From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail202.messagelabs.com (mail202.messagelabs.com [216.82.254.227]) by kanga.kvack.org (Postfix) with ESMTP id 10C476B0047 for ; Wed, 1 Sep 2010 03:24:18 -0400 (EDT) Date: Wed, 1 Sep 2010 08:24:02 +0100 From: Mel Gorman Subject: Re: [PATCH 2/3] mm: page allocator: Calculate a better estimate of NR_FREE_PAGES when memory is low and kswapd is awake Message-ID: <20100901072402.GE13677@csn.ul.ie> References: <1283276257-1793-1-git-send-email-mel@csn.ul.ie> <1283276257-1793-3-git-send-email-mel@csn.ul.ie> <20100901083425.971F.A69D9226@jp.fujitsu.com> MIME-Version: 1.0 Content-Type: text/plain; charset=iso-8859-15 Content-Disposition: inline In-Reply-To: <20100901083425.971F.A69D9226@jp.fujitsu.com> Sender: owner-linux-mm@kvack.org To: KOSAKI Motohiro Cc: 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; } -- Mel Gorman Part-time Phd Student Linux Technology Center University of Limerick IBM Dublin Software Lab -- 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