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+==============
+Cgroup Freezer
+==============
+
+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 cgroup freezer is hierarchical. Freezing a cgroup freezes all
+tasks belonging to the cgroup and all its descendant cgroups. Each
+cgroup has its own state (self-state) and the state inherited from the
+parent (parent-state). Iff both states are THAWED, the cgroup is
+THAWED.
+
+The following cgroupfs files are created by cgroup freezer.
+
+* freezer.state: Read-write.
+
+  When read, returns the effective state of the cgroup - "THAWED",
+  "FREEZING" or "FROZEN". This is the combined self and parent-states.
+  If any is freezing, the cgroup is freezing (FREEZING or FROZEN).
+
+  FREEZING cgroup transitions into FROZEN state when all tasks
+  belonging to the cgroup and its descendants become frozen. Note that
+  a cgroup reverts to FREEZING from FROZEN after a new task is added
+  to the cgroup or one of its descendant cgroups until the new task is
+  frozen.
+
+  When written, sets the self-state of the cgroup. Two values are
+  allowed - "FROZEN" and "THAWED". If FROZEN is written, the cgroup,
+  if not already freezing, enters FREEZING state along with all its
+  descendant cgroups.
+
+  If THAWED is written, the self-state of the cgroup is changed to
+  THAWED.  Note that the effective state may not change to THAWED if
+  the parent-state is still freezing. If a cgroup's effective state
+  becomes THAWED, all its descendants which are freezing because of
+  the cgroup also leave the freezing state.
+
+* freezer.self_freezing: Read only.
+
+  Shows the self-state. 0 if the self-state is THAWED; otherwise, 1.
+  This value is 1 iff the last write to freezer.state was "FROZEN".
+
+* freezer.parent_freezing: Read only.
+
+  Shows the parent-state.  0 if none of the cgroup's ancestors is
+  frozen; otherwise, 1.
+
+The root cgroup is non-freezable and the above interface files don't
+exist.
+
+* 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.