From: "Kirill A. Shutemov" <kirill@shutemov.name>
To: Waiman Long <Waiman.Long@hp.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
Ingo Molnar <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
linux-kernel@vger.kernel.org, linux-mm@kvack.org,
Scott J Norton <scott.norton@hp.com>
Subject: Re: [PATCH] mm, thp: move invariant bug check out of loop in __split_huge_page_map
Date: Mon, 16 Jun 2014 23:59:46 +0300 [thread overview]
Message-ID: <20140616205946.GB14208@node.dhcp.inet.fi> (raw)
In-Reply-To: <20140616204934.GA14208@node.dhcp.inet.fi>
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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next prev parent reply other threads:[~2014-06-16 21:00 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-06-16 19:35 Waiman Long
2014-06-16 20:49 ` Kirill A. Shutemov
2014-06-16 20:59 ` Kirill A. Shutemov [this message]
2014-06-17 3:45 ` Waiman Long
2014-06-17 7:23 ` Kirill A. Shutemov
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