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

[Kirill A. Shutemov]

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@xxxxxx>

But okay:

Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>

-- 
 Kirill A. Shutemov
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