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

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Message-ID: <20181023121055.GS18839@dhcp22.suse.cz>
Date:   Tue, 23 Oct 2018 14:10:55 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Johannes Weiner <hannes@...xchg.org>
Cc:     linux-mm@...ck.org,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        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 Tue 23-10-18 13:42:46, Michal Hocko wrote:
> On Tue 23-10-18 10:01:08, Tetsuo Handa wrote:
> > Michal Hocko wrote:
> > > On Mon 22-10-18 20:45:17, Tetsuo Handa wrote:
> > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > > index e79cb59552d9..a9dfed29967b 100644
> > > > > --- a/mm/memcontrol.c
> > > > > +++ b/mm/memcontrol.c
> > > > > @@ -1380,10 +1380,22 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> > > > >  		.gfp_mask = gfp_mask,
> > > > >  		.order = order,
> > > > >  	};
> > > > > -	bool ret;
> > > > > +	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
> > > > > +	 */
> > > > > +	if (tsk_is_oom_victim(current))
> > > > > +		goto unlock;
> > > > > +
> > > > 
> > > > This is not wrong but is strange. We can use mutex_lock_killable(&oom_lock)
> > > > so that any killed threads no longer wait for oom_lock.
> > > 
> > > tsk_is_oom_victim is stronger because it doesn't depend on
> > > fatal_signal_pending which might be cleared throughout the exit process.
> > > 
> > 
> > I still want to propose this. No need to be memcg OOM specific.
> 
> Well, I maintain what I've said [1] about simplicity and specific fix
> for a specific issue. Especially in the tricky code like this where all
> the consequences are far more subtle than they seem to be.
> 
> This is obviously a matter of taste but I don't see much point discussing
> this back and forth for ever. Unless there is a general agreement that
> the above is less appropriate then I am willing to consider a different
> change but I simply do not have energy to nit pick for ever.
> 
> [1] http://lkml.kernel.org/r/20181022134315.GF18839@dhcp22.suse.cz

In other words. Having a memcg specific fix means, well, a memcg
maintenance burden. Like any other memcg specific oom decisions we
already have. So are you OK with that Johannes or you would like to see
a more generic fix which might turn out to be more complex?
-- 
Michal Hocko
SUSE Labs