container freezer: document the cgroup freezer subsystem.

Describe why we need the freezer subsystem and how to use it in a
documentation file.  Since the cgroups.txt file is focused on the
subsystem-agnostic portions of cgroups make a directory and move the old
cgroups.txt file at the same time.

Signed-off-by: Matt Helsley <matthltc@us.ibm.com>
Cc: Paul Menage <menage@google.com>
Cc: containers@lists.linux-foundation.org
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 99 insertions, 0 deletions

diff --git a/Documentation/cgroups/freezer-subsystem.txt b/Documentation/cgroups/freezer-subsystem.txt
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index 000000000000..c50ab58b72eb
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+++ b/Documentation/cgroups/freezer-subsystem.txt
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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 16990
+
+	<at this point 16990 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.
+
+* Examples of usage :
+
+   # mkdir /containers/freezer
+   # mount -t cgroup -ofreezer freezer  /containers
+   # mkdir /containers/0
+   # echo $some_pid > /containers/0/tasks
+
+to get status of the freezer subsystem :
+
+   # cat /containers/0/freezer.state
+   THAWED
+
+to freeze all tasks in the container :
+
+   # echo FROZEN > /containers/0/freezer.state
+   # cat /containers/0/freezer.state
+   FREEZING
+   # cat /containers/0/freezer.state
+   FROZEN
+
+to unfreeze all tasks in the container :
+
+   # echo THAWED > /containers/0/freezer.state
+   # cat /containers/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 EIO)
+	3) The tasks that blocked the cgroup from entering the "FROZEN"
+		state disappear from the cgroup's set of tasks.