cgroup/cpuset: Don't invalidate sibling partitions on cpuset.cpus conflict

cgroup/cpuset: Don't invalidate sibling partitions on cpuset.cpus conflict

Currently, when setting a cpuset's cpuset.cpus to a value that conflicts
with the cpuset.cpus/cpuset.cpus.exclusive of a sibling partition,
the sibling's partition state becomes invalid. This is overly harsh and
is probably not necessary.

The cpuset.cpus.exclusive control file, if set, will override the
cpuset.cpus of the same cpuset when creating a cpuset partition.
So cpuset.cpus has less priority than cpuset.cpus.exclusive in setting up
a partition.  However, it cannot override a conflicting cpuset.cpus file
in a sibling cpuset and the partition creation process will fail. This
is inconsistent.  That will also make using cpuset.cpus.exclusive less
valuable as a tool to set up cpuset partitions as the users have to
check if such a cpuset.cpus conflict exists or not.

Fix these problems by making sure that once a cpuset.cpus.exclusive
is set without failure, it will always be allowed to form a valid
partition as long as at least one CPU can be granted from its parent
irrespective of the state of the siblings' cpuset.cpus values. Of
course, setting cpuset.cpus.exclusive will fail if it conflicts with
the cpuset.cpus.exclusive or the cpuset.cpus.exclusive.effective value
of a sibling.

Partition can still be created by setting only cpuset.cpus without
setting cpuset.cpus.exclusive. However, any conflicting CPUs in sibling's
cpuset.cpus.exclusive.effective and cpuset.cpus.exclusive values will
be removed from its cpuset.cpus.exclusive.effective as long as there
is still one or more CPUs left and can be granted from its parent. This
CPU stripping is currently done in rm_siblings_excl_cpus().

The new code will now try its best to enable the creation of new
partitions with only cpuset.cpus set without invalidating existing ones.
However it is not guaranteed that all the CPUs requested in cpuset.cpus
will be used in the new partition even when all these CPUs can be
granted from the parent.

This is similar to the fact that cpuset.cpus.effective may not be
able to include all the CPUs requested in cpuset.cpus. In this case,
the parent may not able to grant all the exclusive CPUs requested in
cpuset.cpus to cpuset.cpus.exclusive.effective if some of them have
already been granted to other partitions earlier.

With the creation of multiple sibling partitions by setting
only cpuset.cpus, this does have the side effect that their exact
cpuset.cpus.exclusive.effective settings will depend on the order of
partition creation if there are conflicts. Due to the exclusive nature
of the CPUs in a partition, it is not easy to make it fair other than
the old behavior of invalidating all the conflicting partitions.

For example,
  # echo "0-2" > A1/cpuset.cpus
  # echo "root" > A1/cpuset.cpus.partition
  # cat A1/cpuset.cpus.partition
  root
  # cat A1/cpuset.cpus.exclusive.effective
  0-2
  # echo "2-4" > B1/cpuset.cpus
  # echo "root" > B1/cpuset.cpus.partition
  # cat B1/cpuset.cpus.partition
  root
  # cat B1/cpuset.cpus.exclusive.effective
  3-4
  # cat B1/cpuset.cpus.effective
  3-4

For users who want to be sure that they can get most of the CPUs they
want, cpuset.cpus.exclusive should be used instead if they can set
it successfully without failure. Setting cpuset.cpus.exclusive will
guarantee that sibling conflicts from then onward is no longer possible.

To make this change, we have to separate out the is_cpu_exclusive()
check in cpus_excl_conflict() into a cgroup v1 only
cpuset1_cpus_excl_conflict() helper. The cpus_allowed_validate_change()
helper is now no longer needed and can be removed.

Some existing tests in test_cpuset_prs.sh are updated and new ones are
added to reflect the new behavior. The cgroup-v2.rst doc file is also
updated the clarify what exclusive CPUs will be used when a partition
is created.

Reported-by: Sun Shaojie <sunshaojie@kylinos.cn>
Closes: https://lore.kernel.org/lkml/20251117015708.977585-1-sunshaojie@kylinos.cn/
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>