LXR linux/Documentation/admin-guide/cgroup-v1/freezer-subsystem.rst

   1==============
   2Cgroup Freezer
   3==============
   4
   5The cgroup freezer is useful to batch job management system which start
   6and stop sets of tasks in order to schedule the resources of a machine
   7according to the desires of a system administrator. This sort of program
   8is often used on HPC clusters to schedule access to the cluster as a
   9whole. The cgroup freezer uses cgroups to describe the set of tasks to
  10be started/stopped by the batch job management system. It also provides
  11a means to start and stop the tasks composing the job.
  12
  13The cgroup freezer will also be useful for checkpointing running groups
  14of tasks. The freezer allows the checkpoint code to obtain a consistent
  15image of the tasks by attempting to force the tasks in a cgroup into a
  16quiescent state. Once the tasks are quiescent another task can
  17walk /proc or invoke a kernel interface to gather information about the
  18quiesced tasks. Checkpointed tasks can be restarted later should a
  19recoverable error occur. This also allows the checkpointed tasks to be
  20migrated between nodes in a cluster by copying the gathered information
  21to another node and restarting the tasks there.
  22
  23Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping
  24and resuming tasks in userspace. Both of these signals are observable
  25from within the tasks we wish to freeze. While SIGSTOP cannot be caught,
  26blocked, or ignored it can be seen by waiting or ptracing parent tasks.
  27SIGCONT is especially unsuitable since it can be caught by the task. Any
  28programs designed to watch for SIGSTOP and SIGCONT could be broken by
  29attempting to use SIGSTOP and SIGCONT to stop and resume tasks. We can
  30demonstrate 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
  44This happens because bash can observe both signals and choose how it
  45responds to them.
  46
  47Another example of a program which catches and responds to these
  48signals is gdb. In fact any program designed to use ptrace is likely to
  49have a problem with this method of stopping and resuming tasks.
  50
  51In contrast, the cgroup freezer uses the kernel freezer code to
  52prevent the freeze/unfreeze cycle from becoming visible to the tasks
  53being frozen. This allows the bash example above and gdb to run as
  54expected.
  55
  56The cgroup freezer is hierarchical. Freezing a cgroup freezes all
  57tasks belonging to the cgroup and all its descendant cgroups. Each
  58cgroup has its own state (self-state) and the state inherited from the
  59parent (parent-state). Iff both states are THAWED, the cgroup is
  60THAWED.
  61
  62The 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
  97The root cgroup is non-freezable and the above interface files don't
  98exist.
  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
 107to get status of the freezer subsystem::
 108
 109   # cat /sys/fs/cgroup/freezer/0/freezer.state
 110   THAWED
 111
 112to 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
 120to 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
 126This is the basic mechanism which should do the right thing for user space task
 127in a simple scenario.
 128