memcg: optimize the "Search everything else" loop in mm_update_next_owner()

for_each_process_thread() is sub-optimal. All threads share the same
->mm, we can swicth to the next process once we found a thread with
->mm != NULL and ->mm != mm.
Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Cc: Balbir Singh <bsingharora@gmail.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Michal Hocko <mhocko@suse.cz>
Cc: Peter Chiang <pchiang@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/exit.c

@@ -397,9 +397,15 @@ void mm_update_next_owner(struct mm_struct *mm)
 	/*
 	 * Search through everything else, we should not get here often.
 	 */
-	for_each_process_thread(g, c) {
-		if (!(c->flags & PF_KTHREAD) && c->mm == mm)
-			goto assign_new_owner;
+	for_each_process(g) {
+		if (g->flags & PF_KTHREAD)
+			continue;
+		for_each_thread(g, c) {
+			if (c->mm == mm)
+				goto assign_new_owner;
+			if (c->mm)
+				break;
+		}
 	}
 	read_unlock(&tasklist_lock);
 	/*