Merge branch 'for-5.3' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup

Pull cgroup updates from Tejun Heo:
 "Documentation updates and the addition of cgroup_parse_float() which
  will be used by new controllers including blk-iocost"

* 'for-5.3' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
  docs: cgroup-v1: convert docs to ReST and rename to *.rst
  cgroup: Move cgroup_parse_float() implementation out of CONFIG_SYSFS
  cgroup: add cgroup_parse_float()

1 files changed, 92 insertions, 0 deletions

diff --git a/Documentation/cgroup-v1/pids.rst b/Documentation/cgroup-v1/pids.rst
new file mode 100644
index 000000000000..6acebd9e72c8
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+++ b/Documentation/cgroup-v1/pids.rst
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+=========================
+Process Number Controller
+=========================
+
+Abstract
+--------
+
+The process number controller is used to allow a cgroup hierarchy to stop any
+new tasks from being fork()'d or clone()'d after a certain limit is reached.
+
+Since it is trivial to hit the task limit without hitting any kmemcg limits in
+place, PIDs are a fundamental resource. As such, PID exhaustion must be
+preventable in the scope of a cgroup hierarchy by allowing resource limiting of
+the number of tasks in a cgroup.
+
+Usage
+-----
+
+In order to use the `pids` controller, set the maximum number of tasks in
+pids.max (this is not available in the root cgroup for obvious reasons). The
+number of processes currently in the cgroup is given by pids.current.
+
+Organisational operations are not blocked by cgroup policies, so it is possible
+to have pids.current > pids.max. This can be done by either setting the limit to
+be smaller than pids.current, or attaching enough processes to the cgroup such
+that pids.current > pids.max. However, it is not possible to violate a cgroup
+policy through fork() or clone(). fork() and clone() will return -EAGAIN if the
+creation of a new process would cause a cgroup policy to be violated.
+
+To set a cgroup to have no limit, set pids.max to "max". This is the default for
+all new cgroups (N.B. that PID limits are hierarchical, so the most stringent
+limit in the hierarchy is followed).
+
+pids.current tracks all child cgroup hierarchies, so parent/pids.current is a
+superset of parent/child/pids.current.
+
+The pids.events file contains event counters:
+
+  - max: Number of times fork failed because limit was hit.
+
+Example
+-------
+
+First, we mount the pids controller::
+
+	# mkdir -p /sys/fs/cgroup/pids
+	# mount -t cgroup -o pids none /sys/fs/cgroup/pids
+
+Then we create a hierarchy, set limits and attach processes to it::
+
+	# mkdir -p /sys/fs/cgroup/pids/parent/child
+	# echo 2 > /sys/fs/cgroup/pids/parent/pids.max
+	# echo $$ > /sys/fs/cgroup/pids/parent/cgroup.procs
+	# cat /sys/fs/cgroup/pids/parent/pids.current
+	2
+	#
+
+It should be noted that attempts to overcome the set limit (2 in this case) will
+fail::
+
+	# cat /sys/fs/cgroup/pids/parent/pids.current
+	2
+	# ( /bin/echo "Here's some processes for you." | cat )
+	sh: fork: Resource temporary unavailable
+	#
+
+Even if we migrate to a child cgroup (which doesn't have a set limit), we will
+not be able to overcome the most stringent limit in the hierarchy (in this case,
+parent's)::
+
+	# echo $$ > /sys/fs/cgroup/pids/parent/child/cgroup.procs
+	# cat /sys/fs/cgroup/pids/parent/pids.current
+	2
+	# cat /sys/fs/cgroup/pids/parent/child/pids.current
+	2
+	# cat /sys/fs/cgroup/pids/parent/child/pids.max
+	max
+	# ( /bin/echo "Here's some processes for you." | cat )
+	sh: fork: Resource temporary unavailable
+	#
+
+We can set a limit that is smaller than pids.current, which will stop any new
+processes from being forked at all (note that the shell itself counts towards
+pids.current)::
+
+	# echo 1 > /sys/fs/cgroup/pids/parent/pids.max
+	# /bin/echo "We can't even spawn a single process now."
+	sh: fork: Resource temporary unavailable
+	# echo 0 > /sys/fs/cgroup/pids/parent/pids.max
+	# /bin/echo "We can't even spawn a single process now."
+	sh: fork: Resource temporary unavailable
+	#