linux-kernel - Re: [PATCH] idr: optimize ida_init() to avoid an extra memset

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Date:	Thu, 30 Oct 2014 10:06:45 +0800
From:	Lai Jiangshan <laijs@...fujitsu.com>
To:	Tejun Heo <tj@...nel.org>
CC:	<pang.xunlei@....com.cn>, <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH] idr: optimize ida_init() to avoid an extra memset

On 10/29/2014 10:38 PM, Tejun Heo wrote:
> On Wed, Oct 29, 2014 at 05:26:34PM +0800, pang.xunlei@....com.cn wrote:
>> The memset in ida_init() already handles idr, so there's some
>> redundancy in the following idr_init().
>>
>> This patch removes the memset, and clears ida->free_bitmap instead.
>>
>> Signed-off-by: pang.xunlei <pang.xunlei@....com.cn>
>> ---
>>  lib/idr.c |    3 +--
>>  1 file changed, 1 insertion(+), 2 deletions(-)
>>
>> diff --git a/lib/idr.c b/lib/idr.c
>> index e654aeb..bbe5779 100644
>> --- a/lib/idr.c
>> +++ b/lib/idr.c
>> @@ -1141,8 +1141,7 @@ EXPORT_SYMBOL(ida_simple_remove);
>>   */
>>  void ida_init(struct ida *ida)
>>  {
>> -   memset(ida, 0, sizeof(struct ida));
>>     idr_init(&ida->idr);
>> -
>> +   ida->free_bitmap = NULL;
> 
> I don't know.  Does this matter?  If this *really* matters, I'd much
> rather have memset(&ida->FIRST_FIELD, 0, sizeof(struct ida) - offset
> of FIRST_FIELD) to ensure that all fields get reset or implement an
> internal function like __idr_init_without_zeroing(); however, given
> the size of an idr and the low frequency of the operation, I'd prefer
> to just leave it as-is.
> 

memset(ptr, 0, sizeof()) or kzalloc() is convenient and good for buffers
but bad for structures, objects...

general way for object initialization is:

xxx_init()
{
	explicitly init every field...

	/* maybe complicated, over elaborate */
}

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