3 files changed, 159 insertions, 8 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
+#
diff --git a/Documentation/cgroups/unified-hierarchy.txt b/Documentation/cgroups/unified-hierarchy.txt
index 86847a7647ab..1ee9caf29e57 100644
--- a/Documentation/cgroups/unified-hierarchy.txt
+++ b/Documentation/cgroups/unified-hierarchy.txt
@@ -23,10 +23,13 @@ CONTENTS
 5. Other Changes
   5-1. [Un]populated Notification
   5-2. Other Core Changes
-  5-3. Per-Controller Changes
-    5-3-1. blkio
-    5-3-2. cpuset
-    5-3-3. memory
+  5-3. Controller File Conventions
+    5-3-1. Format
+    5-3-2. Control Knobs
+  5-4. Per-Controller Changes
+    5-4-1. blkio
+    5-4-2. cpuset
+    5-4-3. memory
 6. Planned Changes
   6-1. CAP for resource control
 
@@ -372,14 +375,75 @@ supported and the interface files "release_agent" and
 - The "cgroup.clone_children" file is removed.
 
 
-5-3. Per-Controller Changes
+5-3. Controller File Conventions
 
-5-3-1. blkio
+5-3-1. Format
+
+In general, all controller files should be in one of the following
+formats whenever possible.
+
+- Values only files
+
+  VAL0 VAL1...\n
+
+- Flat keyed files
+
+  KEY0 VAL0\n
+  KEY1 VAL1\n
+  ...
+
+- Nested keyed files
+
+  KEY0 SUB_KEY0=VAL00 SUB_KEY1=VAL01...
+  KEY1 SUB_KEY0=VAL10 SUB_KEY1=VAL11...
+  ...
+
+For a writeable file, the format for writing should generally match
+reading; however, controllers may allow omitting later fields or
+implement restricted shortcuts for most common use cases.
+
+For both flat and nested keyed files, only the values for a single key
+can be written at a time.  For nested keyed files, the sub key pairs
+may be specified in any order and not all pairs have to be specified.
+
+
+5-3-2. Control Knobs
+
+- Settings for a single feature should generally be implemented in a
+  single file.
+
+- In general, the root cgroup should be exempt from resource control
+  and thus shouldn't have resource control knobs.
+
+- If a controller implements ratio based resource distribution, the
+  control knob should be named "weight" and have the range [1, 10000]
+  and 100 should be the default value.  The values are chosen to allow
+  enough and symmetric bias in both directions while keeping it
+  intuitive (the default is 100%).
+
+- If a controller implements an absolute resource guarantee and/or
+  limit, the control knobs should be named "min" and "max"
+  respectively.  If a controller implements best effort resource
+  gurantee and/or limit, the control knobs should be named "low" and
+  "high" respectively.
+
+  In the above four control files, the special token "max" should be
+  used to represent upward infinity for both reading and writing.
+
+- If a setting has configurable default value and specific overrides,
+  the default settings should be keyed with "default" and appear as
+  the first entry in the file.  Specific entries can use "default" as
+  its value to indicate inheritance of the default value.
+
+
+5-4. Per-Controller Changes
+
+5-4-1. blkio
 
 - blk-throttle becomes properly hierarchical.
 
 
-5-3-2. cpuset
+5-4-2. cpuset
 
 - Tasks are kept in empty cpusets after hotplug and take on the masks
   of the nearest non-empty ancestor, instead of being moved to it.
@@ -388,7 +452,7 @@ supported and the interface files "release_agent" and
   masks of the nearest non-empty ancestor.
 
 
-5-3-3. memory
+5-4-3. memory
 
 - use_hierarchy is on by default and the cgroup file for the flag is
   not created.