linux-kernel - Re: [thisops uV3 07/18] highmem: Use this_cpu_xx

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Date:	Tue, 30 Nov 2010 20:13:28 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christoph Lameter <cl@...ux.com>
Cc:	akpm@...ux-foundation.org, Pekka Enberg <penberg@...helsinki.fi>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Catalin Marinas <catalin.marinas@....com>,
	linux-kernel@...r.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Tejun Heo <tj@...nel.org>, linux-mm@...ck.org
Subject: Re: [thisops uV3 07/18] highmem: Use this_cpu_xx_return()
 operations

Le mardi 30 novembre 2010 à 13:07 -0600, Christoph Lameter a écrit :
> pièce jointe document texte brut (this_cpu_highmem)
> Use this_cpu operations to optimize access primitives for highmem.
> 
> The main effect is the avoidance of address calculations through the
> use of a segment prefix.
> 
> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> Cc: Catalin Marinas <catalin.marinas@....com>
> Signed-off-by: Christoph Lameter <cl@...ux.com>
> 
> ---
>  include/linux/highmem.h |    7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> Index: linux-2.6/include/linux/highmem.h
> ===================================================================
> --- linux-2.6.orig/include/linux/highmem.h	2010-11-22 14:43:40.000000000 -0600
> +++ linux-2.6/include/linux/highmem.h	2010-11-22 14:45:02.000000000 -0600
> @@ -81,7 +81,8 @@ DECLARE_PER_CPU(int, __kmap_atomic_idx);
>  
>  static inline int kmap_atomic_idx_push(void)
>  {
> -	int idx = __get_cpu_var(__kmap_atomic_idx)++;
> +	int idx = __this_cpu_inc_return(__kmap_atomic_idx) - 1;
> +
>  #ifdef CONFIG_DEBUG_HIGHMEM
>  	WARN_ON_ONCE(in_irq() && !irqs_disabled());
>  	BUG_ON(idx > KM_TYPE_NR);
> @@ -91,12 +92,12 @@ static inline int kmap_atomic_idx_push(v
>  
>  static inline int kmap_atomic_idx(void)
>  {
> -	return __get_cpu_var(__kmap_atomic_idx) - 1;
> +	return __this_cpu_read(__kmap_atomic_idx) - 1;
>  }
>  
>  static inline int kmap_atomic_idx_pop(void)
>  {
> -	int idx = --__get_cpu_var(__kmap_atomic_idx);
> +	int idx = __this_cpu_dec_return(__kmap_atomic_idx);

__this_cpu_dec_return() is only needed if CONFIG_DEBUG_HIGHMEM

>  #ifdef CONFIG_DEBUG_HIGHMEM
>  	BUG_ON(idx < 0);
>  #endif
> 

You could change kmap_atomic_idx_pop() to return void, and use
__this_cpu_dec(__kmap_atomic_idx)


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