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

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Message-ID: <20180130115846.GA4720@castle.DHCP.thefacebook.com>
Date:   Tue, 30 Jan 2018 11:58:51 +0000
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...nel.org>
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, 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.

Thank you!