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Date: Wed, 13 Jul 2016 22:26:57 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Tejun Heo <tj@kernel.org>
Cc: John Stultz <john.stultz@linaro.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>, Oleg Nesterov <oleg@redhat.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: <20160713202657.GW30154@twins.programming.kicks-ass.net>
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On Wed, Jul 13, 2016 at 04:18:23PM -0400, Tejun Heo wrote:
> Hello, John.
> 
> On Wed, Jul 13, 2016 at 01:13:11PM -0700, John Stultz wrote:
> > On Wed, Jul 13, 2016 at 11:33 AM, Tejun Heo <tj@kernel.org> wrote:
> > > On Wed, Jul 13, 2016 at 02:21:02PM -0400, Tejun Heo wrote:
> > >> One interesting thing to try would be replacing it with a regular
> > >> non-percpu rwsem and see how it behaves.  That should easily tell us
> > >> whether this is from actual contention or artifacts from percpu_rwsem
> > >> implementation.
> > >
> > > So, something like the following.  Can you please see whether this
> > > makes any difference?
> > 
> > Yea. So this brings it down for me closer to what we're seeing with
> > the Dmitry's patch reverting the two problematic commits, usually
> > 10-50us with one early spike at 18ms.
> 
> So, it's a percpu rwsem issue then.  I haven't really followed the
> perpcpu rwsem changes closely.  Oleg, are multi-milisec delay expected
> on down write expected with the current implementation of
> percpu_rwsem?

There is a synchronize_sched() in there, so sorta. That thing is heavily
geared towards readers, as is the only 'sane' choice for global locks.