From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from psmtp.com (na3sys010amx103.postini.com [74.125.245.103]) by kanga.kvack.org (Postfix) with SMTP id AC06B6B0044 for ; Mon, 7 May 2012 03:41:50 -0400 (EDT) Received: by lagz14 with SMTP id z14so4676728lag.14 for ; Mon, 07 May 2012 00:41:48 -0700 (PDT) Date: Mon, 7 May 2012 10:41:45 +0300 (EEST) From: Pekka Enberg Subject: Re: [PATCH 4/4] zsmalloc: zsmalloc: align cache line size In-Reply-To: <4FA33E89.6080206@kernel.org> Message-ID: References: <1336027242-372-1-git-send-email-minchan@kernel.org> <1336027242-372-4-git-send-email-minchan@kernel.org> <4FA28EFD.5070002@vflare.org> <4FA33E89.6080206@kernel.org> MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: owner-linux-mm@kvack.org List-ID: To: Minchan Kim Cc: Nitin Gupta , Greg Kroah-Hartman , Seth Jennings , Dan Magenheimer , Andrew Morton , linux-kernel@vger.kernel.org, linux-mm@kvack.org, cl@linux-foundation.org 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 > >> --- > >> 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: email@kvack.org