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

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Date:	Mon, 16 Jun 2014 23:59:46 +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] mm, thp: move invariant bug check out of loop in
 __split_huge_page_map

On Mon, Jun 16, 2014 at 11:49:34PM +0300, Kirill A. Shutemov wrote:
> On Mon, Jun 16, 2014 at 03:35:48PM -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.

And atomic_read() is *not* atomic operation. It's implemented as
dereferencing though cast to volatile, which suppress compiler
optimization, but doesn't affect what CPU can do with the variable.

So I doubt difference will be measurable anywhere.

-- 
 Kirill A. Shutemov
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