linux-kernel - Re: [PATCH 2/2] memcg, oom: emit oom report when there is no eligible task

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Date:   Tue, 21 Aug 2018 16:06:12 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Greg Thelen <gthelen@...gle.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Dmitry Vyukov <dvyukov@...gle.com>, linux-mm@...ck.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] memcg, oom: emit oom report when there is no
 eligible task

Do you plan to repost these two? They are quite deep in the email thread
so they can easily fall through cracks.

On Wed 08-08-18 18:17:37, Michal Hocko wrote:
> On Wed 08-08-18 10:45:15, Johannes Weiner wrote:
[...]
> > >From bba01122f739b05a689dbf1eeeb4f0e07affd4e7 Mon Sep 17 00:00:00 2001
> > From: Johannes Weiner <hannes@...xchg.org>
> > Date: Wed, 8 Aug 2018 09:59:40 -0400
> > Subject: [PATCH] mm: memcontrol: print proper OOM header when no eligible
> >  victim left
> > 
> > When the memcg OOM killer runs out of killable tasks, it currently
> > prints a WARN with no further OOM context. This has caused some user
> > confusion.
> > 
> > Warnings indicate a kernel problem. In a reported case, however, the
> > situation was triggered by a non-sensical memcg configuration (hard
> > limit set to 0). But without any VM context this wasn't obvious from
> > the report, and it took some back and forth on the mailing list to
> > identify what is actually a trivial issue.
> > 
> > Handle this OOM condition like we handle it in the global OOM killer:
> > dump the full OOM context and tell the user we ran out of tasks.
> > 
> > This way the user can identify misconfigurations easily by themselves
> > and rectify the problem - without having to go through the hassle of
> > running into an obscure but unsettling warning, finding the
> > appropriate kernel mailing list and waiting for a kernel developer to
> > remote-analyze that the memcg configuration caused this.
> > 
> > If users cannot make sense of why the OOM killer was triggered or why
> > it failed, they will still report it to the mailing list, we know that
> > from experience. So in case there is an actual kernel bug causing
> > this, kernel developers will very likely hear about it.
> > 
> > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> 
> Yes this works as well. We would get a dump even for the race we have
> seen but I do not think this is something to lose sleep over. And if it
> triggers too often to be disturbing we can add
> tsk_is_oom_victim(current) check there.
> 
> Acked-by: Michal Hocko <mhocko@...e.com>
> 
> > ---
> >  mm/memcontrol.c |  2 --
> >  mm/oom_kill.c   | 13 ++++++++++---
> >  2 files changed, 10 insertions(+), 5 deletions(-)
> > 
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 4e3c1315b1de..29d9d1a69b36 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -1701,8 +1701,6 @@ static enum oom_status mem_cgroup_oom(struct mem_cgroup *memcg, gfp_t mask, int
> >  	if (mem_cgroup_out_of_memory(memcg, mask, order))
> >  		return OOM_SUCCESS;
> >  
> > -	WARN(1,"Memory cgroup charge failed because of no reclaimable memory! "
> > -		"This looks like a misconfiguration or a kernel bug.");
> >  	return OOM_FAILED;
> >  }
> >  
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 0e10b864e074..07ae222d7830 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -1103,10 +1103,17 @@ bool out_of_memory(struct oom_control *oc)
> >  	}
> >  
> >  	select_bad_process(oc);
> > -	/* Found nothing?!?! Either we hang forever, or we panic. */
> > -	if (!oc->chosen && !is_sysrq_oom(oc) && !is_memcg_oom(oc)) {
> > +	/* Found nothing?!?! */
> > +	if (!oc->chosen) {
> >  		dump_header(oc, NULL);
> > -		panic("Out of memory and no killable processes...\n");
> > +		pr_warn("Out of memory and no killable processes...\n");
> > +		/*
> > +		 * If we got here due to an actual allocation at the
> > +		 * system level, we cannot survive this and will enter
> > +		 * an endless loop in the allocator. Bail out now.
> > +		 */
> > +		if (!is_sysrq_oom(oc) && !is_memcg_oom(oc))
> > +			panic("System is deadlocked on memory\n");
> >  	}
> >  	if (oc->chosen && oc->chosen != (void *)-1UL)
> >  		oom_kill_process(oc, !is_memcg_oom(oc) ? "Out of memory" :
> > -- 
> > 2.18.0
> > 
> 
> -- 
> Michal Hocko
> SUSE Labs

-- 
Michal Hocko
SUSE Labs