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author | Matt Helsley <matthltc@us.ibm.com> | 2008-10-19 07:27:24 +0400 |
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committer | Linus Torvalds <torvalds@linux-foundation.org> | 2008-10-20 19:52:34 +0400 |
commit | bde5ab65581a63e9f4f4bacfae8f201d04d25bed (patch) | |
tree | b159612b2acc416878a1a80a0aea47f7469163d2 /Documentation/cgroups/freezer-subsystem.txt | |
parent | 1aece34833721d64eb33fc15cd923c727296d3d3 (diff) | |
download | linux-bde5ab65581a63e9f4f4bacfae8f201d04d25bed.tar.xz |
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>
Diffstat (limited to 'Documentation/cgroups/freezer-subsystem.txt')
-rw-r--r-- | Documentation/cgroups/freezer-subsystem.txt | 99 |
1 files changed, 99 insertions, 0 deletions
diff --git a/Documentation/cgroups/freezer-subsystem.txt b/Documentation/cgroups/freezer-subsystem.txt new file mode 100644 index 000000000000..c50ab58b72eb --- /dev/null +++ b/Documentation/cgroups/freezer-subsystem.txt @@ -0,0 +1,99 @@ + 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. |