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Currently, when pids.max limit is breached in the hierarchy, the event is counted and reported in the cgroup where the forking task resides. This decouples the limit and the notification caused by the limit making it hard to detect when the actual limit was effected. Redefine the pids.events:max as: the number of times the limit of the cgroup was hit. (Implementation differentiates also "forkfail" event but this is currently not exposed as it would better fit into pids.stat. It also differs from pids.events:max only when pids.max is configured on non-leaf cgroups.) Since it changes semantics of the original "max" event, introduce this change only in the v2 API of the controller and add a cgroup2 mount option to revert to the legacy behavior. Signed-off-by: Michal Koutný <mkoutny@suse.com> Signed-off-by: Tejun Heo <tj@kernel.org>
94 lines
3.2 KiB
ReStructuredText
94 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 in the cgroup because limit was hit in
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self or ancestors.
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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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