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 17:01:14 -0400
From:	Tejun Heo <tj@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	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>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: Severe performance regression w/ 4.4+ on Android due to cgroup
 locking changes

On Wed, Jul 13, 2016 at 10:51:02PM +0200, Peter Zijlstra wrote:
> On Wed, Jul 13, 2016 at 04:39:44PM -0400, Tejun Heo 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.

Given the way read side is used for percpu_rwsem, full memory barrier
on reader side shouldn't matter at all.  The paths are not *that* hot.

> 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.

At the simplest, it can be rwsem equivalence of lglock.

-- 
tejun