linux-kernel - Re: [PATCH] mm, oom: fix oom_unkillable

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Date:   Mon, 17 Jun 2019 09:48:42 -0700
From:   Shakeel Butt <shakeelb@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>, Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm, oom: fix oom_unkillable_task for memcg OOMs

On Mon, Jun 17, 2019 at 9:17 AM Michal Hocko <mhocko@...nel.org> wrote:
>
> On Mon 17-06-19 08:59:54, Shakeel Butt wrote:
> > Currently oom_unkillable_task() checks mems_allowed even for memcg OOMs
> > which does not make sense as memcg OOMs can not be triggered due to
> > numa constraints. Fixing that.
> >
> > Also if memcg is given, oom_unkillable_task() will check the task's
> > memcg membership as well to detect oom killability. However all the
> > memcg related code paths leading to oom_unkillable_task(), other than
> > dump_tasks(), come through mem_cgroup_scan_tasks() which traverses
> > tasks through memcgs. Once dump_tasks() is converted to use
> > mem_cgroup_scan_tasks(), there is no need to do memcg membership check
> > in oom_unkillable_task().
>
> I think this patch just does too much in one go. Could you split out
> the dump_tasks part and the oom_unkillable_task parts into two patches
> please? It should be slightly easier to review.
>

Yes, will do in v2.

> [...]
> > +static bool oom_unkillable_task(struct task_struct *p, struct oom_control *oc)
> >  {
> >       if (is_global_init(p))
> >               return true;
> >       if (p->flags & PF_KTHREAD)
> >               return true;
> > +     if (!oc)
> > +             return false;
>
> Bah, this is just too ugly. AFAICS this is only because oom_score still
> uses oom_unkillable_task which is kinda dubious, no? While you are
> touching this code, can we remove this part as well? I would be really
> surprised if any code really depends on ineligible tasks reporting 0
> oom_score.

I think it is safer to just localize the is_global_init() and
PF_KTHREAD checks in oom_badness() instead of invoking
oom_unkillable_task(). Also I think cpuset_mems_allowed_intersects()
check from /proc/[pid]/oom_score is unintentional.

Shakeel