linux-kernel - Re: [PATCH] i386-pda UP optimization

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Date:	Wed, 29 Nov 2006 01:56:41 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Eric Dumazet <dada1@...mosbay.com>
CC:	akpm@...l.org, Arjan van de Ven <arjan@...radead.org>, ak@...e.de,
	mingo@...e.hu, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] i386-pda UP optimization

Eric Dumazet wrote:
> if !CONFIG_SMP, why even dereferencing boot_pda+PDA_cpu to get 0 ?
> and as PER_CPU(cpu_gdt_descr, %ebx) in !CONFIG_SMP doesnt need the a value in 
> ebx, you can just do :
>
> #define CUR_CPU(reg) /* nothing */
>   

Yep.  On the other hand, I think that's an incredibly rare path anyway,
so it won't make any difference either way.

>> --- a/include/asm-i386/pda.h	Tue Nov 21 18:54:56 2006 -0800
>> +++ b/include/asm-i386/pda.h	Wed Nov 22 02:35:24 2006 -0800
>> @@ -22,6 +22,16 @@ extern struct i386_pda *_cpu_pda[];
>>
>>     
>
> My patch was better IMHO : we dont need to force asm () instructions to 
> perform regular C variable reading/writing in !CONFIG_SMP case.
>
> Using plain C allows compiler to generate a better code.
>   

Probably, but I'm interested in comparing apples with apples; how much
do the actual segment prefixes make a difference?

    J
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