[PATCH RESEND 2/3] memcg: optimize the "Search everything else" loop in mm_update_next_owner()

[Oleg Nesterov <oleg@redhat.com>]

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>
---
 kernel/exit.c |   12 +++++++++---
 1 files changed, 9 insertions(+), 3 deletions(-)

diff --git a/kernel/exit.c b/kernel/exit.c
index e270d2a..429659c 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -358,9 +358,15 @@ retry:
 	/*
 	 * 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);
 	/*
-- 
1.5.5.1