selftests/cgroup: Add test for cpuset affinity on controller disable

Add a new selftest that exposes a bug in cpuset_attach() where thread
CPU affinity is not properly updated when the cpuset controller is
disabled in a threaded cgroup hierarchy.

The test creates a threaded cgroup hierarchy with two child cgroups
(A and B) having different cpuset.cpus constraints:
- Parent: cpuset.cpus=0-1
- Child A: cpuset.cpus=0-1
- Child B: cpuset.cpus=1 (restricted to CPU 1 only)

A multithreaded process is created with threads placed in different
cgroups. When the cpuset controller is disabled on the parent, thread
affinities should be updated to match the parent's cpuset.

Expected behavior:
- thread_a affinity: {0-1} before and after (unchanged)
- thread_b affinity: {1} before, {0-1} after (expanded)

Current buggy behavior:
- thread_b affinity remains {1} after controller disable

Assisted-by: Claude:claude-sonnet-4-5
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Acked-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

0 files changed, 0 insertions, 0 deletions