As reported by sashiko [1], cpuset_update_tasks_nodemask() will do
mpol_rebind_mm() and possibly cpuset_migrate_mm() for all threads of
a multithreaded process. Since commit 3df9ca0a2b ("cpuset: migrate
memory only for threadgroup leaders"), cpuset_attach() had been updated
to rebind and migrate memory only for threadgroup leaders to mark the
group leader as the owner of the mm_struct.
To be consistent and avoid unnecessary performance overhead for heavily
multithreaded processes, follow the cpuset_attach() example and perform
memory rebind and migration only for threadgroup leaders.
Also add a paragraph in cgroup-v2.rst under cpuset.mems that the
threadgroup leader is the memory owner of that threadgroup. Therefore
the non-leading threads shouldn't be in other cgroups whose "cpuset.mems"
doesn't fully overlap that of the group leader.
[1] https://sashiko.dev/#/patchset/20260621032816.1806773-1-longman%40redhat.com
Signed-off-by: Waiman Long <longman@redhat.com>
Reviewed-by: Ridong Chen <ridong.chen@linux.dev>
Signed-off-by: Tejun Heo <tj@kernel.org>
eda17a3a70