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

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Message-ID: <alpine.DEB.2.02.1311261333101.30673@ionos.tec.linutronix.de>
Date:	Tue, 26 Nov 2013 13:34:37 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Ingo Molnar <mingo@...nel.org>, Davidlohr Bueso <davidlohr@...com>,
	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, 26 Nov 2013, 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?
> 
> 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.
> 
> I suppose the idea was to offset that by not requiring locking on the
> wake side.

Aside of that I really would be interrested in an explanation for the
STDDEV going up by factor 5. That's a clear indicator for fishyness.

Thanks,

	tglx
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