linux-kernel - Re: [v8 0/4] cgroup-aware OOM killer

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Message-ID: <20170926133040.uupv3ibkt3jtbotf@dhcp22.suse.cz>
Date:   Tue, 26 Sep 2017 15:30:40 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Johannes Weiner <hannes@...xchg.org>, Tejun Heo <tj@...nel.org>,
        kernel-team@...com, David Rientjes <rientjes@...gle.com>,
        linux-mm@...ck.org, Vladimir Davydov <vdavydov.dev@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Tue 26-09-17 13:13:00, Roman Gushchin wrote:
> On Tue, Sep 26, 2017 at 01:21:34PM +0200, Michal Hocko wrote:
> > On Tue 26-09-17 11:59:25, Roman Gushchin wrote:
> > > On Mon, Sep 25, 2017 at 10:25:21PM +0200, Michal Hocko wrote:
> > > > On Mon 25-09-17 19:15:33, Roman Gushchin wrote:
> > > > [...]
> > > > > I'm not against this model, as I've said before. It feels logical,
> > > > > and will work fine in most cases.
> > > > > 
> > > > > In this case we can drop any mount/boot options, because it preserves
> > > > > the existing behavior in the default configuration. A big advantage.
> > > > 
> > > > I am not sure about this. We still need an opt-in, ragardless, because
> > > > selecting the largest process from the largest memcg != selecting the
> > > > largest task (just consider memcgs with many processes example).
> > > 
> > > As I understand Johannes, he suggested to compare individual processes with
> > > group_oom mem cgroups. In other words, always select a killable entity with
> > > the biggest memory footprint.
> > > 
> > > This is slightly different from my v8 approach, where I treat leaf memcgs
> > > as indivisible memory consumers independent on group_oom setting, so
> > > by default I'm selecting the biggest task in the biggest memcg.
> > 
> > My reading is that he is actually proposing the same thing I've been
> > mentioning. Simply select the biggest killable entity (leaf memcg or
> > group_oom hierarchy) and either kill the largest task in that entity
> > (for !group_oom) or the whole memcg/hierarchy otherwise.
> 
> He wrote the following:
> "So I'm leaning toward the second model: compare all oomgroups and
> standalone tasks in the system with each other, independent of the
> failed hierarchical control structure. Then kill the biggest of them."

I will let Johannes to comment but I believe this is just a
misunderstanding. If we compared only the biggest task from each memcg
then we are basically losing our fairness objective, aren't we?
-- 
Michal Hocko
SUSE Labs