[PATCH v13 10/11] cpuset: Add documentation about the new "cpuset.sched.partition" flag

[Waiman Long <longman@redhat.com>]

The cgroup-v2.rst file is updated to document the purpose of the new
"cpuset.sched.partition" flag and how its usage.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 Documentation/admin-guide/cgroup-v2.rst | 66 +++++++++++++++++++++++++++++++++
 1 file changed, 66 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 533e85c..178cda4 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1686,6 +1686,72 @@ Cpuset Interface Files
 
 	Its value will be affected by memory nodes hotplug events.
 
+  cpuset.sched.partition
+	A read-write single value file which exists on non-root
+	cpuset-enabled cgroups.  It accepts either "0" (off) or "1"
+	(on) when written to.  This flag is set and owned by the
+	parent cgroup.
+
+	If set, it indicates that the current cgroup is the root of a
+	new partition or scheduling domain that comprises itself and
+	all its descendants except those that are separate partition
+	roots themselves and their descendants.  The root cgroup is
+	always a partition root.
+
+	There are constraints on where this flag can be set.  It can
+	only be set in a cgroup if all the following conditions are true.
+
+	1) The "cpuset.cpus" is not empty and the list of CPUs are
+	   exclusive, i.e. they are not shared by any of its siblings.
+	2) The parent cgroup is a partition root.
+	3) The "cpuset.cpus" is also a proper subset of the parent's
+	   "cpuset.cpus.effective".
+	4) There is no child cgroups with cpuset enabled.  This is for
+	   eliminating corner cases that have to be handled if such a
+	   condition is allowed.
+
+	Setting this flag will take the CPUs away from the effective
+	CPUs of the parent cgroup.  Once it is set, this flag cannot
+	be cleared if there are any child cgroups with cpuset enabled.
+
+	A parent partition cannot distribute all its CPUs to its
+	child partitions.  There must be at least one cpu left in the
+	parent partition.
+
+	Once becoming a partition root, changes to "cpuset.cpus" is
+	generally allowed as long as the first condition above is true,
+	the change will not take away all the CPUs from the parent
+	partition and the new "cpuset.cpus" value is a superset of its
+	children's "cpuset.cpus" values.
+
+	Sometimes, external factors like changes to ancestors'
+	"cpuset.cpus" or cpu hotplug can cause the state of the partition
+	root to change.  On read, the "cpuset.sched.partition" file
+	can show the following values.
+
+	"0"  Not a partition root
+	"1"  Partition root
+	"-1" Erroneous partition root
+
+	It is a partition root if the first 2 partition root conditions
+	above are true and at least one CPU from "cpuset.cpus" is
+	granted by the parent cgroup.
+
+	A partition root can become an erroneous partition root if none
+	of CPUs requested in "cpuset.cpus" can be granted by the parent
+	cgroup or the parent cgroup is no longer a partition root.
+	In this case, it is not a real partition even though the
+	restriction of the first partition root condition above will
+	still apply.  All the tasks in the cgroup will be migrated to
+	the nearest ancestor partition.
+
+	An erroneous partition root can be transitioned back to a real
+	partition root if at least one of the requested CPUs can now be
+	granted by its parent.	In this case, the tasks will be migrated
+	back to the newly created partition.  Clearing the partition
+	flag of an erroneous partition root is always allowed even if
+	child cpusets are present.
+
 
 Device controller
 -----------------
-- 
1.8.3.1