that pids.current is larger than
 pids.max.  However, it is not possible to violate a cgroup PID policy
 through fork() or clone(). These will return -EAGAIN if the creation
 of a new process would cause a cgroup policy to be violated.
 
 
 Cpuset
 ------
 
 The "cpuset" controller provides a mechanism for constraining
 the CPU and memory node placement of tasks to only the resources
 specified in the cpuset interface files in a task's current cgroup.
 This is especially valuable on large NUMA systems where placing jobs
 on properly sized subsets of the systems with careful processor and
 memory placement to reduce cross-node memory access and contention
 can improve overall system performance.
 
 The "cpuset" controller is hierarchical.  That means the controller
 cannot use CPUs or memory nodes not allowed in its parent.
 
 
 Cpuset Interface Files
 ~~~~~~~~~~~~~~~~~~~~~~
 
   cpuset.cpus
         A read-write multiple values file which exists on non-root
         cpuset-enabled cgroups.
 
         It lists the requested CPUs to be used by tasks within this
         cgroup.  The actual list of CPUs to be granted, however, is
         subjected to constraints imposed by its parent and can differ
         from the requested CPUs.
 
         The CPU numbers are comma-separated numbers or ranges.
         For example::
 
           # cat cpuset.cpus
           0-4,6,8-10
 
         An empty value indicates that the cgroup is using the same
         setting as the nearest cgroup ancestor with a non-empty
         "cpuset.cpus" or all the available CPUs if none is found.
 
         The value of "cpuset.cpus" stays constant until the next update
         and won't be affected by any CPU hotplug events.
 
   cpuset.cpus.effective
         A read-only multiple values file which exists on all
         cpuset-enabled cgroups.
 
         It lists the onlined CPUs that are actually granted to this
         cgroup by its parent.  These CPUs are allowed to be used by
         tasks within the current cgroup.
 
         If "cpuset.cpus" is empty, the "cpuset.cpus.effective" file shows
         all the CPUs from the parent cgroup that can be available to
         be used by this cgroup.  Otherwise, it should be a subset of
         "cpuset.cpus" unless none of the CPUs listed in "cpuset.cpus"
         can be granted.  In this case, it will be treated just like an
         empty "cpuset.cpus".
 
         Its value will be affected by CPU hotplug events.
 
   cpuset.mems
         A read-write multiple values file which exists on non-root
         cpuset-enabled cgroups.
 
         It lists the requested memory nodes to be used by tasks within
         this cgroup.  The actual list of memory nodes granted, however,
         is subjected to constraints imposed by its parent and can differ
         from the requested memory nodes.
 
         The memory node numbers are comma-separated numbers or ranges.
         For example::
 
           # cat cpuset.mems
           0-1,3
 
         An empty value indicates that the cgroup is using the same
         setting as the nearest cgroup ancestor with a non-empty
         "cpuset.mems" or all the available memory nodes if none
         is found.
 
         The value of "cpuset.mems" stays constant until the next update
         and won't be affected by any memory nodes hotplug events.
 
         Setting a non-empty value to "cpuset.mems" causes memory of
         tasks within the cgroup to be migrated to the designated nodes if
         they are currently using memory outside of the designated nodes.
 
         There is a cost for this memory migration.  The migration
         may not be complete and some memory pages may be left behind.
         So it is recommended that "cpuset.mems" should be set properly
         before spawning new tasks into the cpuset.  Even if there is
         a need to change "cpuset.mems" with active tasks, it shouldn't
         be done frequently.
 
   cpuset.mems.effective
         A read-only multiple values file which exists on all
         cpuset-enabled cgroups.
 
         It lists the onlined memory nodes that are actually granted to
         this cgroup by its parent. These memory nodes are allowed to
         be used by tasks within the current cgroup.
 
         If "cpuset.mems" is empty, it shows all the memory nodes from the
         parent cgroup that will be available to be used by this cgroup.
         Otherwise, it should be a subset of "cpuset.mems" unless none of
         the memory nodes listed in "cpuset.mems" can be granted.  In this
         case, it will be treated just like an empty "cpuset.mems".
 
         Its value will be affected by memory nodes hotplug events.
 
   cpuset.cpus.exclusive
         A read-write multiple values file which exists on non-root
         cpuset-enabled cgroups.
 
         It lists all the exclusive CPUs that are allowed to be used
         to create a new cpuset partition.  Its value is not used
         unless the cgroup becomes a valid partition root.  See the
         "cpuset.cpus.partition" section below for a description of what
         a cpuset partition is.
 
         When the cgroup becomes a partition root, the actual exclusive
         CPUs that are allocated to that partition are listed in
         "cpuset.cpus.exclusive.effective" which may be different
         from "cpuset.cpus.exclusive".  If "cpuset.cpus.exclusive"
         has previously been set, "cpuset.cpus.exclusive.effective"
         is always a subset of it.
 
         Users can manually set it to a value that is different from
         "cpuset.cpus".  One constraint in setting it is that the list of
         CPUs must be exclusive with respect to "cpuset.cpus.exclusive"
         of its sibling.  If "cpuset.cpus.exclusive" of a sibling cgroup
         isn't set, its "cpuset.cpus" value, if set, cannot be a subset
         of it to leave at least one CPU available when the exclusive
         CPUs are taken away.
 
         For a parent cgroup, any one of its exclusive CPUs can only
         be distributed to at most one of its child cgroups.  Having an
         exclusive CPU appearing in two or more of its child cgroups is
         not allowed (the exclusivity rule).  A value that violates the
         exclusivity rule will be rejected with a write error.
 
         The root cgroup is a partition root and all its available CPUs
         are in its exclusive CPU set.
 
   cpuset.cpus.exclusive.effective
         A read-only multiple values file which exists on all non-root
         cpuset-enabled cgroups.
 
         This file shows the effective set of exclusive CPUs that
         can be used to create a partition root.  The content
         of this file will always be a subset of its parent's
         "cpuset.cpus.exclusive.effective" if its parent is not the root
         cgroup.  It will also be a subset of "cpuset.cpus.exclusive"
         if it is set.  If "cpuset.cpus.exclusive" is not set, it is
         treated to have an implicit value of "cpuset.cpus" in the
         formation of local partition.
 
   cpuset.cpus.isolated
         A read-only and root cgroup only multiple values file.
 
         This file shows the set of all isolated CPUs used in existing
         isolated partitions. It will be empty if no isolated partition
         is created.
 
   cpuset.cpus.partition
         A read-write single value file which exists on non-root
         cpuset-enabled cgroups.  This flag is owned by the parent cgroup
         and is not delegatable.
 
         It accepts only the following input values when written to.
 
           ==========    =====================================
           "member"      Non-root member of a partition
           "root"        Partition root
           "isolated"    Partition root without load balancing
           ==========    =====================================
 
         A cpuset partition is a collection of cpuset-enabled cgroups with
         a partition root at the top of the hierarchy and its descendants
         except those that are separate partition roots themselves and
         their descendants.  A partition has exclusive access to the
         set of exclusive CPUs allocated to it.  Other cgroups outside
         of that partition cannot use any CPUs in that set.
 
         There are two types of partitions - local and remote.  A local
         partition is one whose parent cgroup is also a valid partition
         root.  A remote partition is one whose parent cgroup is not a
         valid partition root itself.  Writing to "cpuset.cpus.exclusive"
         is optional for the creation of a local partition as its
         "cpuset.cpus.exclusive" file will assume an implicit value that
         is the same as "cpuset.cpus" if it is not set.  Writing the
         proper "cpuset.cpus.exclusive" values down the cgroup hierarchy
         before the target partition root is mandatory for the creation
         of a remote partition.
 
         Currently, a remote partition cannot be created under a local
         partition.  All the ancestors of a remote partition root except
         the root cgroup cannot be a partition root.
 
         The root cgroup is always a partition root and its state cannot
         be changed.  All other non-root cgroups start out as "member".
 
         When set to "root", the current cgroup is the root of a new
         partition or scheduling domain.  The set of exclusive CPUs is
         determined by the value of its "cpuset.cpus.exclusive.effective".
 
         When set to "isolated", the CPUs in that partition will be in
         an isolated state without any load balancing from the scheduler
         and excluded from the unbound workqueues.  Tasks placed in such
         a partition with multiple CPUs should be carefully distributed
         and bound to each of the individual CPUs for optimal performance.
 
         A partition root ("root" or "isolated") can be in one of the
         two possible states - valid or invalid.  An invalid partition
         root is in a degraded state where some state information may
         be retained, but behaves more like a "member".
 
         All possible state transitions among "member", "root" and
         "isolated" are allowed.
 
         On read, the "cpuset.cpus.partition" file can show the following
         values.
 
           ============================= =====================================
           "member"                      Non-root member of a partition
           "root"                        Partition root
           "isolated"                    Partition root without load balancing
           "root invalid (<reason>)"     Invalid partition root
           "isolated invalid (<reason>)" Invalid isolated partition root
           ============================= =====================================
 
         In the case of an invalid partition root, a descriptive string on
         why the partition is invalid is included within parentheses.
 
         For a local partition root to be valid, the following conditions
         must be met.
 
         1) The parent cgroup is a valid partition root.
         2) The "cpuset.cpus.exclusive.effective" file cannot be empty,
            though it may contain offline CPUs.
         3) The "cpuset.cpus.effective" cannot be empty unless there is
            no task associated with this partition.
 
         For a remote partition root to be valid, all the above conditions
         except the first one must be met.
 
         External events like hotplug or changes to "cpuset.cpus" or
         "cpuset.cpus.exclusive" can cause a valid partition root to
         become invalid and vice versa.  Note that a task cannot be
         moved to a cgroup with empty "cpuset.cpus.effective".
 
         A valid non-root parent partition may distribute out all its CPUs
         to its child local partitions when there is no task associated
         with it.
 
         Care must be taken to change a valid partition root to "member"
         as all its child local partitions, if present, will become
         invalid causing disruption to tasks running in those child
         partitions. These inactivated partitions could be recovered if
         their parent is switched back to a partition root with a proper
         value in "cpuset.cpus" or "cpuset.cpus.exclusive".
 
         Poll and inotify events are triggered whenever the state of
         "cpuset.cpus.partition" changes.  That includes changes caused
         by write to "cpuset.cpus.partition", cpu hotplug or other
         changes that modify the validity status of the partition.
         This will allow user space agents to monitor unexpected changes
         to "cpuset.cpus.partition" without the need to do continuous
         polling.
 
         A user can pre-configure certain CPUs to an isolated state
         with load balancing disabled at boot time with the "isolcpus"
         kernel boot command line option.  If those CPUs are to be put
         into a partition, they have to be used in an isolated partition.
 
 
 Device controller
 -----------------
 
 Device controller manages access to device files. It includes both
 creation of new device files (using mknod), and access to the
 existing device files.
 
 Cgroup v2 device controller has no interface files and is implemented
 on top of cgroup BPF. To control access to device files, a user may
 create bpf programs of type BPF_PROG_TYPE_CGROUP_DEVICE and attach
 them to cgroups with BPF_CGROUP_DEVICE flag. On an attempt to access a
 device file, corresponding BPF programs will be executed, and depending
 on the return value the attempt will succeed or fail with -EPERM.
 
 A BPF_PROG_TYPE_CGROUP_DEVICE program takes a pointer to the
 bpf_cgroup_dev_ctx structure, which describes the device access attempt:
 access type (mknod/read/write) and device (type, major and minor numbers).
 If the program returns 0, the attempt fails with -EPERM, otherwise it
 succeeds.
 
 An example of BPF_PROG_TYPE_CGROUP_DEVICE program may be found in
 tools/testing/selftests/bpf/progs/dev_cgroup.c in the kernel source tree.
 
 
 RDMA
 ----
 
 The "rdma" controller regulates the distribution and accounting of
 RDMA resources.
 
 RDMA Interface Files
 ~~~~~~~~~~~~~~~~~~~~
 
   rdma.max
         A readwrite nested-keyed file that exists for all the cgroups
         except root that describes current configured resource limit
         for a RDMA/IB device.
 
         Lines are keyed by device name and are not ordered.
         Each line contains space separated resource name and its configured
         limit that can be distributed.
 
         The following nested keys are defined.
 
           ==========    =============================
           hca_handle    Maximum number of HCA Handles
           hca_object    Maximum number of HCA Objects
           ==========    =============================
 
         An example for mlx4 and ocrdma device follows::
 
           mlx4_0 hca_handle=2 hca_object=2000
           ocrdma1 hca_handle=3 hca_object=max
 
   rdma.current
         A read-only file that describes current resource usage.
         It exists for all the cgroup except root.
 
         An example for mlx4 and ocrdma device follows::
 
           mlx4_0 hca_handle=1 hca_object=20
           ocrdma1 hca_handle=1 hca_object=23
 
 HugeTLB
 -------
 
 The HugeTLB controller allows to limit the HugeTLB usage per control group and
 enforces the controller limit during page fault.
 
 HugeTLB Interface Files
 ~~~~~~~~~~~~~~~~~~~~~~~
 
   hugetlb.<hugepagesize>.current
         Show current usage for "hugepagesize" hugetlb.  It exists for all
         the cgroup except root.
 
   hugetlb.<hugepagesize>.max
         Set/show the hard limit of "hugepagesize" hugetlb usage.
         The default value is "max".  It exists for all the cgroup except root.
 
   hugetlb.<hugepagesize>.events
         A read-only flat-keyed file which exists on non-root cgroups.
 
           max
                 The number of allocation failure due to HugeTLB limit
 
   hugetlb.<hugepagesize>.events.local
         Similar to hugetlb.<hugepagesize>.events but the fields in the file
         are local to the cgroup i.e. not hierarchical. The file modified event
         generated on this file reflects only the local events.
 
   hugetlb.<hugepagesize>.numa_stat
         Similar to memory.numa_stat, it shows the numa information of the
         hugetlb pages of <hugepagesize> in this cgroup.  Only active in
         use hugetlb pages are included.  The per-node values are in bytes.
 
 Misc
 ----
 
 The Miscellaneous cgroup provides the resource limiting and tracking
 mechanism for the scalar resources which cannot be abstracted like the other
 cgroup resources. Controller is enabled by the CONFIG_CGROUP_MISC config
 option.
 
 A resource can be added to the controller via enum misc_res_type{} in the
 include/linux/misc_cgroup.h file and the corresponding name via misc_res_name[]
 in the kernel/cgroup/misc.c file. Provider of the resource must set its
 capacity prior to using the resource by calling misc_cg_set_capacity().
 
 Once a capacity is set then the resource usage can be updated using charge and
 uncharge APIs. All of the APIs to interact with misc controller are in
 include/linux/misc_cgroup.h.
 
 Misc Interface Files
 ~~~~~~~~~~~~~~~~~~~~
 
 Miscellaneous controller provides 3 interface files. If two misc resources (res_a and res_b) are registered then:
 
   misc.capacity
         A read-only flat-keyed file shown only in the root cgroup.  It shows
         miscellaneous scalar resources available on the platform along with
         their quantities::
 
           $ cat misc.capacity
           res_a 50
           res_b 10
 
   misc.current
         A read-only flat-keyed file shown in the all cgroups.  It shows
         the current usage of the resources in the cgroup and its children.::
 
           $ cat misc.current
           res_a 3
           res_b 0
 
   misc.peak
         A read-only flat-keyed file shown in all cgroups.  It shows the
         historical maximum usage of the resources in the cgroup and its
         children.::
 
           $ cat misc.peak
           res_a 10
           res_b 8
 
   misc.max
         A read-write flat-keyed file shown in the non root cgroups. Allowed
         maximum usage of the resources in the cgroup and its children.::
 
           $ cat misc.max
           res_a max
           res_b 4
 
         Limit can be set by::
 
           # echo res_a 1 > misc.max
 
         Limit can be set to max by::
 
           # echo res_a max > misc.max
 
         Limits can be set higher than the capacity value in the misc.capacity
         file.
 
   misc.events
         A read-only flat-keyed file which exists on non-root cgroups. The
         following entries are defined. Unless specified otherwise, a value
         change in this file generates a file modified event. All fields in
         this file are hierarchical.
 
           max
                 The number of times the cgroup's resource usage was
                 about to go over the max boundary.
 
   misc.events.local
         Similar to misc.events but the fields in the file are local to the
         cgroup i.e. not hierarchical. The file modified event generated on
         this file reflects only the local events.
 
 Migration and Ownership
 ~~~~~~~~~~~~~~~~~~~~~~~
 
 A miscellaneous scalar resource is charged to the cgroup in which it is used
 first, and stays charged to that cgroup until that resource is freed. Migrating
 a process to a different cgroup does not move the charge to the destination
 cgroup where the process has moved.
 
 Others
 ------
 
 perf_event
 ~~~~~~~~~~
 
 perf_event controller, if not mounted on a legacy hierarchy, is
 automatically enabled on the v2 hierarchy so that perf events can
 always be filtered by cgroup v2 path.  The controller can still be
 moved to a legacy hierarchy after v2 hierarchy is populated.
 
 
 Non-normative information
 -------------------------
 
 This section contains information that isn't considered to be a part of
 the stable kernel API and so is subject to change.
 
 
 CPU controller root cgroup process behaviour
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 When distributing CPU cycles in the root cgroup each thread in this
 cgroup is treated as if it was hosted in a separate child cgroup of the
 root cgroup. This child cgroup weight is dependent on its thread nice
 level.
 
 For details of this mapping see sched_prio_to_weight array in
 kernel/sched/core.c file (values from this array should be scaled
 appropriately so the neutral - nice 0 - value is 100 instead of 1024).
 
 
 IO controller root cgroup process behaviour
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
 Root cgroup processes are hosted in an implicit leaf child node.
 When distributing IO resources this implicit child node is taken into
 account as if it was a normal child cgroup of the root cgroup with a
 weight value of 200.
 
 
 Namespace
 =========
 
 Basics
 ------
 
 cgroup namespace provides a mechanism to virtualize the view of the
 "/proc/$PID/cgroup" file and cgroup mounts.  The CLONE_NEWCGROUP clone
 flag can be used with clone(2) and unshare(2) to create a new cgroup
 namespace.  The process running inside the cgroup namespace will have
 its "/proc/$PID/cgroup" output restricted to cgroupns root.  The
 cgroupns root is the cgroup of the process at the time of creation of
 the cgroup namespace.
 
 Without cgroup namespace, the "/proc/$PID/cgroup" file shows the
 complete path of the cgroup of a process.  In a container setup where
 a set of cgroups and namespaces are intended to isolate processes the
 "/proc/$PID/cgroup" file may leak potential system level information
 to the isolated processes.  For example::
 
   # cat /proc/self/cgroup
   0::/batchjobs/container_id1
 
 The path '/batchjobs/container_id1' can be considered as system-data
 and undesirable to expose to the isolated processes.  cgroup namespace
 can be used to restrict visibility of this path.  For example, before
 creating a cgroup namespace, one would see::
 
   # ls -l /proc/self/ns/cgroup
   lrwxrwxrwx 1 root root 0 2014-07-15 10:37 /proc/self/ns/cgroup -> cgroup:[4026531835]
   # cat /proc/self/cgroup
   0::/batchjobs/container_id1
 
 After unsharing a new namespace, the view changes::
 
   # ls -l /proc/self/ns/cgroup
   lrwxrwxrwx 1 root root 0 2014-07-15 10:35 /proc/self/ns/cgroup -> cgroup:[4026532183]
   # cat /proc/self/cgroup
   0::/
 
 When some thread from a multi-threaded process unshares its cgroup
 namespace, the new cgroupns gets applied to the entire process (all
 the threads).  This is natural for the v2 hierarchy; however, for the
 legacy hierarchies, this may be unexpected.
 
 A cgroup namespace is alive as long as there are processes inside or
 mounts pinning it.  When the last usage goes away, the cgroup
 namespace is destroyed.  The cgroupns root and the actual cgroups
 remain.
 
 
 The Root and Views
 ------------------
 
 The 'cgroupns root' for a cgroup namespace is the cgroup in which the
 process calling unshare(2) is running.  For example, if a process in
 /batchjobs/container_id1 cgroup calls unshare, cgroup
 /batchjobs/container_id1 becomes the cgroupns root.  For the
 init_cgroup_ns, this is the real root ('/') cgroup.
 
 The cgroupns root cgroup does not change even if the namespace creator
 process later moves to a different cgroup::
 
   # ~/unshare -c # unshare cgroupns in some cgroup
   # cat /proc/self/cgroup
   0::/
   # mkdir sub_cgrp_1
   # echo 0 > sub_cgrp_1/cgroup.procs
   # cat /proc/self/cgroup
   0::/sub_cgrp_1
 
 Each process gets its namespace-specific view of "/proc/$PID/cgroup"
 
 Processes running inside the cgroup namespace will be able to see
 cgroup paths (in /proc/self/cgroup) only inside their root cgroup.
 From within an unshared cgroupns::
 
   # sleep 100000 &
   [1] 7353
   # echo 7353 > sub_cgrp_1/cgroup.procs
   # cat /proc/7353/cgroup
   0::/sub_cgrp_1
 
 From the initial cgroup namespace, the real cgroup path will be
 visible::
 
   $ cat /proc/7353/cgroup
   0::/batchjobs/container_id1/sub_cgrp_1
 
 From a sibling cgroup namespace (that is, a namespace rooted at a
 different cgroup), the cgroup path relative to its own cgroup
 namespace root will be shown.  For instance, if PID 7353's cgroup
 namespace root is at '/batchjobs/container_id2', then it will see::
 
   # cat /proc/7353/cgroup
   0::/../container_id2/sub_cgrp_1
 
 Note that the relative path always starts with '/' to indicate that
 its relative to the cgroup namespace root of the caller.
 
 
 Migration and setns(2)
 ----------------------
 
 Processes inside a cgroup namespace can move into and out of the
 namespace root if they have proper access to external cgroups.  For
 example, from inside a namespace with cgroupns root at
 /batchjobs/container_id1, and assuming that the global hierarchy is
 still accessible inside cgroupns::
 
   # cat /proc/7353/cgroup
   0::/sub_cgrp_1
   # echo 7353 > batchjobs/container_id2/cgroup.procs
   # cat /proc/7353/cgroup
   0::/../container_id2
 
 Note that this kind of setup is not encouraged.  A task inside cgroup
 namespace should only be exposed to its own cgroupns hierarchy.
 
 setns(2) to another cgroup namespace is allowed when:
 
 (a) the process has CAP_SYS_ADMIN against its current user namespace
 (b) the process has CAP_SYS_ADMIN against the target cgroup
     namespace's userns
 
 No implicit cgroup changes happen with attaching to another cgroup
 namespace.  It is expected that the someone moves the attaching
 process under the target cgroup namespace root.
 
 
 Interaction with Other Namespaces
 ---------------------------------
 
 Namespace specific cgroup hierarchy can be mounted by a process
 running inside a non-init cgroup namespace::
 
   # mount -t cgroup2 none $MOUNT_POINT
 
 This will mount the unified cgroup hierarchy with cgroupns root as the
 filesystem root.  The process needs CAP_SYS_ADMIN against its user and
 mount namespaces.
 
 The virtualization of /proc/self/cgroup file combined with restricting
 the view of cgroup hierarchy by namespace-private cgroupfs mount
 provides a properly isolated cgroup view inside the container.
 
 
 Information on Kernel Programming
 =================================
 
 This section contains kernel programming information in the areas
 where interacting with cgroup is necessary.  cgroup core and
 controllers are not covered.
 
 
 Filesystem Support for Writeback
 --------------------------------
 
 A filesystem can support cgroup writeback by updating
 address_space_operations->writepage[s]() to annotate bio's using the
 following two functions.
 
   wbc_init_bio(@wbc, @bio)
         Should be called for each bio carrying writeback data and
         associates the bio with the inode's owner cgroup and the
         corresponding request queue.  This must be called after
         a queue (device) has been associated with the bio and
         before submission.
 
   wbc_account_cgroup_owner(@wbc, @page, @bytes)
         Should be called for each data segment being written out.
         While this function doesn't care exactly when it's called
         during the writeback session, it's the easiest and most
         natural to call it as data segments are added to a bio.
 
 With writeback bio's annotated, cgroup support can be enabled per
 super_block by setting SB_I_CGROUPWB in ->s_iflags.  This allows for
 selective disabling of cgroup writeback support which is helpful when
 certain filesystem features, e.g. journaled data mode, are
 incompatible.
 
 wbc_init_bio() binds the specified bio to its cgroup.  Depending on
 the configuration, the bio may be executed at a lower priority and if
 the writeback session is holding shared resources, e.g. a journal
 entry, may lead to priority inversion.  There is no one easy solution
 for the problem.  Filesystems can try to work around specific problem
 cases by skipping wbc_init_bio() and using bio_associate_blkg()
 directly.
 
 
 Deprecated v1 Core Features
 ===========================
 
 - Multiple hierarchies including named ones are not supported.
 
 - All v1 mount options are not supported.
 
 - The "tasks" file is removed and "cgroup.procs" is not sorted.
 
 - "cgroup.clone_children" is removed.
 
 - /proc/cgroups is meaningless for v2.  Use "cgroup.controllers" file
   at the root instead.
 
 
 Issues with v1 and Rationales for v2
 ====================================
 
 Multiple Hierarchies
 --------------------
 
 cgroup v1 allowed an arbitrary number of hierarchies and each
 hierarchy could host any number of controllers.  While this seemed to
 provide a high level of flexibility, it wasn't useful in practice.
 
 For example, as there is only one instance of each controller, utility
 type controllers such as freezer which can be useful in all
 hierarchies could only be used in one.  The issue is exacerbated by
 the fact that controllers couldn't be moved to another hierarchy once
 hierarchies were populated.  Another issue was that all controllers
 bound to a hierarchy were forced to have exactly the same view of the
 hierarchy.  It wasn't possible to vary the granularity depending on
 the specific controller.
 
 In practice, these issues heavily limited which controllers could be
 put on the same hierarchy and most configurations resorted to putting
 each controller on its own hierarchy.  Only closely related ones, such
 as the cpu and cpuacct controllers, made sense to be put on the same
 hierarchy.  This often meant that userland ended up managing multiple
 similar hierarchies repeating the same steps on each hierarchy
 whenever a hierarchy management operation was necessary.
 
 Furthermore, support for multiple hierarchies came at a steep cost.
 It greatly complicated cgroup core implementation but more importantly
 the support for multiple hierarchies restricted how cgroup could be
 used in general and what controllers was able to do.
 
 There was no limit on how many hierarchies there might be, which meant
 that a thread's cgroup membership couldn't be described in finite
 length.  The key might contain any number of entries and was unlimited
 in length, which made it highly awkward to manipulate and led to
 addition of controllers which existed only to identify membership,
 which in turn exacerbated the original problem of proliferating number
 of hierarchies.
 
 Also, as a controller couldn't have any expectation regarding the
 topologies of hierarchies other controllers might be on, each
 controller had to assume that all other controllers were attached to
 completely orthogonal hierarchies.  This made it impossible, or at
 least very cumbersome, for controllers to cooperate with each other.
 
 In most use cases, putting controllers on hierarchies which are
 completely orthogonal to each other isn't necessary.  What usually is
 called for is the ability to have differing levels of granularity
 depending on the specific controller.  In other words, hierarchy may
 be collapsed from leaf towards root when viewed from specific
 controllers.  For example, a given configuration might not care about
 how memory is distributed beyond a certain level while still wanting
 to control how CPU cycles are distributed.
 
 
 Thread Granularity
 ------------------
 
 cgroup v1 allowed threads of a process to belong to different cgroups.
 This didn't make sense for some controllers and those controllers
 ended up implementing different ways to ignore such situations but
 much more importantly it blurred the line between API exposed to
 individual applications and system management interface.
 
 Generally, in-process knowledge is available only to the process
 itself; thus, unlike service-level organization of processes,
 categorizing threads of a process requires active participation from
 the application which owns the target process.
 
 cgroup v1 had an ambiguously defined delegation model which got abused
 in combination with thread granularity.  cgroups were delegated to
 individual applications so that they can create and manage their own
 sub-hierarchies and control resource distributions along them.  This
 effectively raised cgroup to the status of a syscall-like API exposed
 to lay programs.
 
 First of all, cgroup has a fundamentally inadequate interface to be
 exposed this way.  For a process to access its own knobs, it has to
 extract the path on the target hierarchy from /proc/self/cgroup,
 construct the path by appending the name of the knob to the path, open
 and then read and/or write to it.  This is not only extremely clunky
 and unusual but also inherently racy.  There is no conventional way to
 define transaction across the required steps and nothing can guarantee
 that the process would actually be operating on its own sub-hierarchy.
 
 cgroup controllers implemented a number of knobs which would never be
 accepted as public APIs because they were just adding control knobs to
 system-management pseudo filesystem.  cgroup ended up with interface
 knobs which were not properly abstracted or refined and directly
 revealed kernel internal details.  These knobs got exposed to
 individual applications through the ill-defined delegation mechanism
 effectively abusing cgroup as a shortcut to implementing public APIs
 without going through the required scrutiny.
 
 This was painful for both userland and kernel.  Userland ended up with
 misbehaving and poorly abstracted interfaces and kernel exposing and
 locked into constructs inadvertently.
 
 
 Competition Between Inner Nodes and Threads
 -------------------------------------------
 
 cgroup v1 allowed threads to be in any cgroups which created an
 interesting problem where threads belonging to a parent cgroup and its
 children cgroups competed for resources.  This was nasty as two
 different types of entities competed and there was no obvious way to
 settle it.  Different controllers did different things.
 
 The cpu controller considered threads and cgroups as equivalents and
 mapped nice levels to cgroup weights.  This worked for some cases but
 fell flat when children wanted to be allocated specific ratios of CPU
 cycles and the number of internal threads fluctuated - the ratios
 constantly changed as the number of competing entities fluctuated.
 There also were other issues.  The mapping from nice level to weight
 wasn't obvious or universal, and there were various other knobs which
 simply weren't available for threads.
 
 The io controller implicitly created a hidden leaf node for each
 cgroup to host the threads.  The hidden leaf had its own copies of all
 the knobs with ``leaf_`` prefixed.  While this allowed equivalent
 control over internal threads, it was with serious drawbacks.  It
 always added an extra layer of nesting which wouldn't be necessary
 otherwise, made the interface messy and significantly complicated the
 implementation.
 
 The memory controller didn't have a way to control what happened
 between internal tasks and child cgroups and the behavior was not
 clearly defined.  There were attempts to add ad-hoc behaviors and
 knobs to tailor the behavior to specific workloads which would have
 led to problems extremely difficult to resolve in the long term.
 
 Multiple controllers struggled with internal tasks and came up with
 different ways to deal with it; unfortunately, all the approaches were
 severely flawed and, furthermore, the widely different behaviors
 made cgroup as a whole highly inconsistent.
 
 This clearly is a problem which needs to be addressed from cgroup core
 in a uniform way.
 
 
 Other Interface Issues
 ----------------------
 
 cgroup v1 grew without oversight and developed a large number of
 idiosyncrasies and inconsistencies.  One issue on the cgroup core side
 was how an empty cgroup was notified - a userland helper binary was
 forked and executed for each event.  The event delivery wasn't
 recursive or delegatable.  The limitations of the mechanism also led
 to in-kernel event delivery filtering mechanism further complicating
 the interface.
 
 Controller interfaces were problematic too.  An extreme example is
 controllers completely ignoring hierarchical organization and treating
 all cgroups as if they were all located directly under the root
 cgroup.  Some controllers exposed a large amount of inconsistent
 implementation details to userland.
 
 There also was no consistency across controllers.  When a new cgroup
 was created, some controllers defaulted to not imposing extra
 restrictions while others disallowed any resource usage until
 explicitly configured.  Configuration knobs for the same type of
 control used widely differing naming schemes and formats.  Statistics
 and information knobs were named arbitrarily and used different
 formats and units even in the same controller.
 
 cgroup v2 establishes common conventions where appropriate and updates
 controllers so that they expose minimal and consistent interfaces.
 
 
 Controller Issues and Remedies
 ------------------------------
 
 Memory
 ~~~~~~
 
 The original lower boundary, the soft limit, is defined as a limit
 that is per default unset.  As a result, the set of cgroups that
 global reclaim prefers is opt-in, rather than opt-out.  The costs for
 optimizing these mostly negative lookups are so high that the
 implementation, despite its enormous size, does not even provide the
 basic desirable behavior.  First off, the soft limit has no
 hierarchical meaning.  All configured groups are organized in a global
 rbtree and treated like equal peers, regardless where they are located
 in the hierarchy.  This makes subtree delegation impossible.  Second,
 the soft limit reclaim pass is so aggressive that it not just
 introduces high allocation latencies into the system, but also impacts
 system performance due to overreclaim, to the point where the feature
 becomes self-defeating.
 
 The memory.low boundary on the other hand is a top-down allocated
 reserve.  A cgroup enjoys reclaim protection when it's within its
 effective low, which makes delegation of subtrees possible. It also
 enjoys having reclaim pressure proportional to its overage when
 above its effective low.
 
 The original high boundary, the hard limit, is defined as a strict
 limit that can not budge, even if the OOM killer has to be called.
 But this generally goes against the goal of making the most out of the
 available memory.  The memory consumption of workloads varies during
 runtime, and that requires users to overcommit.  But doing that with a
 strict upper limit requires either a fairly accurate prediction of the
 working set size or adding slack to the limit.  Since working set size
 estimation is hard and error prone, and getting it wrong results in
 OOM kills, most users tend to err on the side of a looser limit and
 end up wasting precious resources.
 
 The memory.high boundary on the other hand can be set much more
 conservatively.  When hit, it throttles allocations by forcing them
 into direct reclaim to work off the excess, but it never invokes the
 OOM killer.  As a result, a high boundary that is chosen too
 aggressively will not terminate the processes, but instead it will
 lead to gradual performance degradation.  The user can monitor this
 and make corrections until the minimal memory footprint that still
 gives acceptable performance is found.
 
 In extreme cases, with many concurrent allocations and a complete
 breakdown of reclaim progress within the group, the high boundary can
 be exceeded.  But even then it's mostly better to satisfy the
 allocation from the slack available in other groups or the rest of the
 system than killing the group.  Otherwise, memory.max is there to
 limit this type of spillover and ultimately contain buggy or even
 malicious applications.
 
 Setting the original memory.limit_in_bytes below the current usage was
 subject to a race condition, where concurrent charges could cause the
 limit setting to fail. memory.max on the other hand will first set the
 limit to prevent new charges, and then reclaim and OOM kill until the
 new limit is met - or the task writing to memory.max is killed.
 
 The combined memory+swap accounting and limiting is replaced by real
 control over swap space.
 
 The main argument for a combined memory+swap facility in the original
 cgroup design was that global or parental pressure would always be
 able to swap all anonymous memory of a child group, regardless of the
 child's own (possibly untrusted) configuration.  However, untrusted
 groups can sabotage swapping by other means - such as referencing its
 anonymous memory in a tight loop - and an admin can not assume full
 swappability when overcommitting untrusted jobs.
 
 For trusted jobs, on the other hand, a combined counter is not an
 intuitive userspace interface, and it flies in the face of the idea
 that cgroup controllers should account and limit specific physical
 resources.  Swap space is a resource like all others in the system,
 and that's why unified hierarchy allows distributing it separately.

Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.