[PATCH v2 5/6] cgroup/cpuset: Update description of cpuset.cpus.partition in cgroup-v2.rst

Waiman Long
2021-06-21T18:49:23Z

Update Documentation/admin-guide/cgroup-v2.rst on the newly introduced
"isolated" cpuset partition type as well as the ability to create
non-top cpuset partition with no cpu allocated to it.

Signed-off-by: Waiman Long <longman-H+wXaHxf7aLQT0dZR+AlfA@public.gmane.org>
---
Documentation/admin-guide/cgroup-v2.rst | 65 +++++++++++++++++--------
1 file changed, 44 insertions(+), 21 deletions(-)

diff --git a/Documentation/admin-guide/cgroup-v2.rst b/Documentation/admin-guide/cgroup-v2.rst
index b1e81aa8598a..cf40a7f499c0 100644
--- a/Documentation/admin-guide/cgroup-v2.rst
+++ b/Documentation/admin-guide/cgroup-v2.rst
@@ -2010,8 +2010,9 @@ Cpuset Interface Files
It accepts only the following input values when written to.

======== ================================
- "root" a partition root
- "member" a non-root member of a partition
+ "member" Non-root member of a partition
+ "root" Partition root
+ "isolated" Partition root without load balancing
======== ================================

When set to be a partition root, the current cgroup is the
@@ -2020,6 +2021,11 @@ Cpuset Interface Files
partition roots themselves and their descendants. The root
cgroup is always a partition root.

+ With "isolated", the CPUs in that partition root will be in an
+ isolated state without any load balancing from the scheduler.
+ Tasks in such a partition must be explicitly bind to each
+ individual CPU.
+
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.
@@ -2038,12 +2044,25 @@ Cpuset Interface Files
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.
+ A parent partition may distribute all its CPUs to its child
+ partitions as long as it is not the root cgroup and there is no
+ task directly associated with that parent partition. Otherwise,
+ there must be at least one cpu left in the parent partition.
+ A new task cannot be moved to a partition root with no effective
+ cpu.

Once becoming a partition root, changes to "cpuset.cpus" is
- generally allowed as long as the first condition above is true,
+ generally allowed as long as the first condition above is true.
+ Other constraints for this operation are as follows.
+
+ 1) Any newly added CPUs must be a subset of the parent's
+ "cpuset.cpus.effective".
+ 2) Taking away all the CPUs from the parent's "cpuset.cpus.effective"
+ is only allowed if there is no task associated with the
+ parent partition.
+ 3) Deletion of CPUs that have been distributed to child partition
+ roots are not allowed.
+
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.
@@ -2056,6 +2075,7 @@ Cpuset Interface Files
============== ==============================
"member" Non-root member of a partition
"root" Partition root
+ "isolated" Partition root without load balancing
"root invalid" Invalid partition root
============== ==============================

@@ -2063,21 +2083,24 @@ Cpuset Interface Files
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.
+ A partition root becomes invalid if all the CPUs requested in
+ "cpuset.cpus" become unavailable. This can happen if all the
+ CPUs have been offlined, or the state of an ancestor partition
+ root become invalid. 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. In the special case of a parent partition
+ competing with a child partition for the only CPU left, the
+ parent partition wins and the child partition becomes invalid.
+
+ An invalid partition root can be transitioned back to a real
+ partition root if at least one of the requested CPUs become
+ available again. 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
--
2.18.1