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

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Date:   Wed, 4 Oct 2017 21:15:24 +0100
From:   Roman Gushchin <guro@...com>
To:     Shakeel Butt <shakeelb@...gle.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

On Wed, Oct 04, 2017 at 12:48:03PM -0700, Shakeel Butt wrote:
> > +
> > +static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
> > +{
> > +       struct mem_cgroup *iter;
> > +
> > +       oc->chosen_memcg = NULL;
> > +       oc->chosen_points = 0;
> > +
> > +       /*
> > +        * The oom_score is calculated for leaf memory cgroups (including
> > +        * the root memcg).
> > +        */
> > +       rcu_read_lock();
> > +       for_each_mem_cgroup_tree(iter, root) {
> > +               long score;
> > +
> > +               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;
 
 		/*
@@ -2820,7 +2821,7 @@ static void select_victim_memcg(struct mem_cgroup *root, struct oom_control *oc)
 			group_score = 0;
 		}
 
-		if (memcg_has_children(iter))
+		if (memcg_has_children(iter) && iter != root_mem_cgroup)
 			continue;
 
 		score = oom_evaluate_memcg(iter, oc->nodemask, oc->totalpages);

--

> 
> > +
> > +               score = oom_evaluate_memcg(iter, oc->nodemask, oc->totalpages);
> > +
> > +               /*
> > +                * Ignore empty and non-eligible memory cgroups.
> > +                */
> > +               if (score == 0)
> > +                       continue;
> > +
> > +               /*
> > +                * If there are inflight OOM victims, we don't need
> > +                * to look further for new victims.
> > +                */
> > +               if (score == -1) {
> > +                       oc->chosen_memcg = INFLIGHT_VICTIM;
> > +                       mem_cgroup_iter_break(root, iter);
> > +                       break;
> > +               }
> > +
> 
> 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.

Thank you!