On Mon, Oct 15, 2018 at 04:29:35PM -0400, Waiman Long wrote: > 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. What does that mean? The parent cgroup doesn't 'set' anything at all. The user will. > + > + 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. Are those the only conditions under which that -1 can happen? Parent taking away CPUs it previously granted and hotplug? > + > + "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. Effectively or actual? Actual migrating tasks out of the cgroup is irreversible. > + 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. So you need to clarify the above point (I think it is effectively), because otherwise you don't know which tasks to put back.
