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>

1 files changed, 9 insertions, 3 deletions

diff --git a/kernel/exit.c b/kernel/exit.c
index 5ac3c19c245c..750c2e594617 100644
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -397,9 +397,15 @@ 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);
         /*