Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752146AbaFWDFF (ORCPT ); Sun, 22 Jun 2014 23:05:05 -0400 Received: from lgeamrelo01.lge.com ([156.147.1.125]:34620 "EHLO lgeamrelo01.lge.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751392AbaFWDFE (ORCPT ); Sun, 22 Jun 2014 23:05:04 -0400 X-Original-SENDERIP: 10.177.220.169 X-Original-MAILFROM: minchan@kernel.org Date: Mon, 23 Jun 2014 12:05:41 +0900 From: Minchan Kim To: Vlastimil Babka Cc: linux-mm@kvack.org, Andrew Morton , David Rientjes , Mel Gorman , Joonsoo Kim , Michal Nazarewicz , Naoya Horiguchi , Christoph Lameter , Rik van Riel , Zhang Yanfei , linux-kernel@vger.kernel.org Subject: Re: [PATCH v3 09/13] mm, compaction: skip buddy pages by their order in the migrate scanner Message-ID: <20140623030541.GE12413@bbox> References: <1403279383-5862-1-git-send-email-vbabka@suse.cz> <1403279383-5862-10-git-send-email-vbabka@suse.cz> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <1403279383-5862-10-git-send-email-vbabka@suse.cz> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jun 20, 2014 at 05:49:39PM +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. 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. > > Signed-off-by: Vlastimil Babka Acked-by: Minchan Kim -- Kind regards, Minchan Kim -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/