linux-kernel - Re: hackbench regression due to commit 9dfc6e68bfe6e

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Date:	Wed, 07 Apr 2010 19:49:59 +0300
From:	Pekka Enberg <penberg@...helsinki.fi>
To:	Christoph Lameter <cl@...ux-foundation.org>
CC:	"Zhang, Yanmin" <yanmin_zhang@...ux.intel.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	netdev <netdev@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	alex.shi@...el.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Ma, Ling" <ling.ma@...el.com>,
	"Chen, Tim C" <tim.c.chen@...el.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: hackbench regression due to commit 9dfc6e68bfe6e

Christoph Lameter wrote:
> I wonder if this is not related to the kmem_cache_cpu structure straggling
> cache line boundaries under some conditions. On 2.6.33 the kmem_cache_cpu
> structure was larger and therefore tight packing resulted in different
> alignment.
> 
> Could you see how the following patch affects the results. It attempts to
> increase the size of kmem_cache_cpu to a power of 2 bytes. There is also
> the potential that other per cpu fetches to neighboring objects affect the
> situation. We could cacheline align the whole thing.
> 
> ---
>  include/linux/slub_def.h |    5 +++++
>  1 file changed, 5 insertions(+)
> 
> Index: linux-2.6/include/linux/slub_def.h
> ===================================================================
> --- linux-2.6.orig/include/linux/slub_def.h	2010-04-07 11:33:50.000000000 -0500
> +++ linux-2.6/include/linux/slub_def.h	2010-04-07 11:35:18.000000000 -0500
> @@ -38,6 +38,11 @@ struct kmem_cache_cpu {
>  	void **freelist;	/* Pointer to first free per cpu object */
>  	struct page *page;	/* The slab from which we are allocating */
>  	int node;		/* The node of the page (or -1 for debug) */
> +#ifndef CONFIG_64BIT
> +	int dummy1;
> +#endif
> +	unsigned long dummy2;
> +
>  #ifdef CONFIG_SLUB_STATS
>  	unsigned stat[NR_SLUB_STAT_ITEMS];
>  #endif

Would __cacheline_aligned_in_smp do the trick here?
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