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