linux-kernel - Re: [PATCH 2/3] mm: page allocator: Calculate a better estimate of NR_FREE_PAGES when memory is low and kswapd is awake

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Date:	Wed, 1 Sep 2010 15:16:59 -0500 (CDT)
From:	Christoph Lameter <cl@...ux.com>
To:	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>
cc:	Mel Gorman <mel@....ul.ie>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel List <linux-kernel@...r.kernel.org>,
	linux-mm@...ck.org, Rik van Riel <riel@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Minchan Kim <minchan.kim@...il.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Subject: Re: [PATCH 2/3] mm: page allocator: Calculate a better estimate of
 NR_FREE_PAGES when memory is low and kswapd is awake

On Wed, 1 Sep 2010, KOSAKI Motohiro wrote:

> > 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;

Not sure what the point here is. If the delta is going below zero then
there was a concurrent operation updating the counters negatively while
we summed up the counters. It is then safe to assume a value of zero. We
cannot really be more accurate than that.

so

	if (delta < 0)
		delta = 0;

would be correct. See also handling of counter underflow in
vmstat.h:zone_page_state(). As I have said before: I would rather have the
counter handling in one place to avoid creating differences in counter
handling.
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