memcg, oom: remove explicit wakeup in mem_cgroup_oom_synchronize()

Before commit 29ef680ae7c2 ("memcg, oom: move out_of_memory back to the
charge path"), all memcg oom killers were delayed to page fault path. And
the explicit wakeup is used in this case:

thread A:
...
if (locked) { // complete oom-kill, hold the lock
mem_cgroup_oom_unlock(memcg);
...
}
...

thread B:
...

if (locked && !memcg->oom_kill_disable) {
...
} else {
schedule(); // can't acquire the lock
...
}
...

The reason is that thread A kicks off the OOM-killer, which leads to
wakeups from the uncharges of the exiting task. But thread B is not
guaranteed to see them if it enters the OOM path after the OOM kills but
before thread A releases the lock.

Now only oom_kill_disable case is handled from the #PF path. In that case
it is userspace to trigger the wake up not the #PF path itself. All
potential paths to free some charges are responsible to call
memcg_oom_recover() , so the explicit wakeup is not needed in the
mem_cgroup_oom_synchronize() path which doesn't release any memory itself.

Link: https://lkml.kernel.org/r/[e-mail]
Signed-off-by: Haifeng Xu <[e-mail]>
Suggested-by: Michal Hocko <[e-mail]>
Acked-by: Michal Hocko <[e-mail]>
Cc: Johannes Weiner <[e-mail]>
Cc: Roman Gushchin <[e-mail]>
Cc: Shakeel Butt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

mm/memcontrol.c

···
 	mem_cgroup_unmark_under_oom(memcg);
 	finish_wait(&memcg_oom_waitq, &owait.wait);
 
-	if (locked) {
+	if (locked)
 		mem_cgroup_oom_unlock(memcg);
-		/*
-		 * There is no guarantee that an OOM-lock contender
-		 * sees the wakeups triggered by the OOM kill
-		 * uncharges.  Wake any sleepers explicitly.
-		 */
-		memcg_oom_recover(memcg);
-	}
 cleanup:
 	current->memcg_in_oom = NULL;
 	css_put(&memcg->css);