From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f170.google.com (mail-wi0-f170.google.com [209.85.212.170]) by kanga.kvack.org (Postfix) with ESMTP id D308A6B0031 for ; Mon, 16 Jun 2014 17:00:22 -0400 (EDT) Received: by mail-wi0-f170.google.com with SMTP id cc10so5923999wib.5 for ; Mon, 16 Jun 2014 14:00:22 -0700 (PDT) Received: from jenni1.inet.fi (mta-out1.inet.fi. [62.71.2.198]) by mx.google.com with ESMTP id uy7si21025680wjc.123.2014.06.16.14.00.21 for ; Mon, 16 Jun 2014 14:00:21 -0700 (PDT) Date: Mon, 16 Jun 2014 23:59:46 +0300 From: "Kirill A. Shutemov" Subject: Re: [PATCH] mm, thp: move invariant bug check out of loop in __split_huge_page_map Message-ID: <20140616205946.GB14208@node.dhcp.inet.fi> References: <1402947348-60655-1-git-send-email-Waiman.Long@hp.com> <20140616204934.GA14208@node.dhcp.inet.fi> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140616204934.GA14208@node.dhcp.inet.fi> Sender: owner-linux-mm@kvack.org List-ID: To: Waiman Long Cc: Andrew Morton , Mel Gorman , Rik van Riel , Ingo Molnar , Peter Zijlstra , linux-kernel@vger.kernel.org, linux-mm@kvack.org, Scott J Norton 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 -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org