linux-kernel - Re: [RFC PATCH 2/2] memcg: do not report racy no-eligible OOM tasks

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Message-ID: <20181107100810.GA27423@dhcp22.suse.cz>
Date:   Wed, 7 Nov 2018 11:08:10 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     linux-mm@...ck.org, David Rientjes <rientjes@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH 2/2] memcg: do not report racy no-eligible OOM tasks

On Wed 07-11-18 18:45:27, Tetsuo Handa wrote:
> On 2018/11/06 21:42, Michal Hocko wrote:
> > On Tue 06-11-18 18:44:43, Tetsuo Handa wrote:
> > [...]
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index 6e1469b..a97648a 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -1382,8 +1382,13 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >>  	};
> >>  	bool ret;
> >>  
> >> -	mutex_lock(&oom_lock);
> >> -	ret = out_of_memory(&oc);
> >> +	if (mutex_lock_killable(&oom_lock))
> >> +		return true;
> >> +	/*
> >> +	 * A few threads which were not waiting at mutex_lock_killable() can
> >> +	 * fail to bail out. Therefore, check again after holding oom_lock.
> >> +	 */
> >> +	ret = fatal_signal_pending(current) || out_of_memory(&oc);
> >>  	mutex_unlock(&oom_lock);
> >>  	return ret;
> >>  }
> > 
> > If we are goging with a memcg specific thingy then I really prefer
> > tsk_is_oom_victim approach. Or is there any reason why this is not
> > suitable?
> > 
> 
> Why need to wait for mark_oom_victim() called after slow printk() messages?
> 
> If current thread got Ctrl-C and thus current thread can terminate, what is
> nice with waiting for the OOM killer? If there are several OOM events in
> multiple memcg domains waiting for completion of printk() messages? I don't
> see points with waiting for oom_lock, for try_charge() already allows current
> thread to terminate due to fatal_signal_pending() test.

mutex_lock_killable would take care of exiting task already. I would
then still prefer to check for mark_oom_victim because that is not racy
with the exit path clearing signals. I can update my patch to use
_killable lock variant if we are really going with the memcg specific
fix.

Johaness?

-- 
Michal Hocko
SUSE Labs