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author | Aleksa Sarai <cyphar@cyphar.com> | 2015-06-13 03:21:58 +1000 |
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committer | Tejun Heo <tj@kernel.org> | 2015-07-14 17:29:23 -0400 |
commit | 917d8e2d10f40e28aa9e0d824b2e5b8197d79fc2 (patch) | |
tree | 1dbdeecd86257608661fef6345b4cafddee650d8 /Documentation/cgroups | |
parent | 49b786ea146f69c371df18e81ce0a2d5839f865c (diff) | |
download | linux-stable-917d8e2d10f40e28aa9e0d824b2e5b8197d79fc2.tar.gz linux-stable-917d8e2d10f40e28aa9e0d824b2e5b8197d79fc2.tar.bz2 linux-stable-917d8e2d10f40e28aa9e0d824b2e5b8197d79fc2.zip |
cgroup: add documentation for the PIDs controller
Add documentation derived from kernel/cgroup_pids.c to the relevant
Documentation/ directory, along with a few examples of how to use the
PIDs controller as well an explanation of its peculiarities.
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Diffstat (limited to 'Documentation/cgroups')
-rw-r--r-- | Documentation/cgroups/00-INDEX | 2 | ||||
-rw-r--r-- | Documentation/cgroups/pids.txt | 85 |
2 files changed, 87 insertions, 0 deletions
diff --git a/Documentation/cgroups/00-INDEX b/Documentation/cgroups/00-INDEX index 96ce071a3633..3f5a40f57d4a 100644 --- a/Documentation/cgroups/00-INDEX +++ b/Documentation/cgroups/00-INDEX @@ -22,6 +22,8 @@ net_cls.txt - Network classifier cgroups details and usages. net_prio.txt - Network priority cgroups details and usages. +pids.txt + - Process number cgroups details and usages. resource_counter.txt - Resource Counter API. unified-hierarchy.txt diff --git a/Documentation/cgroups/pids.txt b/Documentation/cgroups/pids.txt new file mode 100644 index 000000000000..1a078b5d281a --- /dev/null +++ b/Documentation/cgroups/pids.txt @@ -0,0 +1,85 @@ + 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. + +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 +# |