[RFC][PATCH 6/6] delalloc ENOSPC: improve percpu counter accounting accurate

[Mingming Cao <cmm@us.ibm.com>]

percpu counter: update center counter when sum per-cpu counter

From: Mingming cao <cmm@us.ibm.com>

Delayed allocation need to check free blocks at every write time.
percpu_counter_read_positive() is not quit accurate
 but using percpu_counter_sum_positive() for every write
frequently is quite expensive.

This patch added a new function to update center counter when
sum up per-cpu counters, to increase the accurate rate
for next percpu_counter_read()(which reads only the center counter
and require less calling expensive percpu_counter_sum().(in
ext4_has_free_blocks)

Signed-off-by: Mingming cao <cmm@us.ibm.com>

---
 fs/ext4/balloc.c               |    2 +-
 include/linux/percpu_counter.h |   12 +++++++++---
 lib/percpu_counter.c           |    7 ++++++-
 3 files changed, 16 insertions(+), 5 deletions(-)

Index: linux-2.6.26-rc4/include/linux/percpu_counter.h
===================================================================
--- linux-2.6.26-rc4.orig/include/linux/percpu_counter.h	2008-06-01 15:33:09.000000000 -0700
+++ linux-2.6.26-rc4/include/linux/percpu_counter.h	2008-06-01 15:33:14.000000000 -0700
@@ -35,7 +35,7 @@ int percpu_counter_init_irq(struct percp
 void percpu_counter_destroy(struct percpu_counter *fbc);
 void percpu_counter_set(struct percpu_counter *fbc, s64 amount);
 void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch);
-s64 __percpu_counter_sum(struct percpu_counter *fbc);
+s64 __percpu_counter_sum(struct percpu_counter *fbc, int set);
 
 static inline void percpu_counter_add(struct percpu_counter *fbc, s64 amount)
 {
@@ -44,13 +44,19 @@ static inline void percpu_counter_add(st
 
 static inline s64 percpu_counter_sum_positive(struct percpu_counter *fbc)
 {
-	s64 ret = __percpu_counter_sum(fbc);
+	s64 ret = __percpu_counter_sum(fbc, 0);
 	return ret < 0 ? 0 : ret;
 }
 
+static inline s64 percpu_counter_sum_and_set(struct percpu_counter *fbc)
+{
+	return __percpu_counter_sum(fbc, 1);
+}
+
+
 static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
 {
-	return __percpu_counter_sum(fbc);
+	return __percpu_counter_sum(fbc, 0);
 }
 
 static inline s64 percpu_counter_read(struct percpu_counter *fbc)
Index: linux-2.6.26-rc4/lib/percpu_counter.c
===================================================================
--- linux-2.6.26-rc4.orig/lib/percpu_counter.c	2008-06-01 15:33:09.000000000 -0700
+++ linux-2.6.26-rc4/lib/percpu_counter.c	2008-06-01 15:33:14.000000000 -0700
@@ -52,7 +52,7 @@ EXPORT_SYMBOL(__percpu_counter_add);
  * Add up all the per-cpu counts, return the result.  This is a more accurate
  * but much slower version of percpu_counter_read_positive()
  */
-s64 __percpu_counter_sum(struct percpu_counter *fbc)
+s64 __percpu_counter_sum(struct percpu_counter *fbc, int set)
 {
 	s64 ret;
 	int cpu;
@@ -62,7 +62,12 @@ s64 __percpu_counter_sum(struct percpu_c
 	for_each_online_cpu(cpu) {
 		s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
 		ret += *pcount;
+		if (set)
+			*pcount = 0;
 	}
+	if (set)
+		fbc->count = ret;
+
 	spin_unlock(&fbc->lock);
 	return ret;
 }
Index: linux-2.6.26-rc4/fs/ext4/balloc.c
===================================================================
--- linux-2.6.26-rc4.orig/fs/ext4/balloc.c	2008-06-01 15:33:36.000000000 -0700
+++ linux-2.6.26-rc4/fs/ext4/balloc.c	2008-06-01 15:34:28.000000000 -0700
@@ -1626,7 +1626,7 @@ ext4_fsblk_t ext4_has_free_blocks(struct
 
 	if (free_blocks - root_blocks < FBC_BATCH)
 		free_blocks =
-			percpu_counter_sum_positive(&sbi->s_freeblocks_counter);
+		    percpu_counter_sum_positive_set(&sbi->s_freeblocks_counter);
 	if (free_blocks - root_blocks < nblocks )
 		return free_blocks -root_blocks;
 	return nblocks;