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

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Message-ID: <4FA33E89.6080206@kernel.org>
Date:	Fri, 04 May 2012 11:27:21 +0900
From:	Minchan Kim <minchan@...nel.org>
To:	Nitin Gupta <ngupta@...are.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,
	Pekka Enberg <penberg@...nel.org>
Subject: Re: [PATCH 4/4] zsmalloc: zsmalloc: align cache line size

On 05/03/2012 10:58 PM, Nitin Gupta wrote:

> 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?


AFAIK, SLAB allocates object aligned cache-size so I think that problem cannot happen.
But needs double check.
Cced Pekka.

> 
> Thanks,
> Nitin
> 
> 
>>       if (!pool)
>>           return NULL;
>>
> 
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-- 
Kind regards,
Minchan Kim
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