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

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Message-ID: <20170918061405.pcrf5vauvul4c2nr@dhcp22.suse.cz>
Date:   Mon, 18 Sep 2017 08:14:05 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     David Rientjes <rientjes@...gle.com>, linux-mm@...ck.org,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, kernel-team@...com,
        cgroups@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [v8 0/4] cgroup-aware OOM killer

On Fri 15-09-17 08:23:01, Roman Gushchin wrote:
> On Fri, Sep 15, 2017 at 12:58:26PM +0200, Michal Hocko wrote:
> > On Thu 14-09-17 09:05:48, Roman Gushchin wrote:
> > > On Thu, Sep 14, 2017 at 03:40:14PM +0200, Michal Hocko wrote:
> > > > On Wed 13-09-17 14:56:07, Roman Gushchin wrote:
> > > > > On Wed, Sep 13, 2017 at 02:29:14PM +0200, Michal Hocko wrote:
> > > > [...]
> > > > > > I strongly believe that comparing only leaf memcgs
> > > > > > is more straightforward and it doesn't lead to unexpected results as
> > > > > > mentioned before (kill a small memcg which is a part of the larger
> > > > > > sub-hierarchy).
> > > > > 
> > > > > One of two main goals of this patchset is to introduce cgroup-level
> > > > > fairness: bigger cgroups should be affected more than smaller,
> > > > > despite the size of tasks inside. I believe the same principle
> > > > > should be used for cgroups.
> > > > 
> > > > Yes bigger cgroups should be preferred but I fail to see why bigger
> > > > hierarchies should be considered as well if they are not kill-all. And
> > > > whether non-leaf memcgs should allow kill-all is not entirely clear to
> > > > me. What would be the usecase?
> > > 
> > > We definitely want to support kill-all for non-leaf cgroups.
> > > A workload can consist of several cgroups and we want to clean up
> > > the whole thing on OOM.
> > 
> > Could you be more specific about such a workload? E.g. how can be such a
> > hierarchy handled consistently when its sub-tree gets killed due to
> > internal memory pressure?
> 
> Or just system-wide OOM.
> 
> > Or do you expect that none of the subtree will
> > have hard limit configured?
> 
> And this can also be a case: the whole workload may have hard limit
> configured, while internal memcgs have only memory.low set for "soft"
> prioritization.
> 
> > 
> > But then you just enforce a structural restriction on your configuration
> > because
> > 	root
> >         /  \
> >        A    D
> >       /\   
> >      B  C
> > 
> > is a different thing than
> > 	root
> >         / | \
> >        B  C  D
> >
> 
> I actually don't have a strong argument against an approach to select
> largest leaf or kill-all-set memcg. I think, in practice there will be
> no much difference.

Well, I am worried that the difference will come unexpected when a
deeper hierarchy is needed because of the structural needs.

-- 
Michal Hocko
SUSE Labs