linux-kernel - Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket plist in futex

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Message-ID: <1385477399.12603.20.camel@buesod1.americas.hpqcorp.net>
Date:	Tue, 26 Nov 2013 06:49:59 -0800
From:	Davidlohr Bueso <davidlohr@...com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Jason Low <jason.low2@...com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Mike Galbraith <efault@....de>, Jeff Mahoney <jeffm@...e.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Scott Norton <scott.norton@...com>,
	Tom Vaden <tom.vaden@...com>,
	Aswin Chandramouleeswaran <aswin@...com>,
	Waiman Long <Waiman.Long@...com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket
 plist in futex_wake()

On Tue, 2013-11-26 at 12:56 +0100, Peter Zijlstra wrote:
> On Tue, Nov 26, 2013 at 12:21:40PM +0100, Ingo Molnar wrote:
> > I'm somewhat reluctant to chalk it up to a single mfence - maybe 
> > timings/behavior changed in some substantial way?

That would be nthread mfence calls.

> 
> Ah indeed! We also changed the case where an enqueueing futex sees the
> uval change and bails. It is now far more expensive due to having to
> both queue and unqueue, whereas before it wouldn't queue at all.

Right, but those particular numbers do not measure that path.

Thanks,
Davidlohr

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