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Subject[PATCH v15 11/12] cpuset: Add documentation about the new "cpuset.sched.partition" flag
Date
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 | 73 +++++++++++++++++++++++++++++++++
1 file changed, 73 insertions(+)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index 595b075..f83a523 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -1708,6 +1708,79 @@ 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. This flag is owned by the parent cgroup
+ and is not delegatable.
+
+ It accepts only the following input values when written to.
+
+ "root" or "1" - a paritition root
+ "member" or "0" - a non-root member of a partition
+
+ When set to be a partition root, 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 a partition root 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 it to partition root will take the CPUs away from the
+ effective CPUs of the parent cgroup. Once it is set, this
+ file cannot be reverted back to "member" 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.
+
+ "member" Non-root member of a partition
+ "root" Partition root
+ "root invalid" Invalid 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 invalid 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 itself. In this
+ case, it is not a real partition even though the restriction
+ of the first partition root condition above will still apply.
+ The cpu affinity of all the tasks in the cgroup will then be
+ associated with CPUs in the nearest ancestor partition.
+
+ An invalid 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 cpu
+ affinity of all the tasks in the formerly invalid partition
+ will be associated to the CPUs of the newly formed partition.
+ Changing the partition state of an invalid partition root to
+ "member" is always allowed even if child cpusets are present.
+

Device controller
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1.8.3.1
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