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

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Date:	Wed, 29 Nov 2006 10:30:56 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
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

On Wednesday 29 November 2006 00:12, Jeremy Fitzhardinge wrote:

> Hi Eric,
>
> Could you try this patch out and see if it makes much performance
> difference for you.  You should apply this on top of the %fs patch I
> posted earlier (and use the %fs patch as the baseline for your
> comparisons).

Hi Jeremy

I will try this as soon as possible, thank you.

However I have some remarks browsing your patch.


> +#ifdef CONFIG_SMP
> +#define LOAD_PDA_SEG(reg)	\
> +	movl $(__KERNEL_PDA), reg; \
> +	movl reg, %fs
> +#define CUR_CPU(reg)	movl %fs:PDA_cpu, reg
> +#else
> +#define LOAD_PDA_SEG(reg)
> +#define CUR_CPU(reg)	movl boot_pda+PDA_cpu, reg

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 */


> --- 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.

Eric
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