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Date: Mon, 29 Nov 2010 11:13:12 -0600 (CST)
From: Christoph Lameter <cl@linux.com>
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To: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
cc: akpm@linux-foundation.org, Pekka Enberg <penberg@cs.helsinki.fi>,
        linux-kernel@vger.kernel.org, Eric Dumazet <eric.dumazet@gmail.com>,
        Tejun Heo <tj@kernel.org>
Subject: Re: [thisops uV2 02/10] vmstat: Optimize zone counter modifications
 through the use of this cpu operations
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We could do this with local cmpxchgs like in the following patch. This
would avoid preemption disable and interrupt disable (at least on x86).
Trouble is how do we make this fit for architectures that do not have
cmpxchg?


Index: linux-2.6/mm/vmstat.c
===================================================================
--- linux-2.6.orig/mm/vmstat.c	2010-11-29 10:58:52.000000000 -0600
+++ linux-2.6/mm/vmstat.c	2010-11-29 11:11:34.000000000 -0600
@@ -169,18 +169,23 @@ void __mod_zone_page_state(struct zone *
 {
 	struct per_cpu_pageset __percpu *pcp = zone->pageset;
 	s8 __percpu *p = pcp->vm_stat_diff + item;
-	long x;
-	long t;
+	long o, n, t, z;

-	x = delta + __this_cpu_read(*p);
+	do {
+		z = 0;
+		t = this_cpu_read(pcp->stat_threshold);
+		o = this_cpu_read(*p);
+		n = delta + o;
+
+		if (n > t || n < -t) {
+			/* Overflow must be added to zone counters */
+			z = n;
+			n = 0;
+		}
+	} while (o != n && this_cpu_cmpxchg(*p, o, n) != o);

-	t = __this_cpu_read(pcp->stat_threshold);
-
-	if (unlikely(x > t || x < -t)) {
-		zone_page_state_add(x, zone, item);
-		x = 0;
-	}
-	__this_cpu_write(*p, x);
+	if (z)
+		zone_page_state_add(z, zone, item);
 }
 EXPORT_SYMBOL(__mod_zone_page_state);

@@ -190,11 +195,7 @@ EXPORT_SYMBOL(__mod_zone_page_state);
 void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
 					int delta)
 {
-	unsigned long flags;
-
-	local_irq_save(flags);
 	__mod_zone_page_state(zone, item, delta);
-	local_irq_restore(flags);
 }
 EXPORT_SYMBOL(mod_zone_page_state);