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Diffstat (limited to 'Documentation/cgroup-v1/freezer-subsystem.txt')
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diff --git a/Documentation/cgroup-v1/freezer-subsystem.txt b/Documentation/cgroup-v1/freezer-subsystem.txt deleted file mode 100644 index e831cb2b8394..000000000000 --- a/Documentation/cgroup-v1/freezer-subsystem.txt +++ /dev/null @@ -1,123 +0,0 @@ -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. |