linux-kernel - Re: [RFC -v2] panic_on_oom

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Date:	Wed, 17 Jun 2015 21:31:21 +0900
From:	Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To:	mhocko@...e.cz, linux-mm@...ck.org
Cc:	rientjes@...gle.com, hannes@...xchg.org, tj@...nel.org,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC -v2] panic_on_oom_timeout

Michal Hocko wrote:
> Hi,
> I was thinking about this and I am more and more convinced that we
> shouldn't care about panic_on_oom=2 configuration for now and go with
> the simplest solution first. I have revisited my original patch and
> replaced delayed work by a timer based on the feedback from Tetsuo.
> 

To me, obsolating panic_on_oom > 0 sounds cleaner.

> I think we can rely on timers. A downside would be that we cannot dump
> the full OOM report from the IRQ context because we rely on task_lock
> which is not IRQ safe. But I do not think we really need it. An OOM
> report will be in the log already most of the time and show_mem will
> tell us the current memory situation.
> 
> What do you think?

We can rely on timers, but we can't rely on global timer.

> +	if (sysctl_panic_on_oom_timeout) {
> +		if (sysctl_panic_on_oom > 1) {
> +			pr_warn("panic_on_oom_timeout is ignored for panic_on_oom=2\n");
> +		} else {
> +			/*
> +			 * Only schedule the delayed panic_on_oom when this is
> +			 * the first OOM triggered. oom_lock will protect us
> +			 * from races
> +			 */
> +			if (atomic_read(&oom_victims))
> +				return;
> +
> +			mod_timer(&panic_on_oom_timer,
> +					jiffies + (sysctl_panic_on_oom_timeout * HZ));
> +			return;
> +		}
> +	}

Since this version uses global panic_on_oom_timer, you cannot handle
OOM race like below.

  (1) p1 in memcg1 calls out_of_memory().
  (2) 5 seconds of timeout is started by p1.
  (3) p1 takes 3 seconds for some reason.
  (4) p2 in memcg2 calls out_of_memory().
  (5) p1 calls unmark_oom_victim() but timer continues.
  (6) p2 takes 2 seconds for some reason.
  (7) 5 seconds of timeout expires despite individual delay was less than
      5 seconds.
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