[RFC] Improving scalability of smp_mb__[before|after]_clear_bit

[Russell King - ARM Linux]

On Tue, Jul 12, 2011 at 12:34:59AM -0700, heechul Yun wrote:
> I think L2 cache sync operation, called by mb(), is not necessary for bitops.
> This patch improves lat_pagefault of lmbench by up to 11% on a A9 SMP.
> Higher proceesor
> counts can benefit more.

bitops should only be used on normal memory and not for stuff used for IO,
so this should be safe.

Could you submit it to the patch system please with Catalins ack?

Thanks.

> 
> ---
> diff --git a/arch/arm/include/asm/bitops.h b/arch/arm/include/asm/bitops.h
> index b4892a0..f428059 100644
> --- a/arch/arm/include/asm/bitops.h
> +++ b/arch/arm/include/asm/bitops.h
> @@ -26,8 +26,8 @@
>  #include <linux/compiler.h>
>  #include <asm/system.h>
> 
> -#define smp_mb__before_clear_bit()     mb()
> -#define smp_mb__after_clear_bit()      mb()
> +#define smp_mb__before_clear_bit()     smp_mb()
> +#define smp_mb__after_clear_bit()      smp_mb()
> 
>  /*
>   * These functions are the basis of our bit ops.