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

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Date:   Fri, 15 Sep 2017 08:23:01 -0700
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...nel.org>
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, 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.

The only real concern I have is that then we have to do the same with
oom_priorities (select largest priority tree-wide), and this will limit
an ability to enforce the priority by parent cgroup.

Thanks!