There are reports of users who use thread migrations between cgroups and
they report performance drop after d59cfc09 ("sched, cgroup: replace
signal_struct->group_rwsem with a global percpu_rwsem"). The effect is
pronounced on machines with more CPUs.

The migration is affected by forking noise happening in the background,
after the mentioned commit a migrating thread must wait for all
(forking) processes on the system, not only of its threadgroup.

There are several places that need to synchronize with migration:
	a) do_exit,
	b) de_thread,
	c) copy_process,
	d) cgroup_update_dfl_csses,
	e) parallel migration (cgroup_{proc,thread}s_write).

In the case of self-migrating thread, we relax the synchronization on
cgroup_threadgroup_rwsem to avoid the cost of waiting. d) and e) are
excluded with cgroup_mutex, c) does not matter in case of single thread
migration and the executing thread cannot exec(2) or exit(2) while it is
writing into cgroup.threads. In case of do_exit because of signal
delivery, we either exit before the migration or finish the migration
(of not yet PF_EXITING thread) and die afterwards.

This patch handles only the case of self-migration by writing "0" into
cgroup.threads. For simplicity, we always take cgroup_threadgroup_rwsem
with numeric PIDs.

This change improves migration dependent workload performance similar
to per-signal_struct state.
Signed-off-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>