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

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Date:	Mon, 7 May 2012 10:41:45 +0300 (EEST)
From:	Pekka Enberg <penberg@...nel.org>
To:	Minchan Kim <minchan@...nel.org>
cc:	Nitin Gupta <ngupta@...are.org>,
	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,
	cl@...ux-foundation.org
Subject: Re: [PATCH 4/4] zsmalloc: zsmalloc: align cache line size

On Fri, 4 May 2012, 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?
> 
> AFAIK, SLAB allocates object aligned cache-size so I think that problem cannot happen.
> But needs double check.
> Cced Pekka.

The kmalloc(size) function only gives you the following guarantees:

  (1) The allocated object is _at least_ 'size' bytes.
 
  (2) The returned pointer is aligned to ARCH_KMALLOC_MINALIGN.

Anything beyond that is implementation detail and probably will break if 
you switch between SLAB/SLUB/SLOB.

			Pekka
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