linux-kernel - Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce global impact

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Date:	Wed, 10 Aug 2016 09:56:06 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	John Stultz <john.stultz@...aro.org>
Cc:	Oleg Nesterov <oleg@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...nel.org>,
	lkml <linux-kernel@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Dmitry Shmidt <dimitrysh@...gle.com>,
	Rom Lemarchand <romlem@...gle.com>,
	Colin Cross <ccross@...gle.com>, Todd Kjos <tkjos@...gle.com>
Subject: Re: [PATCH v2] locking/percpu-rwsem: Optimize readers and reduce
 global impact

On Tue, Aug 09, 2016 at 04:47:38PM -0700, John Stultz wrote:
> On Tue, Aug 9, 2016 at 2:51 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > Currently the percpu-rwsem switches to (global) atomic ops while a
> > writer is waiting; which could be quite a while and slows down
> > releasing the readers.
> >
> > This patch cures this problem by ordering the reader-state vs
> > reader-count (see the comments in __percpu_down_read() and
> > percpu_down_write()). This changes a global atomic op into a full
> > memory barrier, which doesn't have the global cacheline contention.
> >
> > This also enables using the percpu-rwsem with rcu_sync disabled in order
> > to bias the implementation differently, reducing the writer latency by
> > adding some cost to readers.
> 
> So this by itself doesn't help us much, but including the following
> from Oleg does help quite a bit:

Correct, Oleg was going to send his rcu_sync rework on top of this. But
since its holiday season things might be tad delayed.