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

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Message-ID: <20141029143838.GC25226@htj.dyndns.org>
Date:	Wed, 29 Oct 2014 10:38:38 -0400
From:	Tejun Heo <tj@...nel.org>
To:	pang.xunlei@....com.cn
Cc:	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>,
	Lai Jiangshan <laijs@...fujitsu.com>
Subject: Re: [PATCH] idr: optimize ida_init() to avoid an extra memset

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.

Thanks.

-- 
tejun
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