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

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Date:	Thu, 14 Jul 2016 18:49:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Tejun Heo <tj@...nel.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 Thu, Jul 14, 2016 at 09:43:40AM -0700, John Stultz wrote:
> On Thu, Jul 14, 2016 at 6:18 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Wed, Jul 13, 2016 at 10:51:02PM +0200, Peter Zijlstra wrote:
> >> So, IIRC, the trade-off is a full memory barrier in read_lock and
> >> read_unlock() vs sync_sched() in write.
> >>
> >> Full memory barriers are expensive and while the combined cost might
> >> well exceed the cost of the sync_sched() it doesn't suffer the latency
> >> issues.
> >>
> >> Not sure if we can frob the two in a single codebase, but I can have a
> >> poke if Oleg or Paul doesn't beat me to it.
> >
> > OK, not too horrible if I say so myself :-)
> >
> > The below is a compile tested only first draft so far. I'll go give it
> > some runtime next.
> 
> Unfortunately it didn't apply cleanly to the 4.4 based tree I'm
> working with, so I had to manually apply the entirety of the
> percpu-rwsem.c changes myself. Hopefully I didn't screw it up.
> 
> So running with this, I'm still seeing some pretty large delays. 80ms
> peak, with lots of >20ms values as well.
> So it doesn't seem to have the positive effect that Paul's change provided.

Well that is weird, did you put a tracepoint/printk in
synchronize_sched() to ensure we don't end up calling that?