linux-kernel - Re: [v4 2/4] mm, oom: cgroup-aware OOM killer

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Date:   Mon, 14 Aug 2017 13:03:49 +0100
From:   Roman Gushchin <guro@...com>
To:     David Rientjes <rientjes@...gle.com>
CC:     Michal Hocko <mhocko@...nel.org>, <linux-mm@...ck.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Tejun Heo <tj@...nel.org>, <kernel-team@...com>,
        <cgroups@...r.kernel.org>, <linux-doc@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [v4 2/4] mm, oom: cgroup-aware OOM killer

On Tue, Aug 08, 2017 at 04:06:38PM -0700, David Rientjes wrote:
> On Tue, 1 Aug 2017, Roman Gushchin wrote:
> 
> > > To the rest of the patch. I have to say I do not quite like how it is
> > > implemented. I was hoping for something much simpler which would hook
> > > into oom_evaluate_task. If a task belongs to a memcg with kill-all flag
> > > then we would update the cumulative memcg badness (more specifically the
> > > badness of the topmost parent with kill-all flag). Memcg will then
> > > compete with existing self contained tasks (oom_badness will have to
> > > tell whether points belong to a task or a memcg to allow the caller to
> > > deal with it). But it shouldn't be much more complex than that.
> > 
> > I'm not sure, it will be any simpler. Basically I'm doing the same:
> > the difference is that you want to iterate over tasks and for each
> > task traverse the memcg tree, update per-cgroup oom score and find
> > the corresponding memcg(s) with the kill-all flag. I'm doing the opposite:
> > traverse the cgroup tree, and for each leaf cgroup iterate over processes.
> > 
> > Also, please note, that even without the kill-all flag the decision is made
> > on per-cgroup level (except tasks in the root cgroup).
> > 
> 
> I think your implementation is preferred and is actually quite simple to 
> follow, and I would encourage you to follow through with it.  It has a 
> similar implementation to what we have done for years to kill a process 
> from a leaf memcg.

Hi David!

Thank you for the support.

> 
> I did notice that oom_kill_memcg_victim() calls directly into 
> __oom_kill_process(), however, so we lack the traditional oom killer 
> output that shows memcg usage and potential tasklist.  I think we should 
> still be dumping this information to the kernel log so that we can see a 
> breakdown of charged memory.

I think the existing output is too verbose for the case, when we kill
a cgroup with many processes inside. But I absolutely agree, that we need
some debug output, I'll add it in v5.

Thanks!