From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753270AbaIHIE1 (ORCPT ); Mon, 8 Sep 2014 04:04:27 -0400 Received: from cantor2.suse.de ([195.135.220.15]:50244 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752852AbaIHIEZ (ORCPT ); Mon, 8 Sep 2014 04:04:25 -0400 Message-ID: <540D6305.8020409@suse.cz> Date: Mon, 08 Sep 2014 10:04:21 +0200 From: Vlastimil Babka User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.7.0 MIME-Version: 1.0 To: Andrew Morton CC: David Rientjes , linux-mm@kvack.org, linux-kernel@vger.kernel.org, Joonsoo Kim , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel , Mel Gorman , Minchan Kim , Zhang Yanfei Subject: Re: [PATCH v6 11/13] mm, compaction: skip buddy pages by their order in the migrate scanner References: <1407142524-2025-1-git-send-email-vbabka@suse.cz> <1407142524-2025-12-git-send-email-vbabka@suse.cz> <20140821151115.bcc66c15d53f7dc89d1b9b73@linux-foundation.org> In-Reply-To: <20140821151115.bcc66c15d53f7dc89d1b9b73@linux-foundation.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 08/22/2014 12:11 AM, Andrew Morton wrote: > On Mon, 4 Aug 2014 10:55:22 +0200 Vlastimil Babka wrote: > >> The migration scanner skips PageBuddy pages, but does not consider their order >> as checking page_order() is generally unsafe without holding the zone->lock, >> and acquiring the lock just for the check wouldn't be a good tradeoff. >> >> Still, this could avoid some iterations over the rest of the buddy page, and >> if we are careful, the race window between PageBuddy() check and page_order() >> is small, and the worst thing that can happen is that we skip too much and miss >> some isolation candidates. This is not that bad, as compaction can already fail >> for many other reasons like parallel allocations, and those have much larger >> race window. >> >> This patch therefore makes the migration scanner obtain the buddy page order >> and use it to skip the whole buddy page, if the order appears to be in the >> valid range. >> >> It's important that the page_order() is read only once, so that the value used >> in the checks and in the pfn calculation is the same. But in theory the >> compiler can replace the local variable by multiple inlines of page_order(). >> Therefore, the patch introduces page_order_unsafe() that uses ACCESS_ONCE to >> prevent this. >> >> Testing with stress-highalloc from mmtests shows a 15% reduction in number of >> pages scanned by migration scanner. The reduction is >60% with __GFP_NO_KSWAPD >> allocations, along with success rates better by few percent. >> This change is also a prerequisite for a later patch which is detecting when >> a cc->order block of pages contains non-buddy pages that cannot be isolated, >> and the scanner should thus skip to the next block immediately. > > What is this "later patch"? Or is the changelog stale? Yes it is stale, that later patch was postponed due to apparent bad effect on fragmentation. I guess we can drop the last paragraph from this commit log.