linux-kernel - Re: [tip:x86/cpu] x86/cpu: Strip any /proc/ cpuinfo model name field whitespace

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Date:	Thu, 28 May 2015 13:39:11 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	Prarit Bhargava <prarit@...hat.com>, Joe Perches <joe@...ches.com>,
	luto@...capital.net, peterz@...radead.org, dvlasenk@...hat.com,
	torvalds@...ux-foundation.org, imammedo@...hat.com,
	brgerst@...il.com, mingo@...nel.org, dave.hansen@...ux.intel.com,
	fenghua.yu@...el.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, bp@...e.de, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/cpu] x86/cpu: Strip any /proc/ cpuinfo model name field
 whitespace

On 05/28/2015 11:33 AM, Borislav Petkov wrote:
> On Thu, May 28, 2015 at 09:57:15AM -0700, H. Peter Anvin wrote:
>> Why?!
>>
>> We are taking about 48 bytes run once per cpu. It isn't worth it to
>> optimize, in fact the extra code size hurts more.
> 
> I wanted to save us the redundant copying of the exact same bytes.
> Because when there's no preceding whitespace, p and q point at the same
> thing so we end up doing *p = *p.
> 
> OTOH, without the optimization, the code is even simpler.
> 
> I can remove it if you wanna - I don't care all that much.
> 

Yes, please.  Actually, with a test inside the loop the way you have it,
the resulting code will almost certainly be slower -- a redundant write
to an already dirty cache line is way cheaper than a branch.

	-hpa

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