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Subject[patch v2 4/4] percpu_counter: use atomic64 for counter
Uses atomic64 for percpu_counter, because it is cheaper than spinlock. This
doesn't slow fast path (percpu_counter_read). atomic64_read equals to fbc->count
for 64-bit system, or equals to spin_lock-read-spin_unlock for 32-bit system.

This can improve some workloads with percpu_counter->lock heavily contented.
For example, vm_committed_as sometimes causes the contention. We should tune
the batch count, but if we can make percpu_counter better, why not? In a 24
CPUs system, 24 processes run stress mmap()/mmunmap(), the atomic method
gives 50x faster.

In percpu_counter_set() and __percpu_counter_sum(), there will be no lock
protecting. This means we might get inprecise count, but we have the same issue
even with lock protecting, because __percpu_counter_add doesn't hold locking
to update cpu local count.

Signed-off-by: Shaohua Li <shaohua.li@intel.com>
---
include/linux/percpu_counter.h | 25 +++----------------------
lib/percpu_counter.c | 40 ++++++++++++++++++++--------------------
2 files changed, 23 insertions(+), 42 deletions(-)

Index: linux/include/linux/percpu_counter.h
===================================================================
--- linux.orig/include/linux/percpu_counter.h 2011-04-13 13:27:22.000000000 +0800
+++ linux/include/linux/percpu_counter.h 2011-04-13 13:47:15.000000000 +0800
@@ -16,8 +16,7 @@
#ifdef CONFIG_SMP

struct percpu_counter {
- spinlock_t lock;
- s64 count;
+ atomic64_t count;
#ifdef CONFIG_HOTPLUG_CPU
struct list_head list; /* All percpu_counters are on a list */
#endif
@@ -26,16 +25,7 @@ struct percpu_counter {

extern int percpu_counter_batch;

-int __percpu_counter_init(struct percpu_counter *fbc, s64 amount,
- struct lock_class_key *key);
-
-#define percpu_counter_init(fbc, value) \
- ({ \
- static struct lock_class_key __key; \
- \
- __percpu_counter_init(fbc, value, &__key); \
- })
-
+int percpu_counter_init(struct percpu_counter *fbc, s64 amount);
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);
@@ -60,16 +50,7 @@ static inline s64 percpu_counter_sum(str

static inline s64 percpu_counter_read(struct percpu_counter *fbc)
{
-#if BITS_PER_LONG == 32
- s64 count;
- unsigned long flags;
- spin_lock_irqsave(&fbc->lock, flags);
- count = fbc->count;
- spin_unlock_irqrestore(&fbc->lock, flags);
- return count;
-#else
- return fbc->count;
-#endif
+ return atomic64_read(&fbc->count);
}

/*
Index: linux/lib/percpu_counter.c
===================================================================
--- linux.orig/lib/percpu_counter.c 2011-04-12 16:22:59.000000000 +0800
+++ linux/lib/percpu_counter.c 2011-04-13 13:38:02.000000000 +0800
@@ -59,13 +59,17 @@ void percpu_counter_set(struct percpu_co
{
int cpu;

- spin_lock(&fbc->lock);
+ /*
+ * Don't really need to disable preempt here, just make sure this is no
+ * big latency because of preemption
+ */
+ preempt_disable();
for_each_possible_cpu(cpu) {
s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
*pcount = 0;
}
- fbc->count = amount;
- spin_unlock(&fbc->lock);
+ atomic64_set(&fbc->count, amount);
+ preempt_enable();
}
EXPORT_SYMBOL(percpu_counter_set);

@@ -76,10 +80,8 @@ void __percpu_counter_add(struct percpu_
preempt_disable();
count = __this_cpu_read(*fbc->counters) + amount;
if (count >= batch || count <= -batch) {
- spin_lock(&fbc->lock);
- fbc->count += count;
+ atomic64_add(count, &fbc->count);
__this_cpu_write(*fbc->counters, 0);
- spin_unlock(&fbc->lock);
} else {
__this_cpu_write(*fbc->counters, count);
}
@@ -93,26 +95,27 @@ EXPORT_SYMBOL(__percpu_counter_add);
*/
s64 __percpu_counter_sum(struct percpu_counter *fbc)
{
- s64 ret;
+ s64 ret = 0;
int cpu;

- spin_lock(&fbc->lock);
- ret = fbc->count;
+ /*
+ * Don't really need to disable preempt here, just make sure this is no
+ * big latency because of preemption
+ */
+ preempt_disable();
for_each_online_cpu(cpu) {
s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
ret += *pcount;
}
- spin_unlock(&fbc->lock);
+ ret += atomic64_read(&fbc->count);
+ preempt_enable();
return ret;
}
EXPORT_SYMBOL(__percpu_counter_sum);

-int __percpu_counter_init(struct percpu_counter *fbc, s64 amount,
- struct lock_class_key *key)
+int percpu_counter_init(struct percpu_counter *fbc, s64 amount)
{
- spin_lock_init(&fbc->lock);
- lockdep_set_class(&fbc->lock, key);
- fbc->count = amount;
+ atomic64_set(&fbc->count, amount);
fbc->counters = alloc_percpu(s32);
if (!fbc->counters)
return -ENOMEM;
@@ -127,7 +130,7 @@ int __percpu_counter_init(struct percpu_
#endif
return 0;
}
-EXPORT_SYMBOL(__percpu_counter_init);
+EXPORT_SYMBOL(percpu_counter_init);

void percpu_counter_destroy(struct percpu_counter *fbc)
{
@@ -171,13 +174,10 @@ static int __cpuinit percpu_counter_hotc
mutex_lock(&percpu_counters_lock);
list_for_each_entry(fbc, &percpu_counters, list) {
s32 *pcount;
- unsigned long flags;

- spin_lock_irqsave(&fbc->lock, flags);
pcount = per_cpu_ptr(fbc->counters, cpu);
- fbc->count += *pcount;
+ atomic64_add(*pcount, &fbc->count);
*pcount = 0;
- spin_unlock_irqrestore(&fbc->lock, flags);
}
mutex_unlock(&percpu_counters_lock);
#endif


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