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diff --git a/Documentation/cgroup-legacy/freezer-subsystem.txt b/Documentation/cgroup-legacy/freezer-subsystem.txt new file mode 100644 index 000000000000..e831cb2b8394 --- /dev/null +++ b/Documentation/cgroup-legacy/freezer-subsystem.txt @@ -0,0 +1,123 @@ +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. |