1 .. _cpusets: 2 3 ======= 4 CPUSETS 5 ======= 6 7 Copyright (C) 2004 BULL SA. 8 9 Written by Simon.Derr@bull.net 10 11 - Portions Copyright (c) 2004-2006 Silicon Graphics, Inc. 12 - Modified by Paul Jackson <pj@sgi.com> 13 - Modified by Christoph Lameter <cl@linux.com> 14 - Modified by Paul Menage <menage@google.com> 15 - Modified by Hidetoshi Seto <seto.hidetoshi@jp.fujitsu.com> 16 17 .. CONTENTS: 18 19 1. Cpusets 20 1.1 What are cpusets ? 21 1.2 Why are cpusets needed ? 22 1.3 How are cpusets implemented ? 23 1.4 What are exclusive cpusets ? 24 1.5 What is memory_pressure ? 25 1.6 What is memory spread ? 26 1.7 What is sched_load_balance ? 27 1.8 What is sched_relax_domain_level ? 28 1.9 How do I use cpusets ? 29 2. Usage Examples and Syntax 30 2.1 Basic Usage 31 2.2 Adding/removing cpus 32 2.3 Setting flags 33 2.4 Attaching processes 34 3. Questions 35 4. Contact 36 37 1. Cpusets 38 ========== 39 40 1.1 What are cpusets ? 41 ---------------------- 42 43 Cpusets provide a mechanism for assigning a set of CPUs and Memory 44 Nodes to a set of tasks. In this document "Memory Node" refers to 45 an on-line node that contains memory. 46 47 Cpusets constrain the CPU and Memory placement of tasks to only 48 the resources within a task's current cpuset. They form a nested 49 hierarchy visible in a virtual file system. These are the essential 50 hooks, beyond what is already present, required to manage dynamic 51 job placement on large systems. 52 53 Cpusets use the generic cgroup subsystem described in 54 Documentation/admin-guide/cgroup-v1/cgroups.rst. 55 56 Requests by a task, using the sched_setaffinity(2) system call to 57 include CPUs in its CPU affinity mask, and using the mbind(2) and 58 set_mempolicy(2) system calls to include Memory Nodes in its memory 59 policy, are both filtered through that task's cpuset, filtering out any 60 CPUs or Memory Nodes not in that cpuset. The scheduler will not 61 schedule a task on a CPU that is not allowed in its cpus_allowed 62 vector, and the kernel page allocator will not allocate a page on a 63 node that is not allowed in the requesting task's mems_allowed vector. 64 65 User level code may create and destroy cpusets by name in the cgroup 66 virtual file system, manage the attributes and permissions of these 67 cpusets and which CPUs and Memory Nodes are assigned to each cpuset, 68 specify and query to which cpuset a task is assigned, and list the 69 task pids assigned to a cpuset. 70 71 72 1.2 Why are cpusets needed ? 73 ---------------------------- 74 75 The management of large computer systems, with many processors (CPUs), 76 complex memory cache hierarchies and multiple Memory Nodes having 77 non-uniform access times (NUMA) presents additional challenges for 78 the efficient scheduling and memory placement of processes. 79 80 Frequently more modest sized systems can be operated with adequate 81 efficiency just by letting the operating system automatically share 82 the available CPU and Memory resources amongst the requesting tasks. 83 84 But larger systems, which benefit more from careful processor and 85 memory placement to reduce memory access times and contention, 86 and which typically represent a larger investment for the customer, 87 can benefit from explicitly placing jobs on properly sized subsets of 88 the system. 89 90 This can be especially valuable on: 91 92 * Web Servers running multiple instances of the same web application, 93 * Servers running different applications (for instance, a web server 94 and a database), or 95 * NUMA systems running large HPC applications with demanding 96 performance characteristics. 97 98 These subsets, or "soft partitions" must be able to be dynamically 99 adjusted, as the job mix changes, without impacting other concurrently 100 executing jobs. The location of the running jobs pages may also be moved 101 when the memory locations are changed. 102 103 The kernel cpuset patch provides the minimum essential kernel 104 mechanisms required to efficiently implement such subsets. It 105 leverages existing CPU and Memory Placement facilities in the Linux 106 kernel to avoid any additional impact on the critical scheduler or 107 memory allocator code. 108 109 110 1.3 How are cpusets implemented ? 111 --------------------------------- 112 113 Cpusets provide a Linux kernel mechanism to constrain which CPUs and 114 Memory Nodes are used by a process or set of processes. 115 116 The Linux kernel already has a pair of mechanisms to specify on which 117 CPUs a task may be scheduled (sched_setaffinity) and on which Memory 118 Nodes it may obtain memory (mbind, set_mempolicy). 119 120 Cpusets extends these two mechanisms as follows: 121 122 - Cpusets are sets of allowed CPUs and Memory Nodes, known to the 123 kernel. 124 - Each task in the system is attached to a cpuset, via a pointer 125 in the task structure to a reference counted cgroup structure. 126 - Calls to sched_setaffinity are filtered to just those CPUs 127 allowed in that task's cpuset. 128 - Calls to mbind and set_mempolicy are filtered to just 129 those Memory Nodes allowed in that task's cpuset. 130 - The root cpuset contains all the systems CPUs and Memory 131 Nodes. 132 - For any cpuset, one can define child cpusets containing a subset 133 of the parents CPU and Memory Node resources. 134 - The hierarchy of cpusets can be mounted at /dev/cpuset, for 135 browsing and manipulation from user space. 136 - A cpuset may be marked exclusive, which ensures that no other 137 cpuset (except direct ancestors and descendants) may contain 138 any overlapping CPUs or Memory Nodes. 139 - You can list all the tasks (by pid) attached to any cpuset. 140 141 The implementation of cpusets requires a few, simple hooks 142 into the rest of the kernel, none in performance critical paths: 143 144 - in init/main.c, to initialize the root cpuset at system boot. 145 - in fork and exit, to attach and detach a task from its cpuset. 146 - in sched_setaffinity, to mask the requested CPUs by what's 147 allowed in that task's cpuset. 148 - in sched.c migrate_live_tasks(), to keep migrating tasks within 149 the CPUs allowed by their cpuset, if possible. 150 - in the mbind and set_mempolicy system calls, to mask the requested 151 Memory Nodes by what's allowed in that task's cpuset. 152 - in page_alloc.c, to restrict memory to allowed nodes. 153 - in vmscan.c, to restrict page recovery to the current cpuset. 154 155 You should mount the "cgroup" filesystem type in order to enable 156 browsing and modifying the cpusets presently known to the kernel. No 157 new system calls are added for cpusets - all support for querying and 158 modifying cpusets is via this cpuset file system. 159 160 The /proc/<pid>/status file for each task has four added lines, 161 displaying the task's cpus_allowed (on which CPUs it may be scheduled) 162 and mems_allowed (on which Memory Nodes it may obtain memory), 163 in the two formats seen in the following example:: 164 165 Cpus_allowed: ffffffff,ffffffff,ffffffff,ffffffff 166 Cpus_allowed_list: 0-127 167 Mems_allowed: ffffffff,ffffffff 168 Mems_allowed_list: 0-63 169 170 Each cpuset is represented by a directory in the cgroup file system 171 containing (on top of the standard cgroup files) the following 172 files describing that cpuset: 173 174 - cpuset.cpus: list of CPUs in that cpuset 175 - cpuset.mems: list of Memory Nodes in that cpuset 176 - cpuset.memory_migrate flag: if set, move pages to cpusets nodes 177 - cpuset.cpu_exclusive flag: is cpu placement exclusive? 178 - cpuset.mem_exclusive flag: is memory placement exclusive? 179 - cpuset.mem_hardwall flag: is memory allocation hardwalled 180 - cpuset.memory_pressure: measure of how much paging pressure in cpuset 181 - cpuset.memory_spread_page flag: if set, spread page cache evenly on allowed nodes 182 - cpuset.memory_spread_slab flag: OBSOLETE. Doesn't have any function. 183 - cpuset.sched_load_balance flag: if set, load balance within CPUs on that cpuset 184 - cpuset.sched_relax_domain_level: the searching range when migrating tasks 185 186 In addition, only the root cpuset has the following file: 187 188 - cpuset.memory_pressure_enabled flag: compute memory_pressure? 189 190 New cpusets are created using the mkdir system call or shell 191 command. The properties of a cpuset, such as its flags, allowed 192 CPUs and Memory Nodes, and attached tasks, are modified by writing 193 to the appropriate file in that cpusets directory, as listed above. 194 195 The named hierarchical structure of nested cpusets allows partitioning 196 a large system into nested, dynamically changeable, "soft-partitions". 197 198 The attachment of each task, automatically inherited at fork by any 199 children of that task, to a cpuset allows organizing the work load 200 on a system into related sets of tasks such that each set is constrained 201 to using the CPUs and Memory Nodes of a particular cpuset. A task 202 may be re-attached to any other cpuset, if allowed by the permissions 203 on the necessary cpuset file system directories. 204 205 Such management of a system "in the large" integrates smoothly with 206 the detailed placement done on individual tasks and memory regions 207 using the sched_setaffinity, mbind and set_mempolicy system calls. 208 209 The following rules apply to each cpuset: 210 211 - Its CPUs and Memory Nodes must be a subset of its parents. 212 - It can't be marked exclusive unless its parent is. 213 - If its cpu or memory is exclusive, they may not overlap any sibling. 214 215 These rules, and the natural hierarchy of cpusets, enable efficient 216 enforcement of the exclusive guarantee, without having to scan all 217 cpusets every time any of them change to ensure nothing overlaps a 218 exclusive cpuset. Also, the use of a Linux virtual file system (vfs) 219 to represent the cpuset hierarchy provides for a familiar permission 220 and name space for cpusets, with a minimum of additional kernel code. 221 222 The cpus and mems files in the root (top_cpuset) cpuset are 223 read-only. The cpus file automatically tracks the value of 224 cpu_online_mask using a CPU hotplug notifier, and the mems file 225 automatically tracks the value of node_states[N_MEMORY]--i.e., 226 nodes with memory--using the cpuset_track_online_nodes() hook. 227 228 The cpuset.effective_cpus and cpuset.effective_mems files are 229 normally read-only copies of cpuset.cpus and cpuset.mems files 230 respectively. If the cpuset cgroup filesystem is mounted with the 231 special "cpuset_v2_mode" option, the behavior of these files will become 232 similar to the corresponding files in cpuset v2. In other words, hotplug 233 events will not change cpuset.cpus and cpuset.mems. Those events will 234 only affect cpuset.effective_cpus and cpuset.effective_mems which show 235 the actual cpus and memory nodes that are currently used by this cpuset. 236 See Documentation/admin-guide/cgroup-v2.rst for more information about 237 cpuset v2 behavior. 238 239 240 1.4 What are exclusive cpusets ? 241 -------------------------------- 242 243 If a cpuset is cpu or mem exclusive, no other cpuset, other than 244 a direct ancestor or descendant, may share any of the same CPUs or 245 Memory Nodes. 246 247 A cpuset that is cpuset.mem_exclusive *or* cpuset.mem_hardwall is "hardwalled", 248 i.e. it restricts kernel allocations for page, buffer and other data 249 commonly shared by the kernel across multiple users. All cpusets, 250 whether hardwalled or not, restrict allocations of memory for user 251 space. This enables configuring a system so that several independent 252 jobs can share common kernel data, such as file system pages, while 253 isolating each job's user allocation in its own cpuset. To do this, 254 construct a large mem_exclusive cpuset to hold all the jobs, and 255 construct child, non-mem_exclusive cpusets for each individual job. 256 Only a small amount of typical kernel memory, such as requests from 257 interrupt handlers, is allowed to be taken outside even a 258 mem_exclusive cpuset. 259 260 261 1.5 What is memory_pressure ? 262 ----------------------------- 263 The memory_pressure of a cpuset provides a simple per-cpuset metric 264 of the rate that the tasks in a cpuset are attempting to free up in 265 use memory on the nodes of the cpuset to satisfy additional memory 266 requests. 267 268 This enables batch managers monitoring jobs running in dedicated 269 cpusets to efficiently detect what level of memory pressure that job 270 is causing. 271 272 This is useful both on tightly managed systems running a wide mix of 273 submitted jobs, which may choose to terminate or re-prioritize jobs that 274 are trying to use more memory than allowed on the nodes assigned to them, 275 and with tightly coupled, long running, massively parallel scientific 276 computing jobs that will dramatically fail to meet required performance 277 goals if they start to use more memory than allowed to them. 278 279 This mechanism provides a very economical way for the batch manager 280 to monitor a cpuset for signs of memory pressure. It's up to the 281 batch manager or other user code to decide what to do about it and 282 take action. 283 284 ==> 285 Unless this feature is enabled by writing "1" to the special file 286 /dev/cpuset/memory_pressure_enabled, the hook in the rebalance 287 code of __alloc_pages() for this metric reduces to simply noticing 288 that the cpuset_memory_pressure_enabled flag is zero. So only 289 systems that enable this feature will compute the metric. 290 291 Why a per-cpuset, running average: 292 293 Because this meter is per-cpuset, rather than per-task or mm, 294 the system load imposed by a batch scheduler monitoring this 295 metric is sharply reduced on large systems, because a scan of 296 the tasklist can be avoided on each set of queries. 297 298 Because this meter is a running average, instead of an accumulating 299 counter, a batch scheduler can detect memory pressure with a 300 single read, instead of having to read and accumulate results 301 for a period of time. 302 303 Because this meter is per-cpuset rather than per-task or mm, 304 the batch scheduler can obtain the key information, memory 305 pressure in a cpuset, with a single read, rather than having to 306 query and accumulate results over all the (dynamically changing) 307 set of tasks in the cpuset. 308 309 A per-cpuset simple digital filter (requires a spinlock and 3 words 310 of data per-cpuset) is kept, and updated by any task attached to that 311 cpuset, if it enters the synchronous (direct) page reclaim code. 312 313 A per-cpuset file provides an integer number representing the recent 314 (half-life of 10 seconds) rate of direct page reclaims caused by 315 the tasks in the cpuset, in units of reclaims attempted per second, 316 times 1000. 317 318 319 1.6 What is memory spread ? 320 --------------------------- 321 There are two boolean flag files per cpuset that control where the 322 kernel allocates pages for the file system buffers and related in 323 kernel data structures. They are called 'cpuset.memory_spread_page' and 324 'cpuset.memory_spread_slab'. 325 326 If the per-cpuset boolean flag file 'cpuset.memory_spread_page' is set, then 327 the kernel will spread the file system buffers (page cache) evenly 328 over all the nodes that the faulting task is allowed to use, instead 329 of preferring to put those pages on the node where the task is running. 330 331 If the per-cpuset boolean flag file 'cpuset.memory_spread_slab' is set, 332 then the kernel will spread some file system related slab caches, 333 such as for inodes and dentries evenly over all the nodes that the 334 faulting task is allowed to use, instead of preferring to put those 335 pages on the node where the task is running. 336 337 The setting of these flags does not affect anonymous data segment or 338 stack segment pages of a task. 339 340 By default, both kinds of memory spreading are off, and memory 341 pages are allocated on the node local to where the task is running, 342 except perhaps as modified by the task's NUMA mempolicy or cpuset 343 configuration, so long as sufficient free memory pages are available. 344 345 When new cpusets are created, they inherit the memory spread settings 346 of their parent. 347 348 Setting memory spreading causes allocations for the affected page 349 or slab caches to ignore the task's NUMA mempolicy and be spread 350 instead. Tasks using mbind() or set_mempolicy() calls to set NUMA 351 mempolicies will not notice any change in these calls as a result of 352 their containing task's memory spread settings. If memory spreading 353 is turned off, then the currently specified NUMA mempolicy once again 354 applies to memory page allocations. 355 356 Both 'cpuset.memory_spread_page' and 'cpuset.memory_spread_slab' are boolean flag 357 files. By default they contain "0", meaning that the feature is off 358 for that cpuset. If a "1" is written to that file, then that turns 359 the named feature on. 360 361 The implementation is simple. 362 363 Setting the flag 'cpuset.memory_spread_page' turns on a per-process flag 364 PFA_SPREAD_PAGE for each task that is in that cpuset or subsequently 365 joins that cpuset. The page allocation calls for the page cache 366 is modified to perform an inline check for this PFA_SPREAD_PAGE task 367 flag, and if set, a call to a new routine cpuset_mem_spread_node() 368 returns the node to prefer for the allocation. 369 370 Similarly, setting 'cpuset.memory_spread_slab' turns on the flag 371 PFA_SPREAD_SLAB, and appropriately marked slab caches will allocate 372 pages from the node returned by cpuset_mem_spread_node(). 373 374 The cpuset_mem_spread_node() routine is also simple. It uses the 375 value of a per-task rotor cpuset_mem_spread_rotor to select the next 376 node in the current task's mems_allowed to prefer for the allocation. 377 378 This memory placement policy is also known (in other contexts) as 379 round-robin or interleave. 380 381 This policy can provide substantial improvements for jobs that need 382 to place thread local data on the corresponding node, but that need 383 to access large file system data sets that need to be spread across 384 the several nodes in the jobs cpuset in order to fit. Without this 385 policy, especially for jobs that might have one thread reading in the 386 data set, the memory allocation across the nodes in the jobs cpuset 387 can become very uneven. 388 389 1.7 What is sched_load_balance ? 390 -------------------------------- 391 392 The kernel scheduler (kernel/sched/core.c) automatically load balances 393 tasks. If one CPU is underutilized, kernel code running on that 394 CPU will look for tasks on other more overloaded CPUs and move those 395 tasks to itself, within the constraints of such placement mechanisms 396 as cpusets and sched_setaffinity. 397 398 The algorithmic cost of load balancing and its impact on key shared 399 kernel data structures such as the task list increases more than 400 linearly with the number of CPUs being balanced. So the scheduler 401 has support to partition the systems CPUs into a number of sched 402 domains such that it only load balances within each sched domain. 403 Each sched domain covers some subset of the CPUs in the system; 404 no two sched domains overlap; some CPUs might not be in any sched 405 domain and hence won't be load balanced. 406 407 Put simply, it costs less to balance between two smaller sched domains 408 than one big one, but doing so means that overloads in one of the 409 two domains won't be load balanced to the other one. 410 411 By default, there is one sched domain covering all CPUs, including those 412 marked isolated using the kernel boot time "isolcpus=" argument. However, 413 the isolated CPUs will not participate in load balancing, and will not 414 have tasks running on them unless explicitly assigned. 415 416 This default load balancing across all CPUs is not well suited for 417 the following two situations: 418 419 1) On large systems, load balancing across many CPUs is expensive. 420 If the system is managed using cpusets to place independent jobs 421 on separate sets of CPUs, full load balancing is unnecessary. 422 2) Systems supporting realtime on some CPUs need to minimize 423 system overhead on those CPUs, including avoiding task load 424 balancing if that is not needed. 425 426 When the per-cpuset flag "cpuset.sched_load_balance" is enabled (the default 427 setting), it requests that all the CPUs in that cpusets allowed 'cpuset.cpus' 428 be contained in a single sched domain, ensuring that load balancing 429 can move a task (not otherwised pinned, as by sched_setaffinity) 430 from any CPU in that cpuset to any other. 431 432 When the per-cpuset flag "cpuset.sched_load_balance" is disabled, then the 433 scheduler will avoid load balancing across the CPUs in that cpuset, 434 --except-- in so far as is necessary because some overlapping cpuset 435 has "sched_load_balance" enabled. 436 437 So, for example, if the top cpuset has the flag "cpuset.sched_load_balance" 438 enabled, then the scheduler will have one sched domain covering all 439 CPUs, and the setting of the "cpuset.sched_load_balance" flag in any other 440 cpusets won't matter, as we're already fully load balancing. 441 442 Therefore in the above two situations, the top cpuset flag 443 "cpuset.sched_load_balance" should be disabled, and only some of the smaller, 444 child cpusets have this flag enabled. 445 446 When doing this, you don't usually want to leave any unpinned tasks in 447 the top cpuset that might use non-trivial amounts of CPU, as such tasks 448 may be artificially constrained to some subset of CPUs, depending on 449 the particulars of this flag setting in descendant cpusets. Even if 450 such a task could use spare CPU cycles in some other CPUs, the kernel 451 scheduler might not consider the possibility of load balancing that 452 task to that underused CPU. 453 454 Of course, tasks pinned to a particular CPU can be left in a cpuset 455 that disables "cpuset.sched_load_balance" as those tasks aren't going anywhere 456 else anyway. 457 458 There is an impedance mismatch here, between cpusets and sched domains. 459 Cpusets are hierarchical and nest. Sched domains are flat; they don't 460 overlap and each CPU is in at most one sched domain. 461 462 It is necessary for sched domains to be flat because load balancing 463 across partially overlapping sets of CPUs would risk unstable dynamics 464 that would be beyond our understanding. So if each of two partially 465 overlapping cpusets enables the flag 'cpuset.sched_load_balance', then we 466 form a single sched domain that is a superset of both. We won't move 467 a task to a CPU outside its cpuset, but the scheduler load balancing 468 code might waste some compute cycles considering that possibility. 469 470 This mismatch is why there is not a simple one-to-one relation 471 between which cpusets have the flag "cpuset.sched_load_balance" enabled, 472 and the sched domain configuration. If a cpuset enables the flag, it 473 will get balancing across all its CPUs, but if it disables the flag, 474 it will only be assured of no load balancing if no other overlapping 475 cpuset enables the flag. 476 477 If two cpusets have partially overlapping 'cpuset.cpus' allowed, and only 478 one of them has this flag enabled, then the other may find its 479 tasks only partially load balanced, just on the overlapping CPUs. 480 This is just the general case of the top_cpuset example given a few 481 paragraphs above. In the general case, as in the top cpuset case, 482 don't leave tasks that might use non-trivial amounts of CPU in 483 such partially load balanced cpusets, as they may be artificially 484 constrained to some subset of the CPUs allowed to them, for lack of 485 load balancing to the other CPUs. 486 487 CPUs in "cpuset.isolcpus" were excluded from load balancing by the 488 isolcpus= kernel boot option, and will never be load balanced regardless 489 of the value of "cpuset.sched_load_balance" in any cpuset. 490 491 1.7.1 sched_load_balance implementation details. 492 ------------------------------------------------ 493 494 The per-cpuset flag 'cpuset.sched_load_balance' defaults to enabled (contrary 495 to most cpuset flags.) When enabled for a cpuset, the kernel will 496 ensure that it can load balance across all the CPUs in that cpuset 497 (makes sure that all the CPUs in the cpus_allowed of that cpuset are 498 in the same sched domain.) 499 500 If two overlapping cpusets both have 'cpuset.sched_load_balance' enabled, 501 then they will be (must be) both in the same sched domain. 502 503 If, as is the default, the top cpuset has 'cpuset.sched_load_balance' enabled, 504 then by the above that means there is a single sched domain covering 505 the whole system, regardless of any other cpuset settings. 506 507 The kernel commits to user space that it will avoid load balancing 508 where it can. It will pick as fine a granularity partition of sched 509 domains as it can while still providing load balancing for any set 510 of CPUs allowed to a cpuset having 'cpuset.sched_load_balance' enabled. 511 512 The internal kernel cpuset to scheduler interface passes from the 513 cpuset code to the scheduler code a partition of the load balanced 514 CPUs in the system. This partition is a set of subsets (represented 515 as an array of struct cpumask) of CPUs, pairwise disjoint, that cover 516 all the CPUs that must be load balanced. 517 518 The cpuset code builds a new such partition and passes it to the 519 scheduler sched domain setup code, to have the sched domains rebuilt 520 as necessary, whenever: 521 522 - the 'cpuset.sched_load_balance' flag of a cpuset with non-empty CPUs changes, 523 - or CPUs come or go from a cpuset with this flag enabled, 524 - or 'cpuset.sched_relax_domain_level' value of a cpuset with non-empty CPUs 525 and with this flag enabled changes, 526 - or a cpuset with non-empty CPUs and with this flag enabled is removed, 527 - or a cpu is offlined/onlined. 528 529 This partition exactly defines what sched domains the scheduler should 530 setup - one sched domain for each element (struct cpumask) in the 531 partition. 532 533 The scheduler remembers the currently active sched domain partitions. 534 When the scheduler routine partition_sched_domains() is invoked from 535 the cpuset code to update these sched domains, it compares the new 536 partition requested with the current, and updates its sched domains, 537 removing the old and adding the new, for each change. 538 539 540 1.8 What is sched_relax_domain_level ? 541 -------------------------------------- 542 543 In sched domain, the scheduler migrates tasks in 2 ways; periodic load 544 balance on tick, and at time of some schedule events. 545 546 When a task is woken up, scheduler try to move the task on idle CPU. 547 For example, if a task A running on CPU X activates another task B 548 on the same CPU X, and if CPU Y is X's sibling and performing idle, 549 then scheduler migrate task B to CPU Y so that task B can start on 550 CPU Y without waiting task A on CPU X. 551 552 And if a CPU run out of tasks in its runqueue, the CPU try to pull 553 extra tasks from other busy CPUs to help them before it is going to 554 be idle. 555 556 Of course it takes some searching cost to find movable tasks and/or 557 idle CPUs, the scheduler might not search all CPUs in the domain 558 every time. In fact, in some architectures, the searching ranges on 559 events are limited in the same socket or node where the CPU locates, 560 while the load balance on tick searches all. 561 562 For example, assume CPU Z is relatively far from CPU X. Even if CPU Z 563 is idle while CPU X and the siblings are busy, scheduler can't migrate 564 woken task B from X to Z since it is out of its searching range. 565 As the result, task B on CPU X need to wait task A or wait load balance 566 on the next tick. For some applications in special situation, waiting 567 1 tick may be too long. 568 569 The 'cpuset.sched_relax_domain_level' file allows you to request changing 570 this searching range as you like. This file takes int value which 571 indicates size of searching range in levels approximately as follows, 572 otherwise initial value -1 that indicates the cpuset has no request. 573 574 ====== =========================================================== 575 -1 no request. use system default or follow request of others. 576 0 no search. 577 1 search siblings (hyperthreads in a core). 578 2 search cores in a package. 579 3 search cpus in a node [= system wide on non-NUMA system] 580 4 search nodes in a chunk of node [on NUMA system] 581 5 search system wide [on NUMA system] 582 ====== =========================================================== 583 584 Not all levels can be present and values can change depending on the 585 system architecture and kernel configuration. Check 586 /sys/kernel/debug/sched/domains/cpu*/domain*/ for system-specific 587 details. 588 589 The system default is architecture dependent. The system default 590 can be changed using the relax_domain_level= boot parameter. 591 592 This file is per-cpuset and affect the sched domain where the cpuset 593 belongs to. Therefore if the flag 'cpuset.sched_load_balance' of a cpuset 594 is disabled, then 'cpuset.sched_relax_domain_level' have no effect since 595 there is no sched domain belonging the cpuset. 596 597 If multiple cpusets are overlapping and hence they form a single sched 598 domain, the largest value among those is used. Be careful, if one 599 requests 0 and others are -1 then 0 is used. 600 601 Note that modifying this file will have both good and bad effects, 602 and whether it is acceptable or not depends on your situation. 603 Don't modify this file if you are not sure. 604 605 If your situation is: 606 607 - The migration costs between each cpu can be assumed considerably 608 small(for you) due to your special application's behavior or 609 special hardware support for CPU cache etc. 610 - The searching cost doesn't have impact(for you) or you can make 611 the searching cost enough small by managing cpuset to compact etc. 612 - The latency is required even it sacrifices cache hit rate etc. 613 then increasing 'sched_relax_domain_level' would benefit you. 614 615 616 1.9 How do I use cpusets ? 617 -------------------------- 618 619 In order to minimize the impact of cpusets on critical kernel 620 code, such as the scheduler, and due to the fact that the kernel 621 does not support one task updating the memory placement of another 622 task directly, the impact on a task of changing its cpuset CPU 623 or Memory Node placement, or of changing to which cpuset a task 624 is attached, is subtle. 625 626 If a cpuset has its Memory Nodes modified, then for each task attached 627 to that cpuset, the next time that the kernel attempts to allocate 628 a page of memory for that task, the kernel will notice the change 629 in the task's cpuset, and update its per-task memory placement to 630 remain within the new cpusets memory placement. If the task was using 631 mempolicy MPOL_BIND, and the nodes to which it was bound overlap with 632 its new cpuset, then the task will continue to use whatever subset 633 of MPOL_BIND nodes are still allowed in the new cpuset. If the task 634 was using MPOL_BIND and now none of its MPOL_BIND nodes are allowed 635 in the new cpuset, then the task will be essentially treated as if it 636 was MPOL_BIND bound to the new cpuset (even though its NUMA placement, 637 as queried by get_mempolicy(), doesn't change). If a task is moved 638 from one cpuset to another, then the kernel will adjust the task's 639 memory placement, as above, the next time that the kernel attempts 640 to allocate a page of memory for that task. 641 642 If a cpuset has its 'cpuset.cpus' modified, then each task in that cpuset 643 will have its allowed CPU placement changed immediately. Similarly, 644 if a task's pid is written to another cpuset's 'tasks' file, then its 645 allowed CPU placement is changed immediately. If such a task had been 646 bound to some subset of its cpuset using the sched_setaffinity() call, 647 the task will be allowed to run on any CPU allowed in its new cpuset, 648 negating the effect of the prior sched_setaffinity() call. 649 650 In summary, the memory placement of a task whose cpuset is changed is 651 updated by the kernel, on the next allocation of a page for that task, 652 and the processor placement is updated immediately. 653 654 Normally, once a page is allocated (given a physical page 655 of main memory) then that page stays on whatever node it 656 was allocated, so long as it remains allocated, even if the 657 cpusets memory placement policy 'cpuset.mems' subsequently changes. 658 If the cpuset flag file 'cpuset.memory_migrate' is set true, then when 659 tasks are attached to that cpuset, any pages that task had 660 allocated to it on nodes in its previous cpuset are migrated 661 to the task's new cpuset. The relative placement of the page within 662 the cpuset is preserved during these migration operations if possible. 663 For example if the page was on the second valid node of the prior cpuset 664 then the page will be placed on the second valid node of the new cpuset. 665 666 Also if 'cpuset.memory_migrate' is set true, then if that cpuset's 667 'cpuset.mems' file is modified, pages allocated to tasks in that 668 cpuset, that were on nodes in the previous setting of 'cpuset.mems', 669 will be moved to nodes in the new setting of 'mems.' 670 Pages that were not in the task's prior cpuset, or in the cpuset's 671 prior 'cpuset.mems' setting, will not be moved. 672 673 There is an exception to the above. If hotplug functionality is used 674 to remove all the CPUs that are currently assigned to a cpuset, 675 then all the tasks in that cpuset will be moved to the nearest ancestor 676 with non-empty cpus. But the moving of some (or all) tasks might fail if 677 cpuset is bound with another cgroup subsystem which has some restrictions 678 on task attaching. In this failing case, those tasks will stay 679 in the original cpuset, and the kernel will automatically update 680 their cpus_allowed to allow all online CPUs. When memory hotplug 681 functionality for removing Memory Nodes is available, a similar exception 682 is expected to apply there as well. In general, the kernel prefers to 683 violate cpuset placement, over starving a task that has had all 684 its allowed CPUs or Memory Nodes taken offline. 685 686 There is a second exception to the above. GFP_ATOMIC requests are 687 kernel internal allocations that must be satisfied, immediately. 688 The kernel may drop some request, in rare cases even panic, if a 689 GFP_ATOMIC alloc fails. If the request cannot be satisfied within 690 the current task's cpuset, then we relax the cpuset, and look for 691 memory anywhere we can find it. It's better to violate the cpuset 692 than stress the kernel. 693 694 To start a new job that is to be contained within a cpuset, the steps are: 695 696 1) mkdir /sys/fs/cgroup/cpuset 697 2) mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset 698 3) Create the new cpuset by doing mkdir's and write's (or echo's) in 699 the /sys/fs/cgroup/cpuset virtual file system. 700 4) Start a task that will be the "founding father" of the new job. 701 5) Attach that task to the new cpuset by writing its pid to the 702 /sys/fs/cgroup/cpuset tasks file for that cpuset. 703 6) fork, exec or clone the job tasks from this founding father task. 704 705 For example, the following sequence of commands will setup a cpuset 706 named "Charlie", containing just CPUs 2 and 3, and Memory Node 1, 707 and then start a subshell 'sh' in that cpuset:: 708 709 mount -t cgroup -ocpuset cpuset /sys/fs/cgroup/cpuset 710 cd /sys/fs/cgroup/cpuset 711 mkdir Charlie 712 cd Charlie 713 /bin/echo 2-3 > cpuset.cpus 714 /bin/echo 1 > cpuset.mems 715 /bin/echo $$ > tasks 716 sh 717 # The subshell 'sh' is now running in cpuset Charlie 718 # The next line should display '/Charlie' 719 cat /proc/self/cpuset 720 721 There are ways to query or modify cpusets: 722 723 - via the cpuset file system directly, using the various cd, mkdir, echo, 724 cat, rmdir commands from the shell, or their equivalent from C. 725 - via the C library libcpuset. 726 - via the C library libcgroup. 727 (https://github.com/libcgroup/libcgroup/) 728 - via the python application cset. 729 (http://code.google.com/p/cpuset/) 730 731 The sched_setaffinity calls can also be done at the shell prompt using 732 SGI's runon or Robert Love's taskset. The mbind and set_mempolicy 733 calls can be done at the shell prompt using the numactl command 734 (part of Andi Kleen's numa package). 735 736 2. Usage Examples and Syntax 737 ============================ 738 739 2.1 Basic Usage 740 --------------- 741 742 Creating, modifying, using the cpusets can be done through the cpuset 743 virtual filesystem. 744 745 To mount it, type: 746 # mount -t cgroup -o cpuset cpuset /sys/fs/cgroup/cpuset 747 748 Then under /sys/fs/cgroup/cpuset you can find a tree that corresponds to the 749 tree of the cpusets in the system. For instance, /sys/fs/cgroup/cpuset 750 is the cpuset that holds the whole system. 751 752 If you want to create a new cpuset under /sys/fs/cgroup/cpuset:: 753 754 # cd /sys/fs/cgroup/cpuset 755 # mkdir my_cpuset 756 757 Now you want to do something with this cpuset:: 758 759 # cd my_cpuset 760 761 In this directory you can find several files:: 762 763 # ls 764 cgroup.clone_children cpuset.memory_pressure 765 cgroup.event_control cpuset.memory_spread_page 766 cgroup.procs cpuset.memory_spread_slab 767 cpuset.cpu_exclusive cpuset.mems 768 cpuset.cpus cpuset.sched_load_balance 769 cpuset.mem_exclusive cpuset.sched_relax_domain_level 770 cpuset.mem_hardwall notify_on_release 771 cpuset.memory_migrate tasks 772 773 Reading them will give you information about the state of this cpuset: 774 the CPUs and Memory Nodes it can use, the processes that are using 775 it, its properties. By writing to these files you can manipulate 776 the cpuset. 777 778 Set some flags:: 779 780 # /bin/echo 1 > cpuset.cpu_exclusive 781 782 Add some cpus:: 783 784 # /bin/echo 0-7 > cpuset.cpus 785 786 Add some mems:: 787 788 # /bin/echo 0-7 > cpuset.mems 789 790 Now attach your shell to this cpuset:: 791 792 # /bin/echo $$ > tasks 793 794 You can also create cpusets inside your cpuset by using mkdir in this 795 directory:: 796 797 # mkdir my_sub_cs 798 799 To remove a cpuset, just use rmdir:: 800 801 # rmdir my_sub_cs 802 803 This will fail if the cpuset is in use (has cpusets inside, or has 804 processes attached). 805 806 Note that for legacy reasons, the "cpuset" filesystem exists as a 807 wrapper around the cgroup filesystem. 808 809 The command:: 810 811 mount -t cpuset X /sys/fs/cgroup/cpuset 812 813 is equivalent to:: 814 815 mount -t cgroup -ocpuset,noprefix X /sys/fs/cgroup/cpuset 816 echo "/sbin/cpuset_release_agent" > /sys/fs/cgroup/cpuset/release_agent 817 818 2.2 Adding/removing cpus 819 ------------------------ 820 821 This is the syntax to use when writing in the cpus or mems files 822 in cpuset directories:: 823 824 # /bin/echo 1-4 > cpuset.cpus -> set cpus list to cpus 1,2,3,4 825 # /bin/echo 1,2,3,4 > cpuset.cpus -> set cpus list to cpus 1,2,3,4 826 827 To add a CPU to a cpuset, write the new list of CPUs including the 828 CPU to be added. To add 6 to the above cpuset:: 829 830 # /bin/echo 1-4,6 > cpuset.cpus -> set cpus list to cpus 1,2,3,4,6 831 832 Similarly to remove a CPU from a cpuset, write the new list of CPUs 833 without the CPU to be removed. 834 835 To remove all the CPUs:: 836 837 # /bin/echo "" > cpuset.cpus -> clear cpus list 838 839 2.3 Setting flags 840 ----------------- 841 842 The syntax is very simple:: 843 844 # /bin/echo 1 > cpuset.cpu_exclusive -> set flag 'cpuset.cpu_exclusive' 845 # /bin/echo 0 > cpuset.cpu_exclusive -> unset flag 'cpuset.cpu_exclusive' 846 847 2.4 Attaching processes 848 ----------------------- 849 850 :: 851 852 # /bin/echo PID > tasks 853 854 Note that it is PID, not PIDs. You can only attach ONE task at a time. 855 If you have several tasks to attach, you have to do it one after another:: 856 857 # /bin/echo PID1 > tasks 858 # /bin/echo PID2 > tasks 859 ... 860 # /bin/echo PIDn > tasks 861 862 863 3. Questions 864 ============ 865 866 Q: 867 what's up with this '/bin/echo' ? 868 869 A: 870 bash's builtin 'echo' command does not check calls to write() against 871 errors. If you use it in the cpuset file system, you won't be 872 able to tell whether a command succeeded or failed. 873 874 Q: 875 When I attach processes, only the first of the line gets really attached ! 876 877 A: 878 We can only return one error code per call to write(). So you should also 879 put only ONE pid. 880 881 4. Contact 882 ========== 883 884 Web: http://www.bullopensource.org/cpuset
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