linux-kernel - Re: Severe performance regression w/ 4.4+ on Android due to cgroup locking changes

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Date:	Wed, 13 Jul 2016 22:26:57 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Tejun Heo <tj@...nel.org>
Cc:	John Stultz <john.stultz@...aro.org>,
	Ingo Molnar <mingo@...hat.com>,
	lkml <linux-kernel@...r.kernel.org>,
	Dmitry Shmidt <dimitrysh@...gle.com>,
	Rom Lemarchand <romlem@...gle.com>,
	Colin Cross <ccross@...gle.com>, Todd Kjos <tkjos@...gle.com>,
	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
 locking changes

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@...nel.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.