linux-kernel - Re: [PATCH 4/4] zsmalloc: zsmalloc: align cache line size

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Date:	Thu, 03 May 2012 09:58:21 -0400
From:	Nitin Gupta <ngupta@...are.org>
To:	Minchan Kim <minchan@...nel.org>
CC:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH 4/4] zsmalloc: zsmalloc: align cache line size

On 5/3/12 2:40 AM, Minchan Kim wrote:
> It's a overkill to align pool size with PAGE_SIZE to avoid
> false-sharing. This patch aligns it with just cache line size.
>
> Signed-off-by: Minchan Kim<minchan@...nel.org>
> ---
>   drivers/staging/zsmalloc/zsmalloc-main.c |    6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/staging/zsmalloc/zsmalloc-main.c b/drivers/staging/zsmalloc/zsmalloc-main.c
> index 51074fa..3991b03 100644
> --- a/drivers/staging/zsmalloc/zsmalloc-main.c
> +++ b/drivers/staging/zsmalloc/zsmalloc-main.c
> @@ -489,14 +489,14 @@ fail:
>
>   struct zs_pool *zs_create_pool(const char *name, gfp_t flags)
>   {
> -	int i, error, ovhd_size;
> +	int i, error;
>   	struct zs_pool *pool;
>
>   	if (!name)
>   		return NULL;
>
> -	ovhd_size = roundup(sizeof(*pool), PAGE_SIZE);
> -	pool = kzalloc(ovhd_size, GFP_KERNEL);
> +	pool = kzalloc(ALIGN(sizeof(*pool), cache_line_size()),
> +				GFP_KERNEL);

a basic question:
  Is rounding off allocation size to cache_line_size enough to ensure 
that the object is cache-line-aligned? Isn't it possible that even 
though the object size is multiple of cache-line, it may still not be 
properly aligned and end up sharing cache line with some other 
read-mostly object?

Thanks,
Nitin


>   	if (!pool)
>   		return NULL;
>

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