linux-kernel - Re: [PATCH 4/4, v2] x86: enlightenment for ticket spin locks

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Message-ID: <4C2B180E.9060104@goop.org>
Date:	Wed, 30 Jun 2010 12:10:22 +0200
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jan Beulich <JBeulich@...ell.com>
CC:	Peter Zijlstra <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	ksrinivasan <ksrinivasan@...ell.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"hpa@...or.com" <hpa@...or.com>
Subject: Re: [PATCH 4/4, v2] x86: enlightenment for ticket spin locks -	 improve
 yield behavior on Xen

On 06/30/2010 10:49 AM, Jan Beulich wrote:
>>>> On 30.06.10 at 10:11, Peter Zijlstra <peterz@...radead.org> wrote:
>>>>         
>> On Tue, 2010-06-29 at 15:35 +0100, Jan Beulich wrote:
>>     
>>> The (only) additional overhead this introduces for native execution is
>>> the writing of the owning CPU in the lock acquire paths. 
>>>       
>> Uhm, and growing the size of spinlock_t to 6 (or 8 bytes when aligned)
>> bytes when NR_CPUS>256.
>>     
> Indeed, I should have mentioned that. Will do so in an eventual
> next version.
>   

Rather than increasing the lock size, why not just disable the
enlightenment if the number of (possible) cpus is > 256?  I don't think
a VM will ever have that many cpus, so it will only apply in the case of
booting the kernel on large physical machine.

    J
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