linux-kernel - Re: [PATCH 5/5] mm, oom_reaper: implement OOM victims queuing

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Date:	Tue, 16 Feb 2016 16:53:22 +0100
From:	Michal Hocko <mhocko@...nel.org>
To:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:	akpm@...ux-foundation.org, rientjes@...gle.com, mgorman@...e.de,
	oleg@...hat.com, torvalds@...ux-foundation.org, hughd@...gle.com,
	andrea@...nel.org, riel@...hat.com, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 5/5] mm, oom_reaper: implement OOM victims queuing

On Tue 16-02-16 20:11:24, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > Unless we are under global OOM then this doesn't matter much because the
> > allocation request should succeed at some point in time and memcg
> > charges are bypassed for tasks with pending fatal signals. So we can
> > make a forward progress.
> 
> Hmm, then I wonder how memcg OOM livelock occurs. Anyway, OK for now.
> 
> But current OOM reaper forgot a protection for list item "double add" bug.
> Precisely speaking, this is not a OOM reaper's bug.
[...]
> For oom_kill_allocating_task = 1 case (despite the name, it still tries to kill
> children first),

Yes this is a long term behavior and I cannot say I would be happy about
that because it clearly breaks the defined semantic.

> the OOM killer does not wait for OOM victim to clear TIF_MEMDIE
> because select_bad_process() is not called. Therefore, if an OOM victim fails to
> terminate because the OOM reaper failed to reap enough memory, the kernel is
> flooded with OOM killer messages trying to kill that stuck victim (with OOM
> reaper lockup due to list corruption).

Hmmm, I didn't consider this possibility. For now I would simply disable
oom_reaper for sysctl_oom_kill_allocating_task. oom_kill_allocating_task
needs some more changes IMO. a) we shouldn't kill children as a
heuristic b) we should go and panic if the current task is TIF_MEMDIE
already because that means that we cannot do anything about OOM. But I
think this should be handled separately.

Whould be the following acceptable for now?
---
diff --git a/mm/oom_kill.c b/mm/oom_kill.c
index 7e9953a64489..357cee067950 100644
--- a/mm/oom_kill.c
+++ b/mm/oom_kill.c
@@ -678,7 +678,14 @@ void oom_kill_process(struct oom_control *oc, struct task_struct *p,
 	unsigned int victim_points = 0;
 	static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
 					      DEFAULT_RATELIMIT_BURST);
-	bool can_oom_reap = true;
+	bool can_oom_reap;
+	
+	/*
+	 * oom_kill_allocating_task doesn't follow normal OOM exclusion
+	 * and so the same task might enter oom_kill_process which oom_reaper
+	 * cannot handle currently.
+	 */
+	can_oom_reap = !sysctl_oom_kill_allocating_task;
 
 	/*
 	 * If the task is already exiting, don't alarm the sysadmin or kill
-- 
Michal Hocko
SUSE Labs