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Message-Id: <1434129718-8823-1-git-send-email-cyphar@cyphar.com> Date: Sat, 13 Jun 2015 03:21:58 +1000 From: Aleksa Sarai <cyphar@...har.com> To: tj@...nel.org, lizefan@...wei.com, mingo@...hat.com, peterz@...radead.org Cc: richard@....at, fweisbec@...il.com, linux-kernel@...r.kernel.org, cgroups@...r.kernel.org, Aleksa Sarai <cyphar@...har.com> Subject: [PATCH] cgroup: add documentation for the PIDs controller The attached patch adds documentation concerning the PIDs controller. This should be applied alongside the rest of this patchset[1]. [1]: https://lkml.org/lkml/2015/6/9/320 8<----------------------------------------------------------------------------- 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@...har.com> --- Documentation/cgroups/00-INDEX | 2 + Documentation/cgroups/pids.txt | 85 ++++++++++++++++++++++++++++++++++++++++++ 2 files changed, 87 insertions(+) create mode 100644 Documentation/cgroups/pids.txt diff --git a/Documentation/cgroups/00-INDEX b/Documentation/cgroups/00-INDEX index 96ce071..3f5a40f 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 0000000..1a078b5 --- /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 +# -- 2.4.2 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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