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

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