linux-kernel - Re: [PATCH v9 05/19] qspinlock: Optimize for smaller NR

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Date:	Thu, 17 Apr 2014 17:33:06 -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 05/19] qspinlock: Optimize for smaller NR_CPUS

On 04/17/2014 11:51 AM, Peter Zijlstra wrote:
> On Thu, Apr 17, 2014 at 11:03:57AM -0400, Waiman Long wrote:
>> @@ -48,6 +53,9 @@
>>    * We can further change the first spinner to spin on a bit in the lock word
>>    * instead of its node; whereby avoiding the need to carry a node from lock to
>>    * unlock, and preserving API.
>> + *
>> + * N.B. The current implementation only supports architectures that allow
>> + *      atomic operations on smaller 8-bit and 16-bit data types.
>>    */
> Only for the _Q_PENDING_BITS == 8 case, the other case should still be
> fine.

Yes, but _Q_PENDING_BITS is controlled by NR_CPUS which, for almost all 
the distributions, is less than 16K which means _Q_PENDING_BITS will 
always be set to 8, especially for non-x86 architectures.

-Longman
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