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@xxxxxxxxxx> --- 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 533e85cb851b..178cda473a26 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 ----------------- -- 2.18.0
