From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752172AbdBMKvW (ORCPT ); Mon, 13 Feb 2017 05:51:22 -0500 Received: from outbound-smtp03.blacknight.com ([81.17.249.16]:57205 "EHLO outbound-smtp03.blacknight.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750853AbdBMKvU (ORCPT ); Mon, 13 Feb 2017 05:51:20 -0500 Date: Mon, 13 Feb 2017 10:51:18 +0000 From: Mel Gorman To: Vlastimil Babka Cc: linux-mm@kvack.org, Johannes Weiner , Joonsoo Kim , David Rientjes , linux-kernel@vger.kernel.org, kernel-team@fb.com Subject: Re: [PATCH v2 03/10] mm, page_alloc: split smallest stolen page in fallback Message-ID: <20170213105118.f3y5y2xaf2kdnxv7@techsingularity.net> References: <20170210172343.30283-1-vbabka@suse.cz> <20170210172343.30283-4-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20170210172343.30283-4-vbabka@suse.cz> User-Agent: Mutt/1.6.2 (2016-07-01) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Feb 10, 2017 at 06:23:36PM +0100, Vlastimil Babka wrote: > The __rmqueue_fallback() function is called when there's no free page of > requested migratetype, and we need to steal from a different one. There are > various heuristics to make this event infrequent and reduce permanent > fragmentation. The main one is to try stealing from a pageblock that has the > most free pages, and possibly steal them all at once and convert the whole > pageblock. Precise searching for such pageblock would be expensive, so instead > the heuristics walks the free lists from MAX_ORDER down to requested order and > assumes that the block with highest-order free page is likely to also have the > most free pages in total. > > Chances are that together with the highest-order page, we steal also pages of > lower orders from the same block. But then we still split the highest order > page. This is wasteful and can contribute to fragmentation instead of avoiding > it. > The original intent was that if an allocation request was stealing a pageblock that taking the largest one would reduce the likelihood of a steal in the near future by the same type. > This patch thus changes __rmqueue_fallback() to just steal the page(s) and put > them on the freelist of the requested migratetype, and only report whether it > was successful. Then we pick (and eventually split) the smallest page with > __rmqueue_smallest(). This all happens under zone lock, so nobody can steal it > from us in the process. This should reduce fragmentation due to fallbacks. At > worst we are only stealing a single highest-order page and waste some cycles by > moving it between lists and then removing it, but fallback is not exactly hot > path so that should not be a concern. As a side benefit the patch removes some > duplicate code by reusing __rmqueue_smallest(). > > Signed-off-by: Vlastimil Babka But conceptually this is better so Acked-by: Mel Gorman -- Mel Gorman SUSE Labs