93 lines
3.2 KiB
ReStructuredText
93 lines
3.2 KiB
ReStructuredText
=========================
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Process Number Controller
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=========================
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Abstract
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--------
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The process number controller is used to allow a cgroup hierarchy to stop any
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new tasks from being fork()'d or clone()'d after a certain limit is reached.
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Since it is trivial to hit the task limit without hitting any kmemcg limits in
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place, PIDs are a fundamental resource. As such, PID exhaustion must be
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preventable in the scope of a cgroup hierarchy by allowing resource limiting of
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the number of tasks in a cgroup.
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Usage
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-----
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In order to use the `pids` controller, set the maximum number of tasks in
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pids.max (this is not available in the root cgroup for obvious reasons). The
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number of processes currently in the cgroup is given by pids.current.
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Organisational operations are not blocked by cgroup policies, so it is possible
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to have pids.current > pids.max. This can be done by either setting the limit to
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be smaller than pids.current, or attaching enough processes to the cgroup such
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that pids.current > pids.max. However, it is not possible to violate a cgroup
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policy through fork() or clone(). fork() and clone() will return -EAGAIN if the
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creation of a new process would cause a cgroup policy to be violated.
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To set a cgroup to have no limit, set pids.max to "max". This is the default for
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all new cgroups (N.B. that PID limits are hierarchical, so the most stringent
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limit in the hierarchy is followed).
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pids.current tracks all child cgroup hierarchies, so parent/pids.current is a
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superset of parent/child/pids.current.
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The pids.events file contains event counters:
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- max: Number of times fork failed because limit was hit.
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Example
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-------
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First, we mount the pids controller::
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# mkdir -p /sys/fs/cgroup/pids
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# mount -t cgroup -o pids none /sys/fs/cgroup/pids
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Then we create a hierarchy, set limits and attach processes to it::
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# mkdir -p /sys/fs/cgroup/pids/parent/child
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# echo 2 > /sys/fs/cgroup/pids/parent/pids.max
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# echo $$ > /sys/fs/cgroup/pids/parent/cgroup.procs
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# cat /sys/fs/cgroup/pids/parent/pids.current
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2
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#
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It should be noted that attempts to overcome the set limit (2 in this case) will
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fail::
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# cat /sys/fs/cgroup/pids/parent/pids.current
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2
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# ( /bin/echo "Here's some processes for you." | cat )
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sh: fork: Resource temporary unavailable
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#
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Even if we migrate to a child cgroup (which doesn't have a set limit), we will
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not be able to overcome the most stringent limit in the hierarchy (in this case,
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parent's)::
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# echo $$ > /sys/fs/cgroup/pids/parent/child/cgroup.procs
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# cat /sys/fs/cgroup/pids/parent/pids.current
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2
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# cat /sys/fs/cgroup/pids/parent/child/pids.current
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2
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# cat /sys/fs/cgroup/pids/parent/child/pids.max
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max
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# ( /bin/echo "Here's some processes for you." | cat )
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sh: fork: Resource temporary unavailable
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#
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We can set a limit that is smaller than pids.current, which will stop any new
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processes from being forked at all (note that the shell itself counts towards
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pids.current)::
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# echo 1 > /sys/fs/cgroup/pids/parent/pids.max
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# /bin/echo "We can't even spawn a single process now."
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sh: fork: Resource temporary unavailable
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# echo 0 > /sys/fs/cgroup/pids/parent/pids.max
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# /bin/echo "We can't even spawn a single process now."
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sh: fork: Resource temporary unavailable
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#
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