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 15:33:02 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	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>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
 locking changes

On Wed, Jul 13, 2016 at 06:01:28PM -0400, Tejun Heo wrote:
> Hello, Paul.
> 
> On Wed, Jul 13, 2016 at 02:18:41PM -0700, Paul E. McKenney wrote:
> > On Wed, Jul 13, 2016 at 05:05:26PM -0400, Tejun Heo wrote:
> > > On Wed, Jul 13, 2016 at 02:03:15PM -0700, Paul E. McKenney wrote:
> > > > Take the patch that I just sent out and make the choice of normal
> > > > vs. expedited depend on CONFIG_PREEMPT_RT or whatever the -rt guys are
> > > > calling it these days.  Is there a low-latency Kconfig option other
> > > > than CONFIG_NO_HZ_FULL?
> > > 
> > > Sounds like a plan to me.
> > 
> > I like the way we like each other's idea.  Mutually assured laziness?  ;-)
> 
> Heh, indeed. :)
> 
> Technically, I think the lglock approach would be better here given
> the combination of requirements; however, it's quite a bit more code
> which would likely require some sophistications down the line (like
> blocking new readers first at the start of down_write).  If we have to
> go there, we'll go there but for now I think it'd be simpler to
> conditionally switch to the expedited operations.  It can be a config
> option which is selected by !RT as you suggested.  If anyone hits an
> actual issue with that, we can go for the lglock thing.

Fair enough!  ;-)

							Thanx, Paul