[PATCH 3/2] memcg: Facilitate termination of memcg OOM victims.

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>

If memcg OOM events in different domains are pending, already OOM-killed
threads needlessly wait for pending memcg OOM events in different domains.
An out_of_memory() call is slow because it involves printk(). With slow
serial consoles, out_of_memory() might take more than a second. Therefore,
allowing killed processes to quickly call mmput() from exit_mm() from
do_exit() will help calling __mmput() (which can reclaim more memory than
the OOM reaper can reclaim) quickly.

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
---
 mm/memcontrol.c | 17 +++++++++++------
 1 file changed, 11 insertions(+), 6 deletions(-)

diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 90eb2e2..a7d3ba9 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -1389,14 +1389,19 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
 	};
 	bool ret = true;
 
-	mutex_lock(&oom_lock);
-
 	/*
-	 * multi-threaded tasks might race with oom_reaper and gain
-	 * MMF_OOM_SKIP before reaching out_of_memory which can lead
-	 * to out_of_memory failure if the task is the last one in
-	 * memcg which would be a false possitive failure reported
+	 * Multi-threaded tasks might race with oom_reaper() and gain
+	 * MMF_OOM_SKIP before reaching out_of_memory(). But if current
+	 * thread was already killed or is ready to terminate, there is
+	 * no need to call out_of_memory() nor wait for oom_reaoer() to
+	 * set MMF_OOM_SKIP. These three checks minimize possibility of
+	 * needlessly calling out_of_memory() and try to call exit_mm()
+	 * as soon as possible.
 	 */
+	if (mutex_lock_killable(&oom_lock))
+		return true;
+	if (fatal_signal_pending(current))
+		goto unlock;
 	if (tsk_is_oom_victim(current))
 		goto unlock;
 
-- 
1.8.3.1