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 13:52:11 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Tejun Heo <tj@...nel.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 10:26:57PM +0200, Peter Zijlstra wrote:
> 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.

Then one diagnostic step to take would be to replace that
synchronize_sched() with synchronize_sched_expedited(), and see if that
gets rid of the delays.

Not a particularly real-time-friendly fix, but certainly a good check
on our various assumptions.

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/rcu/sync.c b/kernel/rcu/sync.c
index be922c9f3d37..211acddc7e21 100644
--- a/kernel/rcu/sync.c
+++ b/kernel/rcu/sync.c
@@ -38,19 +38,19 @@ static const struct {
 #endif
 } gp_ops[] = {
 	[RCU_SYNC] = {
-		.sync = synchronize_rcu,
+		.sync = synchronize_rcu_expedited,
 		.call = call_rcu,
 		.wait = rcu_barrier,
 		__INIT_HELD(rcu_read_lock_held)
 	},
 	[RCU_SCHED_SYNC] = {
-		.sync = synchronize_sched,
+		.sync = synchronize_sched_expedited,
 		.call = call_rcu_sched,
 		.wait = rcu_barrier_sched,
 		__INIT_HELD(rcu_read_lock_sched_held)
 	},
 	[RCU_BH_SYNC] = {
-		.sync = synchronize_rcu_bh,
+		.sync = synchronize_rcu_bh_expedited,
 		.call = call_rcu_bh,
 		.wait = rcu_barrier_bh,
 		__INIT_HELD(rcu_read_lock_bh_held)