Re: [PATCH] Reduce vm_stat cacheline contention in __vm_enough_memory

[Christoph Lameter <cl@gentwo.org>]

Also the whole thing could be optimized by concentrating updates to the
vm_stat array at one point in time. If any local per cpu differential
overflows then update all the counters in the same cacheline for which we have per cpu
differentials.

That will defer another acquisition of the cacheline for the next delta
overflowing. After an update all the per cpu differentials would be zero.

This could be added to zone_page_state_add....


Something like this patch? (Restriction of the updates to the same
cacheline missing. Just does everything and the zone_page_state may need
uninlining now)

---
 include/linux/vmstat.h |   19 ++++++++++++++++---
 mm/vmstat.c            |   10 ++++------
 2 files changed, 20 insertions(+), 9 deletions(-)

Index: linux-2.6/include/linux/vmstat.h
===================================================================
--- linux-2.6.orig/include/linux/vmstat.h	2011-10-14 09:58:03.000000000 -0500
+++ linux-2.6/include/linux/vmstat.h	2011-10-14 10:08:00.000000000 -0500
@@ -90,10 +90,23 @@ static inline void vm_events_fold_cpu(in
 extern atomic_long_t vm_stat[NR_VM_ZONE_STAT_ITEMS];

 static inline void zone_page_state_add(long x, struct zone *zone,
-				 enum zone_stat_item item)
+				 enum zone_stat_item item, s8 new_value)
 {
-	atomic_long_add(x, &zone->vm_stat[item]);
-	atomic_long_add(x, &vm_stat[item]);
+	enum zone_stat_item i;
+
+	for (i = 0; i < NR_VM_EVENT_ITEMS; i++) {
+		long y;
+
+		if (i == item)
+			y = this_cpu_xchg(zone->pageset->vm_stat_diff[i], new_value) + x;
+		else
+			y = this_cpu_xchg(zone->pageset->vm_stat_diff[i], 0);
+
+		if (y) {
+			atomic_long_add(y, &zone->vm_stat[item]);
+			atomic_long_add(y, &vm_stat[item]);
+		}
+	}
 }

 static inline unsigned long global_page_state(enum zone_stat_item item)
Index: linux-2.6/mm/vmstat.c
===================================================================
--- linux-2.6.orig/mm/vmstat.c	2011-10-14 10:04:20.000000000 -0500
+++ linux-2.6/mm/vmstat.c	2011-10-14 10:08:39.000000000 -0500
@@ -221,7 +221,7 @@ void __mod_zone_page_state(struct zone *
 	t = __this_cpu_read(pcp->stat_threshold);

 	if (unlikely(x > t || x < -t)) {
-		zone_page_state_add(x, zone, item);
+		zone_page_state_add(x, zone, item, 0);
 		x = 0;
 	}
 	__this_cpu_write(*p, x);
@@ -262,8 +262,7 @@ void __inc_zone_state(struct zone *zone,
 	if (unlikely(v > t)) {
 		s8 overstep = t >> 1;

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

@@ -284,8 +283,7 @@ void __dec_zone_state(struct zone *zone,
 	if (unlikely(v < - t)) {
 		s8 overstep = t >> 1;

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

@@ -343,7 +341,7 @@ static inline void mod_state(struct zone
 	} while (this_cpu_cmpxchg(*p, o, n) != o);

 	if (z)
-		zone_page_state_add(z, zone, item);
+		zone_page_state_add(z, zone, item, 0);
 }

 void mod_zone_page_state(struct zone *zone, enum zone_stat_item item,

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