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Date: Thu, 14 Jul 2016 19:41:12 +0200
From: Oleg Nesterov <oleg@redhat.com>
To: John Stultz <john.stultz@linaro.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Tejun Heo <tj@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, lkml <linux-kernel@vger.kernel.org>,
        Dmitry Shmidt <dimitrysh@google.com>,
        Rom Lemarchand <romlem@google.com>, Colin Cross <ccross@google.com>,
        Todd Kjos <tkjos@google.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
	locking changes
Message-ID: <20160714174111.GA15816@redhat.com>
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On 07/14, John Stultz wrote:
>
> I'm not supposed to be applying this on-top of
> Paul's change, right?

Right, unless I am totally confused,

> > Just in case, could you try the patch below? Of course, without other
> > optimizations from Peter, this change makes cgroup_threadgroup_rwsem
> > much worse than a plain rw_semaphore.
> >
> > Oleg.
> >
> > --- x/kernel/cgroup.c
> > +++ x/kernel/cgroup.c
> > @@ -5605,6 +5605,8 @@ int __init cgroup_init(void)
> >         BUG_ON(cgroup_init_cftypes(NULL, cgroup_dfl_base_files));
> >         BUG_ON(cgroup_init_cftypes(NULL, cgroup_legacy_base_files));
> >
> > +       rcu_sync_enter(&cgroup_threadgroup_rwsem.rss);
> > +
> 
> 
> So adding this does make a huge difference ontop of Peter's patch.

Ah, sorry for confusion. I meant, you could try this one-liner without
any other changes.

But we will need the "slow mode optimization" part from Peter's patch
anyway, otherwise percpu_rw_semaphore simply makes no sense for
cgroup_threadgroup_rwsem.

Oleg.