From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail191.messagelabs.com (mail191.messagelabs.com [216.82.242.19]) by kanga.kvack.org (Postfix) with SMTP id 0F6916B0047 for ; Wed, 1 Sep 2010 03:33:33 -0400 (EDT) Received: from m4.gw.fujitsu.co.jp ([10.0.50.74]) by fgwmail7.fujitsu.co.jp (Fujitsu Gateway) with ESMTP id o817XTFh008146 for (envelope-from kosaki.motohiro@jp.fujitsu.com); Wed, 1 Sep 2010 16:33:30 +0900 Received: from smail (m4 [127.0.0.1]) by outgoing.m4.gw.fujitsu.co.jp (Postfix) with ESMTP id 84F4845DE6E for ; Wed, 1 Sep 2010 16:33:29 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (s4.gw.fujitsu.co.jp [10.0.50.94]) by m4.gw.fujitsu.co.jp (Postfix) with ESMTP id 550E645DE60 for ; Wed, 1 Sep 2010 16:33:29 +0900 (JST) Received: from s4.gw.fujitsu.co.jp (localhost.localdomain [127.0.0.1]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id 2F6861DB8040 for ; Wed, 1 Sep 2010 16:33:29 +0900 (JST) Received: from m105.s.css.fujitsu.com (m105.s.css.fujitsu.com [10.249.87.105]) by s4.gw.fujitsu.co.jp (Postfix) with ESMTP id AE2761DB8037 for ; Wed, 1 Sep 2010 16:33:25 +0900 (JST) From: KOSAKI Motohiro Subject: Re: [PATCH 2/3] mm: page allocator: Calculate a better estimate of NR_FREE_PAGES when memory is low and kswapd is awake In-Reply-To: <20100901072402.GE13677@csn.ul.ie> References: <20100901083425.971F.A69D9226@jp.fujitsu.com> <20100901072402.GE13677@csn.ul.ie> Message-Id: <20100901163146.9755.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset="US-ASCII" Content-Transfer-Encoding: 7bit Date: Wed, 1 Sep 2010 16:33:24 +0900 (JST) Sender: owner-linux-mm@kvack.org To: Mel Gorman Cc: kosaki.motohiro@jp.fujitsu.com, Andrew Morton , Linux Kernel List , linux-mm@kvack.org, Rik van Riel , Johannes Weiner , Minchan Kim , Christoph Lameter , KAMEZAWA Hiroyuki List-ID: > On Wed, Sep 01, 2010 at 08:37:41AM +0900, KOSAKI Motohiro wrote: > > > +#ifdef CONFIG_SMP > > > +/* Called when a more accurate view of NR_FREE_PAGES is needed */ > > > +unsigned long zone_nr_free_pages(struct zone *zone) > > > +{ > > > + unsigned long nr_free_pages = zone_page_state(zone, NR_FREE_PAGES); > > > + > > > + /* > > > + * While kswapd is awake, it is considered the zone is under some > > > + * memory pressure. Under pressure, there is a risk that > > > + * per-cpu-counter-drift will allow the min watermark to be breached > > > + * potentially causing a live-lock. While kswapd is awake and > > > + * free pages are low, get a better estimate for free pages > > > + */ > > > + if (nr_free_pages < zone->percpu_drift_mark && > > > + !waitqueue_active(&zone->zone_pgdat->kswapd_wait)) { > > > + int cpu; > > > + > > > + for_each_online_cpu(cpu) { > > > + struct per_cpu_pageset *pset; > > > + > > > + pset = per_cpu_ptr(zone->pageset, cpu); > > > + nr_free_pages += pset->vm_stat_diff[NR_FREE_PAGES]; > > > > If my understanding is correct, we have no lock when reading pset->vm_stat_diff. > > It mean nr_free_pages can reach negative value at very rarely race. boundary > > check is necessary? > > > > True, well spotted. > > How about the following? It records a delta and checks if delta is negative > and would cause underflow. > > unsigned long zone_nr_free_pages(struct zone *zone) > { > unsigned long nr_free_pages = zone_page_state(zone, NR_FREE_PAGES); > long delta = 0; > > /* > * While kswapd is awake, it is considered the zone is under some > * memory pressure. Under pressure, there is a risk that > * per-cpu-counter-drift will allow the min watermark to be breached > * potentially causing a live-lock. While kswapd is awake and > * free pages are low, get a better estimate for free pages > */ > if (nr_free_pages < zone->percpu_drift_mark && > !waitqueue_active(&zone->zone_pgdat->kswapd_wait)) { > int cpu; > > for_each_online_cpu(cpu) { > struct per_cpu_pageset *pset; > > pset = per_cpu_ptr(zone->pageset, cpu); > delta += pset->vm_stat_diff[NR_FREE_PAGES]; > } > } > > /* Watch for underflow */ > if (delta < 0 && abs(delta) > nr_free_pages) > delta = -nr_free_pages; > > return nr_free_pages + delta; > } Looks good to me :) Reviewed-by: KOSAKI Motohiro Thanks. -- 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