From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f44.google.com (mail-pa0-f44.google.com [209.85.220.44]) by kanga.kvack.org (Postfix) with ESMTP id E8FE06B0035 for ; Thu, 21 Aug 2014 18:11:18 -0400 (EDT) Received: by mail-pa0-f44.google.com with SMTP id eu11so15070376pac.3 for ; Thu, 21 Aug 2014 15:11:18 -0700 (PDT) Received: from mail.linuxfoundation.org (mail.linuxfoundation.org. [140.211.169.12]) by mx.google.com with ESMTPS id km10si38098426pbd.132.2014.08.21.15.11.17 for (version=TLSv1.2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 21 Aug 2014 15:11:17 -0700 (PDT) Date: Thu, 21 Aug 2014 15:11:15 -0700 From: Andrew Morton 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> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: 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 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? -- 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