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

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Message-ID: <20140618122442.GB5957@node.dhcp.inet.fi>
Date:	Wed, 18 Jun 2014 15:24:42 +0300
From:	"Kirill A. Shutemov" <kirill@...temov.name>
To:	Waiman Long <Waiman.Long@...com>
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 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.

> Signed-off-by: Waiman Long <Waiman.Long@...com>

But okay:

Acked-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>

-- 
 Kirill A. Shutemov
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