linux-kernel - Re: [PATCH v9 06/19] qspinlock: prolong the stay in the pending bit path

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Message-ID: <535169CA.8060209@hp.com>
Date:	Fri, 18 Apr 2014 14:07:06 -0400
From:	Waiman Long <waiman.long@...com>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, linux-arch@...r.kernel.org,
	x86@...nel.org, linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org,
	xen-devel@...ts.xenproject.org, kvm@...r.kernel.org,
	Paolo Bonzini <paolo.bonzini@...il.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Raghavendra K T <raghavendra.kt@...ux.vnet.ibm.com>,
	David Vrabel <david.vrabel@...rix.com>,
	Oleg Nesterov <oleg@...hat.com>,
	Gleb Natapov <gleb@...hat.com>,
	Scott J Norton <scott.norton@...com>,
	Chegu Vinod <chegu_vinod@...com>
Subject: Re: [PATCH v9 06/19] qspinlock: prolong the stay in the pending bit
 path

On 04/18/2014 04:33 AM, Peter Zijlstra wrote:
> On Thu, Apr 17, 2014 at 09:46:04PM -0400, Waiman Long wrote:
>> BTW, I didn't test out your atomic_test_and_set() change. Did it provide a
>> noticeable performance benefit when compared with cmpxchg()?
> I've not tested that I think. I had a hard time showing that cmpxchg
> loops were slower, but once I did, I simply replaced all cmpxchg loops
> with unconditional ops where possible.
>
> The machine that was big enough to show it lived in a lab half way
> around the world and using it was a right pain in the ass, so I didn't
> use it more than I absolutely had to.

Thank for letting me know about it. The cmpxchg() slowdown that I 
observed happens when it is heavily contended. In other words, if many 
CPUs are trying to grab it at roughly the same time. If there is just a 
few contending CPUs, the difference shouldn't be too big.

-Longman
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