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
#