linux-kernel - Re: [v10 3/6] mm, oom: cgroup-aware OOM killer

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Date:   Wed, 4 Oct 2017 14:24:26 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Roman Gushchin <guro@...com>
Cc:     Linux MM <linux-mm@...ck.org>, Michal Hocko <mhocko@...nel.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>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, kernel-team@...com,
        Cgroups <cgroups@...r.kernel.org>, linux-doc@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [v10 3/6] mm, oom: cgroup-aware OOM killer

>> > +               if (memcg_has_children(iter))
>> > +                       continue;
>>
>> && iter != root_mem_cgroup ?
>
> Oh, sure. I had a stupid bug in my test script, which prevented me from
> catching this. Thanks!
>
> This should fix the problem.
> --
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 2e82625bd354..b3848bce4c86 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -2807,7 +2807,8 @@ static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
>                  * We don't consider non-leaf non-oom_group memory cgroups
>                  * as OOM victims.
>                  */
> -               if (memcg_has_children(iter) && !mem_cgroup_oom_group(iter))
> +               if (memcg_has_children(iter) && iter != root_mem_cgroup &&
> +                   !mem_cgroup_oom_group(iter))
>                         continue;

I think you are mixing the 3rd and 4th patch. The root_mem_cgroup
check should be in 3rd while oom_group stuff should be in 4th.


>>
>> Shouldn't there be a CSS_ONLINE check? Also instead of css_get at the
>> end why not css_tryget_online() here and css_put for the previous
>> selected one.
>
> Hm, why do we need to check this? I do not see, how we can choose
> an OFFLINE memcg as a victim, tbh. Please, explain the problem.
>

Sorry about the confusion. There are two things. First, should we do a
css_get on the newly selected memcg within the for loop when we still
have a reference to it?

Second, for the OFFLINE memcg, you are right oom_evaluate_memcg() will
return 0 for offlined memcgs. Maybe no need to call
oom_evaluate_memcg() for offlined memcgs.