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Date: Wed, 1 Sep 2010 19:54:04 -0500 (CDT)
From: Christoph Lameter <cl@linux.com>
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To: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
cc: Mel Gorman <mel@csn.ul.ie>, Andrew Morton <akpm@linux-foundation.org>,
        Linux Kernel List <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Rik van Riel <riel@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>,
        Minchan Kim <minchan.kim@gmail.com>,
        KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.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
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On Wed, 1 Sep 2010, Christoph Lameter wrote:

> The effect needs to be the same as retrieving a global or
> zone ZVC counter. Which is currently implemented in the following way:
>
> static inline unsigned long zone_page_state(struct zone *zone,
>                                         enum zone_stat_item item)
> {
>         long x = atomic_long_read(&zone->vm_stat[item]);
> #ifdef CONFIG_SMP
>         if (x < 0)
>                 x = 0;
> #endif
>         return x;
> }
>

Here is a patch that defined a snapshot function that works in the same
way:

Subject: Add a snapshot function for vm statistics

Add a snapshot function that can more accurately determine
the current value of a zone counter.

Signed-off-by: Christoph Lameter <cl@linux.com>


Index: linux-2.6/include/linux/vmstat.h
===================================================================
--- linux-2.6.orig/include/linux/vmstat.h	2010-09-01 19:45:23.506071189 -0500
+++ linux-2.6/include/linux/vmstat.h	2010-09-01 19:53:02.978979081 -0500
@@ -170,6 +170,28 @@
 	return x;
 }

+/*
+ * More accurate version that also considers the currently pending
+ * deltas. For that we need to loop over all cpus to find the current
+ * deltas. There is no synchronization so the result cannot be
+ * exactly accurate either.
+ */
+static inline unsigned long zone_page_state_snapshot(struct zone *zone,
+					enum zone_stat_item item)
+{
+	int cpu;
+	long x = atomic_long_read(&zone->vm_stat[item]);
+
+#ifdef CONFIG_SMP
+	for_each_online_cpu(cpu)
+		x += per_cpu_ptr(zone->pageset, cpu)->vm_stat_diff[item];
+
+	if (x < 0)
+		x = 0;
+#endif
+	return x;
+}
+
 extern unsigned long global_reclaimable_pages(void);
 extern unsigned long zone_reclaimable_pages(struct zone *zone);