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Date: Tue, 26 Nov 2013 13:34:37 +0100 (CET)
From: Thomas Gleixner <tglx@linutronix.de>
To: Peter Zijlstra <peterz@infradead.org>
cc: Ingo Molnar <mingo@kernel.org>, Davidlohr Bueso <davidlohr@hp.com>,
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        Waiman Long <Waiman.Long@hp.com>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Subject: Re: [RFC patch 0/5] futex: Allow lockless empty check of hashbucket
 plist in futex_wake()
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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