linux-kernel - Re: [PATCH] mm: Add unlikely for current

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Message-ID: <alpine.DEB.2.02.1306161103020.22688@chino.kir.corp.google.com>
Date:	Sun, 16 Jun 2013 11:04:50 -0700 (PDT)
From:	David Rientjes <rientjes@...gle.com>
To:	Zhang Yanfei <zhangyanfei.yes@...il.com>
cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Linux MM <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: Add unlikely for current_order test

On Sat, 15 Jun 2013, Zhang Yanfei wrote:

> From: Zhang Yanfei <zhangyanfei@...fujitsu.com>
> 
> Since we have an unlikely for the "current_order >= pageblock_order / 2"
> test above, adding an unlikely for this "current_order >= pageblock_order"
> test seems more appropriate.
> 

I don't understand the justification at all, current_order being unlikely 
greater than or equal to pageblock_order / 2 doesn't imply at all that 
it's unlikely that current_order is greater than or equal to 
pageblock_order.

> Signed-off-by: Zhang Yanfei <zhangyanfei@...fujitsu.com>
> ---
>  mm/page_alloc.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index c3edb62..1b6d7de 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1071,7 +1071,7 @@ __rmqueue_fallback(struct zone *zone, int order, int start_migratetype)
>  			rmv_page_order(page);
>  
>  			/* Take ownership for orders >= pageblock_order */
> -			if (current_order >= pageblock_order &&
> +			if (unlikely(current_order >= pageblock_order) &&
>  			    !is_migrate_cma(migratetype))
>  				change_pageblock_range(page, current_order,
>  							start_migratetype);
--
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