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 08:08:45 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	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 Thu, Jul 14, 2016 at 02:04:28PM +0200, Peter Zijlstra wrote:
> > I think it probably makes sense to make this the default on !RT at
> > least with a separate patch w/o stable cc'd.  While most use cases
> > will be fine with the latency on write path, it also means that the
> > reader side is blocked for the duration which can hurt.  rwsem implies
> > a lot more readers and thus more read lock operations than writes.
> > It's weird to trade off higher latency for lower cpu usage when it
> > would also slow down all readers.
> 
> NAK, no expedited muck by default. There's more than just RT that
> doesn't like IPI sprays.

Can you elaborate?  If that's the case, we have the wrong implemention
for percpu-rwsem where very long delays for writers induce the same
level of delays to all readers.  If expedited by default isn't
workable, we should move away from rcu_sync for percpu_rwsem.

Thanks.

-- 
tejun