linux-kernel - Re: [PATCH v2 2/2] mm, thp: replace smp_mb after atomic_add by smp_mb__after

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Message-ID: <20140618121731.GA5957@node.dhcp.inet.fi>
Date:	Wed, 18 Jun 2014 15:17:31 +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 2/2] mm, thp: replace smp_mb after atomic_add by
 smp_mb__after_atomic

On Tue, Jun 17, 2014 at 06:37:59PM -0400, Waiman Long wrote:
> In some architectures like x86, atomic_add() is a full memory
> barrier. In that case, an additional smp_mb() is just a waste of time.
> This patch replaces that smp_mb() by smp_mb__after_atomic() which
> will avoid the redundant memory barrier in some architectures.
> 
> With a 3.16-rc1 based kernel, this patch reduced the execution time
> of breaking 1000 transparent huge pages from 38,245us to 30,964us. A
> reduction of 19% which is quite sizeable. It also reduces the %cpu
> time of the __split_huge_page_refcount function in the perf profile
> from 2.18% to 1.15%.
> 
> Signed-off-by: Waiman Long <Waiman.Long@...com>

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

-- 
 Kirill A. Shutemov
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