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Date:   Wed, 18 Mar 2020 13:31:07 +0100
From:   Michal Hocko <mhocko@kernel.org>
To:     Roman Gushchin <guro@fb.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        kernel-team@fb.com, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] mm: memcg: make memory.oom.group tolerable to task
 migration
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On Tue 17-03-20 13:36:45, Roman Gushchin wrote:
[...]
> > > And the we can put something like
> > > 	if (WARN_ON_ONCE(!mem_cgroup_is_descendant(memcg, oom_domain)))
> > > 		goto out;
> > > to mem_cgroup_get_oom_group?
> > 
> > This would be a user triggerable warning and that sounds like a bad idea
> > to me. We should just live with races. The only question I still do not
> > have a proper answer for is how much we do care. If we do not care all
> > that much about the original memcg then go with your patch. But if we
> > want to be slightly more careful then we should note the memcg in
> > oom_evaluate_task and use it when killing.
> 
> But it won't close the race, right?
>
> oom_evaluate_task() can race with a task migration too, so we can record
> the old or the new cgroup.

Are you sure? I thought that cgroups iterator code would take care of
those races. The documentation doesn't tell much in that respect. Maybe
it would be good to add a clarification there.
 
> Then I'd stick with my original patch which solves the main problem here:
> unnecessary killing of too many tasks.

OK, I am fine with that. I couldn't convince myself that the other part
of the problem is serious enough. Maybe we will find workloads which do
care and we can add that later on.
-- 
Michal Hocko
SUSE Labs