linux-kernel - Re: [patch -mm v2 2/3] mm, memcg: replace cgroup aware oom killer mount option with tunable

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Date:   Tue, 30 Jan 2018 13:08:52 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Roman Gushchin <guro@...com>
Cc:     Tejun Heo <tj@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        David Rientjes <rientjes@...gle.com>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        kernel-team@...com, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [patch -mm v2 2/3] mm, memcg: replace cgroup aware oom killer
 mount option with tunable

On Tue 30-01-18 11:58:51, Roman Gushchin wrote:
> On Tue, Jan 30, 2018 at 09:54:45AM +0100, Michal Hocko wrote:
> > On Mon 29-01-18 11:11:39, Tejun Heo wrote:
> 
> Hello, Michal!
> 
> > diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
> > index 2eaed1e2243d..67bdf19f8e5b 100644
> > --- a/Documentation/cgroup-v2.txt
> > +++ b/Documentation/cgroup-v2.txt
> > @@ -1291,8 +1291,14 @@ This affects both system- and cgroup-wide OOMs. For a cgroup-wide OOM
> >  the memory controller considers only cgroups belonging to the sub-tree
> >  of the OOM'ing cgroup.
> >  
> > -The root cgroup is treated as a leaf memory cgroup, so it's compared
> > -with other leaf memory cgroups and cgroups with oom_group option set.
>                                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> IMO, this statement is important. Isn't it?
> 
> > +Leaf cgroups are compared based on their cumulative memory usage. The
> > +root cgroup is treated as a leaf memory cgroup as well, so it's
> > +compared with other leaf memory cgroups. Due to internal implementation
> > +restrictions the size of the root cgroup is a cumulative sum of
> > +oom_badness of all its tasks (in other words oom_score_adj of each task
> > +is obeyed). Relying on oom_score_adj (appart from OOM_SCORE_ADJ_MIN)
> > +can lead to overestimating of the root cgroup consumption and it is
> 
> Hm, and underestimating too. Also OOM_SCORE_ADJ_MIN isn't any different
> in this case. Say, all tasks except a small one have OOM_SCORE_ADJ set to
> -999, this means the root croup has extremely low chances to be elected.
> 
> > +therefore discouraged. This might change in the future, though.
> 
> Other than that looks very good to me.

This?

diff --git a/Documentation/cgroup-v2.txt b/Documentation/cgroup-v2.txt
index 2eaed1e2243d..34ad80ee90f2 100644
--- a/Documentation/cgroup-v2.txt
+++ b/Documentation/cgroup-v2.txt
@@ -1291,8 +1291,15 @@ This affects both system- and cgroup-wide OOMs. For a cgroup-wide OOM
 the memory controller considers only cgroups belonging to the sub-tree
 of the OOM'ing cgroup.
 
-The root cgroup is treated as a leaf memory cgroup, so it's compared
-with other leaf memory cgroups and cgroups with oom_group option set.
+Leaf cgroups and cgroups with oom_group option set are compared based
+on their cumulative memory usage. The root cgroup is treated as a
+leaf memory cgroup as well, so it's compared with other leaf memory
+cgroups. Due to internal implementation restrictions the size of
+the root cgroup is a cumulative sum of oom_badness of all its tasks
+(in other words oom_score_adj of each task is obeyed). Relying on
+oom_score_adj (appart from OOM_SCORE_ADJ_MIN) can lead to over or
+underestimating of the root cgroup consumption and it is therefore
+discouraged. This might change in the future, though.
 
 If there are no cgroups with the enabled memory controller,
 the OOM killer is using the "traditional" process-based approach.
-- 
Michal Hocko
SUSE Labs