linux-kernel - Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler optimization

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Date:	Fri, 2 Aug 2013 18:27:22 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Joonsoo Kim <iamjoonsoo.kim@....com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, Joonsoo Kim <js1304@...il.com>,
	Minchan Kim <minchan@...nel.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>
Subject: Re: [PATCH 1/4] mm, page_alloc: add likely macro to help compiler
 optimization

On Fri 02-08-13 11:07:56, Joonsoo Kim wrote:
> We rarely allocate a page with ALLOC_NO_WATERMARKS and it is used
> in slow path. For making fast path more faster, add likely macro to
> help compiler optimization.

The code is different in mmotm tree (see mm: page_alloc: rearrange
watermark checking in get_page_from_freelist)

Besides that, make sure you provide numbers which prove your claims
about performance optimizations.

> Signed-off-by: Joonsoo Kim <iamjoonsoo.kim@....com>
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index b100255..86ad44b 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1901,7 +1901,7 @@ zonelist_scan:
>  			goto this_zone_full;
>  
>  		BUILD_BUG_ON(ALLOC_NO_WATERMARKS < NR_WMARK);
> -		if (!(alloc_flags & ALLOC_NO_WATERMARKS)) {
> +		if (likely(!(alloc_flags & ALLOC_NO_WATERMARKS))) {
>  			unsigned long mark;
>  			int ret;
>  
> -- 
> 1.7.9.5
> 
> --
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-- 
Michal Hocko
SUSE Labs
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