linux-kernel - Re: [v6 2/4] mm, oom: cgroup-aware OOM killer

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Date:   Thu, 24 Aug 2017 14:58:42 +0100
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     <linux-mm@...ck.org>, Vladimir Davydov <vdavydov.dev@...il.com>,
        "Johannes Weiner" <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, <kernel-team@...com>,
        <cgroups@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [v6 2/4] mm, oom: cgroup-aware OOM killer

On Thu, Aug 24, 2017 at 02:58:11PM +0200, Michal Hocko wrote:
> On Thu 24-08-17 13:28:46, Roman Gushchin wrote:
> > Hi Michal!
> > 
> There is nothing like a "better victim". We are pretty much in a
> catastrophic situation when we try to survive by killing a userspace.

Not necessary, it can be a cgroup OOM.

> We try to kill the largest because that assumes that we return the
> most memory from it. Now I do understand that you want to treat the
> memcg as a single killable entity but I find it really questionable
> to do a per-memcg metric and then do not treat it like that and kill
> only a single task. Just imagine a single memcg with zillions of taks
> each very small and you select it as the largest while a small taks
> itself doesn't help to help to get us out of the OOM.

I don't think it's different from a non-containerized state: if you
have a zillion of small tasks in the system, you'll meet the same issues.

> > > I guess I have asked already and we haven't reached any consensus. I do
> > > not like how you treat memcgs and tasks differently. Why cannot we have
> > > a memcg score a sum of all its tasks?
> > 
> > It sounds like a more expensive way to get almost the same with less accuracy.
> > Why it's better?
> 
> because then you are comparing apples to apples?

Well, I can say that I compare some number of pages against some other number
of pages. And the relation between a page and memcg is more obvious, than a
relation between a page and a process.

Both ways are not ideal, and sum of the processes is not ideal too.
Especially, if you take oom_score_adj into account. Will you respect it?

I've started actually with such approach, but then found it weird.

> Besides that you have
> to check each task for over-killing anyway. So I do not see any
> performance merits here.

It's an implementation detail, and we can hopefully get rid of it at some point.

> 
> > > How do you want to compare memcg score with tasks score?
> > 
> > I have to do it for tasks in root cgroups, but it shouldn't be a common case.
> 
> How come? I can easily imagine a setup where only some memcgs which
> really do need a kill-all semantic while all others can live with single
> task killed perfectly fine.

I mean taking a unified cgroup hierarchy into an account, there should not
be lot of tasks in the root cgroup, if any.