linux-kernel - Re: [PATCH v2 1/2] mm, thp: move invariant bug check out of loop in __split_huge_page

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Date:	Wed, 18 Jun 2014 11:31:04 -0400
From:	Waiman Long <waiman.long@...com>
To:	"Kirill A. Shutemov" <kirill@...temov.name>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	linux-kernel@...r.kernel.org, linux-mm@...ck.org,
	Scott J Norton <scott.norton@...com>
Subject: Re: [PATCH v2 1/2] mm, thp: move invariant bug check out of loop
 in __split_huge_page_map

On 06/18/2014 08:24 AM, Kirill A. Shutemov wrote:
> On Tue, Jun 17, 2014 at 06:37:58PM -0400, Waiman Long wrote:
>> In the __split_huge_page_map() function, the check for
>> page_mapcount(page) is invariant within the for loop. Because of the
>> fact that the macro is implemented using atomic_read(), the redundant
>> check cannot be optimized away by the compiler leading to unnecessary
>> read to the page structure.
>>
>> This patch moves the invariant bug check out of the loop so that it
>> will be done only once. On a 3.16-rc1 based kernel, the execution
>> time of a microbenchmark that broke up 1000 transparent huge pages
>> using munmap() had an execution time of 38,245us and 38,548us with
>> and without the patch respectively. The performance gain is about 1%.
> For this low difference it would be nice to average over few runs +
> stddev. It can easily can be a noise.

The timing data was the average of 5 runs with a SD of 100-200us.
>> Signed-off-by: Waiman Long<Waiman.Long@...com>
> But okay:
>
> Acked-by: Kirill A. Shutemov<kirill.shutemov@...ux.intel.com>
>

Thank for the review.

-Longman
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