vmstat: Optimize zone counter modifications through the use of this cpu operations

[Christoph Lameter <cl@xxxxxxxxx>]

this cpu operations can be used to slightly optimize the functions. The
changes will avoid some address calculations and replace them with the
use of the percpu segment register.

If one would have this_cpu_inc_return and this_cpu_dec_return then it
would be possible to optimize inc_zone_page_state and dec_zone_page_state even
more.

before:

linux-2.6$ size mm/vmstat.o
   text	   data	    bss	    dec	    hex	filename
   8914	    606	    264	   9784	   2638	mm/vmstat.o

after:

linux-2.6$ size mm/vmstat.o
   text	   data	    bss	    dec	    hex	filename
   8904	    606	    264	   9774	   262e	mm/vmstat.o

Signed-off-by: Christoph Lameter <cl@xxxxxxxxx>

---
 mm/vmstat.c |   56 ++++++++++++++++++++++++++++++++------------------------
 1 file changed, 32 insertions(+), 24 deletions(-)

Index: linux-2.6/mm/vmstat.c
===================================================================
--- linux-2.6.orig/mm/vmstat.c	2010-11-09 11:18:22.000000000 -0600
+++ linux-2.6/mm/vmstat.c	2010-11-09 11:19:06.000000000 -0600
@@ -167,18 +167,20 @@ static void refresh_zone_stat_thresholds
 void __mod_zone_page_state(struct zone *zone, enum zone_stat_item item,
 				int delta)
 {
-	struct per_cpu_pageset *pcp = this_cpu_ptr(zone->pageset);
-
-	s8 *p = pcp->vm_stat_diff + item;
+	struct per_cpu_pageset * __percpu pcp = zone->pageset;
+	s8 * __percpu p = pcp->vm_stat_diff + item;
 	long x;
+	long t;
+
+	x = delta + __this_cpu_read(*p);

-	x = delta + *p;
+	t = __this_cpu_read(pcp->stat_threshold);

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

@@ -221,16 +223,19 @@ EXPORT_SYMBOL(mod_zone_page_state);
  */
 void __inc_zone_state(struct zone *zone, enum zone_stat_item item)
 {
-	struct per_cpu_pageset *pcp = this_cpu_ptr(zone->pageset);
-	s8 *p = pcp->vm_stat_diff + item;
-
-	(*p)++;
+	struct per_cpu_pageset * __percpu pcp = zone->pageset;
+	s8 * __percpu p = pcp->vm_stat_diff + item;
+	int v, t;
+
+	__this_cpu_inc(*p);
+
+	v = __this_cpu_read(*p);
+	t = __this_cpu_read(pcp->stat_threshold);
+	if (unlikely(v > t)) {
+		int overstep = t / 2;

-	if (unlikely(*p > pcp->stat_threshold)) {
-		int overstep = pcp->stat_threshold / 2;
-
-		zone_page_state_add(*p + overstep, zone, item);
-		*p = -overstep;
+		zone_page_state_add(v + overstep, zone, item);
+		__this_cpu_write(*p, overstep);
 	}
 }

@@ -242,16 +247,19 @@ EXPORT_SYMBOL(__inc_zone_page_state);

 void __dec_zone_state(struct zone *zone, enum zone_stat_item item)
 {
-	struct per_cpu_pageset *pcp = this_cpu_ptr(zone->pageset);
-	s8 *p = pcp->vm_stat_diff + item;
-
-	(*p)--;
-
-	if (unlikely(*p < - pcp->stat_threshold)) {
-		int overstep = pcp->stat_threshold / 2;
+	struct per_cpu_pageset * __percpu pcp = zone->pageset;
+	s8 * __percpu p = pcp->vm_stat_diff + item;
+	int v, t;
+
+	__this_cpu_dec(*p);
+
+	v = __this_cpu_read(*p);
+	t = __this_cpu_read(pcp->stat_threshold);
+	if (unlikely(v < - t)) {
+		int overstep = t / 2;

-		zone_page_state_add(*p - overstep, zone, item);
-		*p = overstep;
+		zone_page_state_add(v - overstep, zone, item);
+		__this_cpu_write(*p, overstep);
 	}
 }

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