From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pd0-f171.google.com (mail-pd0-f171.google.com [209.85.192.171]) by kanga.kvack.org (Postfix) with ESMTP id DEAAA6B0035 for ; Thu, 12 Dec 2013 14:40:24 -0500 (EST) Received: by mail-pd0-f171.google.com with SMTP id z10so1056630pdj.2 for ; Thu, 12 Dec 2013 11:40:24 -0800 (PST) Received: from blackbird.sr71.net (www.sr71.net. [198.145.64.142]) by mx.google.com with ESMTP id fu1si1988606pbc.284.2013.12.12.11.40.22 for ; Thu, 12 Dec 2013 11:40:23 -0800 (PST) Message-ID: <52AA10E5.9040708@sr71.net> Date: Thu, 12 Dec 2013 11:39:17 -0800 From: Dave Hansen MIME-Version: 1.0 Subject: Re: [RFC][PATCH 2/3] mm: slab: move around slab ->freelist for cmpxchg References: <20131211224022.AA8CF0B9@viggo.jf.intel.com> <20131211224025.70B40B9C@viggo.jf.intel.com> <00000142e7ea519d-8906d225-c99c-44b5-b381-b573c75fd097-000000@email.amazonses.com> In-Reply-To: <00000142e7ea519d-8906d225-c99c-44b5-b381-b573c75fd097-000000@email.amazonses.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Christoph Lameter Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, kirill.shutemov@linux.intel.com, Andi Kleen On 12/12/2013 09:46 AM, Christoph Lameter wrote: > On Wed, 11 Dec 2013, Dave Hansen wrote: >> The write-argument to cmpxchg_double() must be 16-byte aligned. >> We used to align 'struct page' itself in order to guarantee this, >> but that wastes 8-bytes per page. Instead, we take 8-bytes >> internal to the page before page->counters and move freelist >> between there and the existing 8-bytes after counters. That way, >> no matter how 'stuct page' itself is aligned, we can ensure that >> we have a 16-byte area with which to to this cmpxchg. > > Well this adds additional branching to the fast paths. I don't think it *HAS* to inherently. The reason here is really that we swap the _order_ of the arguments to the cmpxchg() since their order in memory changes. Essentially, we do: | flags | freelist | counters | | | flags | | counters | freelist | I did this so I wouldn't have to make a helper for ->counters. But, if we also move counters around, we can do: | flags | counters | freelist | | | flags | | counters | freelist | I believe we can do that all with plain pointer arithmetic and masks so that it won't cost any branches. -- 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