cgroup: Optimize single thread migration

There are reports of users who use thread migrations between cgroups and
they report performance drop after d59cfc09c32a ("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>

1 files changed, 3 insertions, 2 deletions

diff --git a/kernel/cgroup/cgroup-internal.h b/kernel/cgroup/cgroup-internal.h
index 809e34a3c017..90d1710fef6c 100644
--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -231,9 +231,10 @@ int cgroup_migrate(struct task_struct *leader, bool threadgroup,
 
 int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 		       bool threadgroup);
-struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup)
+struct task_struct *cgroup_procs_write_start(char *buf, bool threadgroup,
+					     bool *locked)
 	__acquires(&cgroup_threadgroup_rwsem);
-void cgroup_procs_write_finish(struct task_struct *task)
+void cgroup_procs_write_finish(struct task_struct *task, bool locked)
 	__releases(&cgroup_threadgroup_rwsem);
 
 void cgroup_lock_and_drain_offline(struct cgroup *cgrp);