1 ============== 2 Cgroup Freezer 3 ============== 4 5 The cgroup freezer is useful to batch job management system which start 6 and stop sets of tasks in order to schedule the resources of a machine 7 according to the desires of a system administrator. This sort of program 8 is often used on HPC clusters to schedule access to the cluster as a 9 whole. The cgroup freezer uses cgroups to describe the set of tasks to 10 be started/stopped by the batch job management system. It also provides 11 a means to start and stop the tasks composing the job. 12 13 The cgroup freezer will also be useful for checkpointing running groups 14 of tasks. The freezer allows the checkpoint code to obtain a consistent 15 image of the tasks by attempting to force the tasks in a cgroup into a 16 quiescent state. Once the tasks are quiescent another task can 17 walk /proc or invoke a kernel interface to gather information about the 18 quiesced tasks. Checkpointed tasks can be restarted later should a 19 recoverable error occur. This also allows the checkpointed tasks to be 20 migrated between nodes in a cluster by copying the gathered information 21 to another node and restarting the tasks there. 22 23 Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping 24 and resuming tasks in userspace. Both of these signals are observable 25 from within the tasks we wish to freeze. While SIGSTOP cannot be caught, 26 blocked, or ignored it can be seen by waiting or ptracing parent tasks. 27 SIGCONT is especially unsuitable since it can be caught by the task. Any 28 programs designed to watch for SIGSTOP and SIGCONT could be broken by 29 attempting to use SIGSTOP and SIGCONT to stop and resume tasks. We can 30 demonstrate this problem using nested bash shells:: 31 32 $ echo $$ 33 16644 34 $ bash 35 $ echo $$ 36 16690 37 38 From a second, unrelated bash shell: 39 $ kill -SIGSTOP 16690 40 $ kill -SIGCONT 16690 41 42 <at this point 16690 exits and causes 16644 to exit too> 43 44 This happens because bash can observe both signals and choose how it 45 responds to them. 46 47 Another example of a program which catches and responds to these 48 signals is gdb. In fact any program designed to use ptrace is likely to 49 have a problem with this method of stopping and resuming tasks. 50 51 In contrast, the cgroup freezer uses the kernel freezer code to 52 prevent the freeze/unfreeze cycle from becoming visible to the tasks 53 being frozen. This allows the bash example above and gdb to run as 54 expected. 55 56 The cgroup freezer is hierarchical. Freezing a cgroup freezes all 57 tasks belonging to the cgroup and all its descendant cgroups. Each 58 cgroup has its own state (self-state) and the state inherited from the 59 parent (parent-state). Iff both states are THAWED, the cgroup is 60 THAWED. 61 62 The following cgroupfs files are created by cgroup freezer. 63 64 * freezer.state: Read-write. 65 66 When read, returns the effective state of the cgroup - "THAWED", 67 "FREEZING" or "FROZEN". This is the combined self and parent-states. 68 If any is freezing, the cgroup is freezing (FREEZING or FROZEN). 69 70 FREEZING cgroup transitions into FROZEN state when all tasks 71 belonging to the cgroup and its descendants become frozen. Note that 72 a cgroup reverts to FREEZING from FROZEN after a new task is added 73 to the cgroup or one of its descendant cgroups until the new task is 74 frozen. 75 76 When written, sets the self-state of the cgroup. Two values are 77 allowed - "FROZEN" and "THAWED". If FROZEN is written, the cgroup, 78 if not already freezing, enters FREEZING state along with all its 79 descendant cgroups. 80 81 If THAWED is written, the self-state of the cgroup is changed to 82 THAWED. Note that the effective state may not change to THAWED if 83 the parent-state is still freezing. If a cgroup's effective state 84 becomes THAWED, all its descendants which are freezing because of 85 the cgroup also leave the freezing state. 86 87 * freezer.self_freezing: Read only. 88 89 Shows the self-state. 0 if the self-state is THAWED; otherwise, 1. 90 This value is 1 iff the last write to freezer.state was "FROZEN". 91 92 * freezer.parent_freezing: Read only. 93 94 Shows the parent-state. 0 if none of the cgroup's ancestors is 95 frozen; otherwise, 1. 96 97 The root cgroup is non-freezable and the above interface files don't 98 exist. 99 100 * Examples of usage:: 101 102 # mkdir /sys/fs/cgroup/freezer 103 # mount -t cgroup -ofreezer freezer /sys/fs/cgroup/freezer 104 # mkdir /sys/fs/cgroup/freezer/0 105 # echo $some_pid > /sys/fs/cgroup/freezer/0/tasks 106 107 to get status of the freezer subsystem:: 108 109 # cat /sys/fs/cgroup/freezer/0/freezer.state 110 THAWED 111 112 to freeze all tasks in the container:: 113 114 # echo FROZEN > /sys/fs/cgroup/freezer/0/freezer.state 115 # cat /sys/fs/cgroup/freezer/0/freezer.state 116 FREEZING 117 # cat /sys/fs/cgroup/freezer/0/freezer.state 118 FROZEN 119 120 to unfreeze all tasks in the container:: 121 122 # echo THAWED > /sys/fs/cgroup/freezer/0/freezer.state 123 # cat /sys/fs/cgroup/freezer/0/freezer.state 124 THAWED 125 126 This is the basic mechanism which should do the right thing for user space task 127 in a simple scenario.
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