Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752576AbaG2BFi (ORCPT ); Mon, 28 Jul 2014 21:05:38 -0400 Received: from mail-ig0-f181.google.com ([209.85.213.181]:34495 "EHLO mail-ig0-f181.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751730AbaG2BFh (ORCPT ); Mon, 28 Jul 2014 21:05:37 -0400 Date: Mon, 28 Jul 2014 18:05:34 -0700 (PDT) From: David Rientjes X-X-Sender: rientjes@chino.kir.corp.google.com To: Vlastimil Babka cc: Andrew Morton , linux-kernel@vger.kernel.org, linux-mm:; Subject: Re: [PATCH v5 11/14] mm, compaction: skip buddy pages by their order in the migrate scanner In-Reply-To: <1406553101-29326-12-git-send-email-vbabka@suse.cz> Message-ID: References: <1406553101-29326-1-git-send-email-vbabka@suse.cz> <1406553101-29326-12-git-send-email-vbabka@suse.cz> User-Agent: Alpine 2.02 (DEB 1266 2009-07-14) MIME-Version: 1.0 Content-Type: TEXT/PLAIN; charset=US-ASCII Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, 28 Jul 2014, 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. > > Signed-off-by: Vlastimil Babka > Reviewed-by: Zhang Yanfei > Acked-by: Minchan Kim > Acked-by: Mel Gorman > Cc: Joonsoo Kim > Cc: Michal Nazarewicz > Cc: Naoya Horiguchi > Cc: Christoph Lameter > Cc: Rik van Riel > Cc: David Rientjes Acked-by: David Rientjes Seems I'm overruled in the definition of page_order_unsafe(). Owell, you have more than one caller so I guess it makes sense. -- 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/