124 lines
4.8 KiB
Text
124 lines
4.8 KiB
Text
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The cgroup freezer is useful to batch job management system which start
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and stop sets of tasks in order to schedule the resources of a machine
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according to the desires of a system administrator. This sort of program
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is often used on HPC clusters to schedule access to the cluster as a
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whole. The cgroup freezer uses cgroups to describe the set of tasks to
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be started/stopped by the batch job management system. It also provides
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a means to start and stop the tasks composing the job.
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The cgroup freezer will also be useful for checkpointing running groups
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of tasks. The freezer allows the checkpoint code to obtain a consistent
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image of the tasks by attempting to force the tasks in a cgroup into a
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quiescent state. Once the tasks are quiescent another task can
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walk /proc or invoke a kernel interface to gather information about the
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quiesced tasks. Checkpointed tasks can be restarted later should a
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recoverable error occur. This also allows the checkpointed tasks to be
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migrated between nodes in a cluster by copying the gathered information
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to another node and restarting the tasks there.
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Sequences of SIGSTOP and SIGCONT are not always sufficient for stopping
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and resuming tasks in userspace. Both of these signals are observable
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from within the tasks we wish to freeze. While SIGSTOP cannot be caught,
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blocked, or ignored it can be seen by waiting or ptracing parent tasks.
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SIGCONT is especially unsuitable since it can be caught by the task. Any
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programs designed to watch for SIGSTOP and SIGCONT could be broken by
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attempting to use SIGSTOP and SIGCONT to stop and resume tasks. We can
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demonstrate this problem using nested bash shells:
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$ echo $$
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16644
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$ bash
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$ echo $$
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16690
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From a second, unrelated bash shell:
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$ kill -SIGSTOP 16690
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$ kill -SIGCONT 16690
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<at this point 16690 exits and causes 16644 to exit too>
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This happens because bash can observe both signals and choose how it
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responds to them.
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Another example of a program which catches and responds to these
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signals is gdb. In fact any program designed to use ptrace is likely to
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have a problem with this method of stopping and resuming tasks.
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In contrast, the cgroup freezer uses the kernel freezer code to
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prevent the freeze/unfreeze cycle from becoming visible to the tasks
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being frozen. This allows the bash example above and gdb to run as
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expected.
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The cgroup freezer is hierarchical. Freezing a cgroup freezes all
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tasks belonging to the cgroup and all its descendant cgroups. Each
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cgroup has its own state (self-state) and the state inherited from the
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parent (parent-state). Iff both states are THAWED, the cgroup is
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THAWED.
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The following cgroupfs files are created by cgroup freezer.
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* freezer.state: Read-write.
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When read, returns the effective state of the cgroup - "THAWED",
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"FREEZING" or "FROZEN". This is the combined self and parent-states.
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If any is freezing, the cgroup is freezing (FREEZING or FROZEN).
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FREEZING cgroup transitions into FROZEN state when all tasks
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belonging to the cgroup and its descendants become frozen. Note that
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a cgroup reverts to FREEZING from FROZEN after a new task is added
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to the cgroup or one of its descendant cgroups until the new task is
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frozen.
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When written, sets the self-state of the cgroup. Two values are
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allowed - "FROZEN" and "THAWED". If FROZEN is written, the cgroup,
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if not already freezing, enters FREEZING state along with all its
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descendant cgroups.
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If THAWED is written, the self-state of the cgroup is changed to
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THAWED. Note that the effective state may not change to THAWED if
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the parent-state is still freezing. If a cgroup's effective state
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becomes THAWED, all its descendants which are freezing because of
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the cgroup also leave the freezing state.
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* freezer.self_freezing: Read only.
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Shows the self-state. 0 if the self-state is THAWED; otherwise, 1.
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This value is 1 iff the last write to freezer.state was "FROZEN".
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* freezer.parent_freezing: Read only.
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Shows the parent-state. 0 if none of the cgroup's ancestors is
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frozen; otherwise, 1.
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The root cgroup is non-freezable and the above interface files don't
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exist.
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* Examples of usage :
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# mkdir /sys/fs/cgroup/freezer
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# mount -t cgroup -ofreezer freezer /sys/fs/cgroup/freezer
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# mkdir /sys/fs/cgroup/freezer/0
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# echo $some_pid > /sys/fs/cgroup/freezer/0/tasks
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to get status of the freezer subsystem :
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# cat /sys/fs/cgroup/freezer/0/freezer.state
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THAWED
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to freeze all tasks in the container :
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# echo FROZEN > /sys/fs/cgroup/freezer/0/freezer.state
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# cat /sys/fs/cgroup/freezer/0/freezer.state
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FREEZING
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# cat /sys/fs/cgroup/freezer/0/freezer.state
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FROZEN
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to unfreeze all tasks in the container :
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# echo THAWED > /sys/fs/cgroup/freezer/0/freezer.state
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# cat /sys/fs/cgroup/freezer/0/freezer.state
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THAWED
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This is the basic mechanism which should do the right thing for user space task
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in a simple scenario.
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