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

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Date:   Thu, 5 Oct 2017 12:45:51 +0100
From:   Roman Gushchin <guro@...com>
To:     Michal Hocko <mhocko@...nel.org>
CC:     Shakeel Butt <shakeelb@...gle.com>, Linux MM <linux-mm@...ck.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 Thu, Oct 05, 2017 at 01:12:30PM +0200, Michal Hocko wrote:
> On Thu 05-10-17 11:27:07, Roman Gushchin wrote:
> > On Wed, Oct 04, 2017 at 02:24:26PM -0700, Shakeel Butt wrote:
> [...]
> > > 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?
> > 
> > We're holding rcu_read_lock, it should be enough. We're bumping css counter
> > just before releasing rcu lock.
> 
> yes
> 
> > > 
> > > 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.
> > 
> > Sounds like a good optimization, which can be done on top of the current
> > patchset.
> 
> You could achive this by checking whether a memcg has tasks rather than
> explicitly checking for children memcgs as I've suggested already.

Using cgroup_has_tasks() will require additional locking, so I'm not sure
it worth it.