From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755018AbaHUWLS (ORCPT ); Thu, 21 Aug 2014 18:11:18 -0400 Received: from mail.linuxfoundation.org ([140.211.169.12]:54979 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1754730AbaHUWLR (ORCPT ); Thu, 21 Aug 2014 18:11:17 -0400 Date: Thu, 21 Aug 2014 15:11:15 -0700 From: Andrew Morton To: Vlastimil Babka 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 Message-Id: <20140821151115.bcc66c15d53f7dc89d1b9b73@linux-foundation.org> In-Reply-To: <1407142524-2025-12-git-send-email-vbabka@suse.cz> References: <1407142524-2025-1-git-send-email-vbabka@suse.cz> <1407142524-2025-12-git-send-email-vbabka@suse.cz> X-Mailer: Sylpheed 3.2.0beta5 (GTK+ 2.24.10; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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?