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

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Message-Id: <4C2B45C60200007800008CA6@vpn.id2.novell.com>
Date:	Wed, 30 Jun 2010 12:25:26 +0100
From:	"Jan Beulich" <JBeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
Cc:	"mingo@...e.hu" <mingo@...e.hu>,
	"Peter Zijlstra" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"Ky Srinivasan" <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 30.06.10 at 12:10, Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> 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.

While that would be an option, I think the decision to go either
way should really be left to the user (after all that's why there
is a config option in the first place).

Jan

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