103 lines
4.0 KiB
Plaintext
103 lines
4.0 KiB
Plaintext
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 16990
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<at this point 16990 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 freezer subsystem in the container filesystem defines a file named
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freezer.state. Writing "FROZEN" to the state file will freeze all tasks in the
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cgroup. Subsequently writing "THAWED" will unfreeze the tasks in the cgroup.
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Reading will return the current state.
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Note freezer.state doesn't exist in root cgroup, which means root cgroup
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is non-freezable.
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* Examples of usage :
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# mkdir /containers
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# mount -t cgroup -ofreezer freezer /containers
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# mkdir /containers/0
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# echo $some_pid > /containers/0/tasks
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to get status of the freezer subsystem :
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# cat /containers/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 > /containers/0/freezer.state
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# cat /containers/0/freezer.state
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FREEZING
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# cat /containers/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 > /containers/0/freezer.state
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# cat /containers/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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It's important to note that freezing can be incomplete. In that case we return
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EBUSY. This means that some tasks in the cgroup are busy doing something that
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prevents us from completely freezing the cgroup at this time. After EBUSY,
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the cgroup will remain partially frozen -- reflected by freezer.state reporting
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"FREEZING" when read. The state will remain "FREEZING" until one of these
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things happens:
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1) Userspace cancels the freezing operation by writing "THAWED" to
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the freezer.state file
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2) Userspace retries the freezing operation by writing "FROZEN" to
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the freezer.state file (writing "FREEZING" is not legal
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and returns EINVAL)
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3) The tasks that blocked the cgroup from entering the "FROZEN"
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state disappear from the cgroup's set of tasks.
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