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Date: Fri, 11 Dec 2015 11:25:54 -0500
From: Tejun Heo <tj@kernel.org>
Subject: Re: [PATCH 1/2] memcg: flatten task_struct->memcg_oom
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To: Peter Zijlstra <peterz@infradead.org>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>, Ingo Molnar <mingo@redhat.com>, Sasha Levin <sasha.levin@oracle.com>, Andrew Morton <akpm@linux-foundation.org>, Johannes Weiner <hannes@cmpxchg.org>, mhocko@kernel.org, cgroups@vger.kernel.org, "linux-mm@kvack.org" <linux-mm@kvack.org>, vdavydov@parallels.com, kernel-team@fb.com, Dmitry Vyukov <dvyukov@google.com>

Hello, Peter, Ingo.

On Wed, Nov 25, 2015 at 06:44:49PM +0100, Peter Zijlstra wrote:
> On Wed, Nov 25, 2015 at 06:31:41PM +0300, Andrey Ryabinin wrote:
> > > +       /* scheduler bits, serialized by scheduler locks */
> > >         unsigned sched_reset_on_fork:1;
> > >         unsigned sched_contributes_to_load:1;
> > >         unsigned sched_migrated:1;
> > > +       unsigned __padding_sched:29;
> > 
> > AFAIK the order of bit fields is implementation defined, so GCC could
> > sort all these bits as it wants.
> 
> We're relying on it doing DTRT in other places, so I'm fairly confident
> this'll work, otoh
> 
> > You could use unnamed zero-widht bit-field to force padding:
> > 
> >          unsigned :0; //force aligment to the next boundary.
> 
> That's a nice trick I was not aware of, thanks!

Has this been fixed yet?  While I'm not completely sure and I don't
think there's a way to be certain after the fact, we have a single
report of a machine which is showing ~4G as loadavg and one plausible
explanation could be that one of the ->nr_uninterruptible counters
underflowed from sched_contributes_to_load getting corrupted, so it'd
be great to get this one fixed soon.

Thanks.

-- 
tejun

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