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-The cgroup freezer is useful to batch job management system which start
-and stop sets of tasks in order to schedule the resources of a machine
-according to the desires of a system administrator. This sort of program
-is often used on HPC clusters to schedule access to the cluster as a
-whole. The cgroup freezer uses cgroups to describe the set of tasks to
-be started/stopped by the batch job management system. It also provides
-a means to start and stop the tasks composing the job.
-
-The cgroup freezer will also be useful for checkpointing running groups
-of tasks. The freezer allows the checkpoint code to obtain a consistent
-image of the tasks by attempting to force the tasks in a cgroup into a
-quiescent state. Once the tasks are quiescent another task can
-walk /proc or invoke a kernel interface to gather information about the
-quiesced tasks. Checkpointed tasks can be restarted later should a
-recoverable error occur. This also allows the checkpointed tasks to be
-migrated between nodes in a cluster by copying the gathered information
-to another node and restarting the tasks there.
-
-Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping
-and resuming tasks in userspace. Both of these signals are observable
-from within the tasks we wish to freeze. While SIGSTOP cannot be caught,
-blocked, or ignored it can be seen by waiting or ptracing parent tasks.
-SIGCONT is especially unsuitable since it can be caught by the task. Any
-programs designed to watch for SIGSTOP and SIGCONT could be broken by
-attempting to use SIGSTOP and SIGCONT to stop and resume tasks. We can
-demonstrate this problem using nested bash shells:
-
- $ echo $$
- 16644
- $ bash
- $ echo $$
- 16690
-
- From a second, unrelated bash shell:
- $ kill -SIGSTOP 16690
- $ kill -SIGCONT 16690
-
- <at this point 16690 exits and causes 16644 to exit too>
-
-This happens because bash can observe both signals and choose how it
-responds to them.
-
-Another example of a program which catches and responds to these
-signals is gdb. In fact any program designed to use ptrace is likely to
-have a problem with this method of stopping and resuming tasks.
-
-In contrast, the cgroup freezer uses the kernel freezer code to
-prevent the freeze/unfreeze cycle from becoming visible to the tasks
-being frozen. This allows the bash example above and gdb to run as
-expected.
-
-The freezer subsystem in the container filesystem defines a file named
-freezer.state. Writing "FROZEN" to the state file will freeze all tasks in the
-cgroup. Subsequently writing "THAWED" will unfreeze the tasks in the cgroup.
-Reading will return the current state.
-
-Note freezer.state doesn't exist in root cgroup, which means root cgroup
-is non-freezable.
-
-* Examples of usage :
-
- # mkdir /sys/fs/cgroup/freezer
- # mount -t cgroup -ofreezer freezer /sys/fs/cgroup/freezer
- # mkdir /sys/fs/cgroup/freezer/0
- # echo $some_pid > /sys/fs/cgroup/freezer/0/tasks
-
-to get status of the freezer subsystem :
-
- # cat /sys/fs/cgroup/freezer/0/freezer.state
- THAWED
-
-to freeze all tasks in the container :
-
- # echo FROZEN > /sys/fs/cgroup/freezer/0/freezer.state
- # cat /sys/fs/cgroup/freezer/0/freezer.state
- FREEZING
- # cat /sys/fs/cgroup/freezer/0/freezer.state
- FROZEN
-
-to unfreeze all tasks in the container :
-
- # echo THAWED > /sys/fs/cgroup/freezer/0/freezer.state
- # cat /sys/fs/cgroup/freezer/0/freezer.state
- THAWED
-
-This is the basic mechanism which should do the right thing for user space task
-in a simple scenario.
-
-It's important to note that freezing can be incomplete. In that case we return
-EBUSY. This means that some tasks in the cgroup are busy doing something that
-prevents us from completely freezing the cgroup at this time. After EBUSY,
-the cgroup will remain partially frozen -- reflected by freezer.state reporting
-"FREEZING" when read. The state will remain "FREEZING" until one of these
-things happens:
-
- 1) Userspace cancels the freezing operation by writing "THAWED" to
- the freezer.state file
- 2) Userspace retries the freezing operation by writing "FROZEN" to
- the freezer.state file (writing "FREEZING" is not legal
- and returns EINVAL)
- 3) The tasks that blocked the cgroup from entering the "FROZEN"
- state disappear from the cgroup's set of tasks.