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 16:18:23 -0400
From:	Tejun Heo <tj@...nel.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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

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?

Thanks.

-- 
tejun