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

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Message-ID: <20170914133407.e7gstxssq6j5lo25@dhcp22.suse.cz>
Date:   Thu, 14 Sep 2017 15:34:07 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     David Rientjes <rientjes@...gle.com>
Cc:     Roman Gushchin <guro@...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 Wed 13-09-17 13:46:08, David Rientjes wrote:
> On Wed, 13 Sep 2017, Michal Hocko wrote:
> 
> > > > This patchset makes the OOM killer cgroup-aware.
> > > > 
> > > > v8:
> > > >   - Do not kill tasks with OOM_SCORE_ADJ -1000
> > > >   - Make the whole thing opt-in with cgroup mount option control
> > > >   - Drop oom_priority for further discussions
> > > 
> > > Nack, we specifically require oom_priority for this to function correctly, 
> > > otherwise we cannot prefer to kill from low priority leaf memcgs as 
> > > required.
> > 
> > While I understand that your usecase might require priorities I do not
> > think this part missing is a reason to nack the cgroup based selection
> > and kill-all parts. This can be done on top. The only important part
> > right now is the current selection semantic - only leaf memcgs vs. size
> > of the hierarchy). 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).
> > 
> 
> The problem is that we cannot enable the cgroup-aware oom killer and 
> oom_group behavior because, without oom priorities, we have no ability to 
> influence the cgroup that it chooses.  It is doing two things: providing 
> more fairness amongst cgroups by selecting based on cumulative usage 
> rather than single large process (good!), and effectively is removing all 
> userspace control of oom selection (bad).  We want the former, but it 
> needs to be coupled with support so that we can protect vital cgroups, 
> regardless of their usage.

I understand that your usecase needs a more fine grained control over
the selection but that alone is not a reason to nack the implementation
which doesn't provide it (yet).

> It is certainly possible to add oom priorities on top before it is merged, 
> but I don't see why it isn't part of the patchset.

Because the semantic of the priority for non-leaf memcgs is not fully
clear and I would rather have the core of the functionality merged
before this is sorted out.

> We need it before its 
> merged to avoid users playing with /proc/pid/oom_score_adj to prevent any 
> killing in the most preferable memcg when they could have simply changed 
> the oom priority.

I am sorry but I do not really understand your concern. Are you
suggesting that users would start oom disable all tasks in a memcg to
give it a higher priority? Even if that was the case why should such an
abuse be a blocker for generic memcg aware oom killer being merged?
-- 
Michal Hocko
SUSE Labs