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Date: Mon, 16 May 2011 08:11:36 +0200
From: Eric Dumazet <eric.dumazet@...il.com>
To: Shaohua Li <shaohua.li@...el.com>
Cc: Tejun Heo <tj@...nel.org>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
"cl@...ux.com" <cl@...ux.com>,
"npiggin@...nel.dk" <npiggin@...nel.dk>
Subject: Re: [patch V3] percpu_counter: scalability works
Le lundi 16 mai 2011 à 08:58 +0800, Shaohua Li a écrit :
> so if _sum starts and ends here, _sum can still get deviation.
This makes no sense at all. If you have so many cpus 'here' right before
you increment fbc->sum_cnt, then no matter how precise and super
cautious you are in your _sum() implementation, as soon as you exit from
sum(), other cpus already changed the percpu counter global value.
> @@ -76,10 +74,20 @@ 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;
> + while (1) {
> + atomic_inc_return(&fbc->add_start);
> + if (atomic_read(&fbc->sum_start) != 0)
> + atomic_dec(&fbc->add_start);
> + else
> + break;
> + while (atomic_read(&fbc->sum_start) != 0)
> + cpu_relax();
> + }
> +
> + atomic64_add(count, &fbc->count);
> __this_cpu_write(*fbc->counters, 0);
> - spin_unlock(&fbc->lock);
> +
> + atomic_dec(&fbc->add_start);
> } else {
> __this_cpu_write(*fbc->counters, count);
> }
>
This is way too heavy. You have 3 atomic ops here and a very slow
atomic_inc_return() in fast path [ not all machines are x86].
Not all percpu_counters are used in degenerated way. Most of them hit
the global count not very often.
Your version slows down a very common case (one cpu only calling _add()
several times, for example network stack in input path)
fbc->counters being in same cache line than fbc->add_start/sum_start and
all, I bet everything will be very slow during a _sum() on a 4096 cpu
machine, especially if this _sum() is interrupted by some long lasting
interrupt.
I believe the 'deviation' risk is almost null with my patch.
Remember percpu_counter is not an exact counter but a very lazy one.
(Only requirement is to not have drift)
The risk is small especially if we move the :
__this_cpu_write(*fbc->counters, 0);
before the :
atomic64_add(count, &fbc->count);
and then do the sequence increment _after_ this.
Here is my V4 : We dont need the second fbc->slowcount, given sum() get
fbc->count after the folding, not before : If some cpus enter _add()
while _sum() is running they'll seem sum_cnt signal and change
fbc->count immediately.
I also make following sequence in _add() :
__this_cpu_write(*fbc->counters, 0);
atomic64_add(count, &pcrw->count);
pcrw->sequence++;
include/linux/percpu_counter.h | 25 +++++++--
lib/percpu_counter.c | 78 ++++++++++++++++++++-----------
2 files changed, 70 insertions(+), 33 deletions(-)
diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 46f6ba5..e3e62b1 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -15,13 +15,24 @@
#ifdef CONFIG_SMP
-struct percpu_counter {
- spinlock_t lock;
- s64 count;
+/*
+ * For performance reasons, we keep this part in a separate cache line
+ */
+struct percpu_counter_rw {
+ atomic64_t count;
+ unsigned int sequence;
+
+ /* since we have plenty room, store list here, even if never used */
#ifdef CONFIG_HOTPLUG_CPU
struct list_head list; /* All percpu_counters are on a list */
+ struct percpu_counter *fbc;
#endif
- s32 __percpu *counters;
+} ____cacheline_aligned_in_smp;
+
+struct percpu_counter {
+ atomic_t sum_cnt; /* count of in flight sum() */
+ struct percpu_counter_rw *pcrw;
+ s32 __percpu *counters;
};
extern int percpu_counter_batch;
@@ -60,7 +71,9 @@ static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
static inline s64 percpu_counter_read(struct percpu_counter *fbc)
{
- return fbc->count;
+ struct percpu_counter_rw *pcrw = fbc->pcrw;
+
+ return atomic64_read(&pcrw->count);
}
/*
@@ -70,7 +83,7 @@ static inline s64 percpu_counter_read(struct percpu_counter *fbc)
*/
static inline s64 percpu_counter_read_positive(struct percpu_counter *fbc)
{
- s64 ret = fbc->count;
+ s64 ret = percpu_counter_read(fbc);
barrier(); /* Prevent reloads of fbc->count */
if (ret >= 0)
diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index 28f2c33..27292ba 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -9,6 +9,7 @@
#include <linux/cpu.h>
#include <linux/module.h>
#include <linux/debugobjects.h>
+#include <linux/slab.h>
static LIST_HEAD(percpu_counters);
static DEFINE_MUTEX(percpu_counters_lock);
@@ -58,28 +59,32 @@ static inline void debug_percpu_counter_deactivate(struct percpu_counter *fbc)
void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
{
int cpu;
+ struct percpu_counter_rw *pcrw = fbc->pcrw;
- spin_lock(&fbc->lock);
for_each_possible_cpu(cpu) {
s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
*pcount = 0;
}
- fbc->count = amount;
- spin_unlock(&fbc->lock);
+ atomic64_set(&pcrw->count, amount);
}
EXPORT_SYMBOL(percpu_counter_set);
void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
{
s64 count;
+ struct percpu_counter_rw *pcrw = fbc->pcrw;
+
+ if (atomic_read(&fbc->sum_cnt)) {
+ atomic64_add(amount, &pcrw->count);
+ return;
+ }
preempt_disable();
count = __this_cpu_read(*fbc->counters) + amount;
if (count >= batch || count <= -batch) {
- spin_lock(&fbc->lock);
- fbc->count += count;
__this_cpu_write(*fbc->counters, 0);
- spin_unlock(&fbc->lock);
+ atomic64_add(count, &pcrw->count);
+ pcrw->sequence++;
} else {
__this_cpu_write(*fbc->counters, count);
}
@@ -95,14 +100,25 @@ s64 __percpu_counter_sum(struct percpu_counter *fbc)
{
s64 ret;
int cpu;
+ unsigned int seq;
+ struct percpu_counter_rw *pcrw = fbc->pcrw;
- spin_lock(&fbc->lock);
- ret = fbc->count;
- for_each_online_cpu(cpu) {
- s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
- ret += *pcount;
- }
- spin_unlock(&fbc->lock);
+ atomic_inc(&fbc->sum_cnt);
+ do {
+ seq = pcrw->sequence;
+ smp_rmb();
+
+ ret = 0;
+ for_each_online_cpu(cpu) {
+ s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
+ ret += *pcount;
+ }
+ ret += atomic64_read(&pcrw->count);
+
+ smp_rmb();
+ } while (pcrw->sequence != seq);
+
+ atomic_dec(&fbc->sum_cnt);
return ret;
}
EXPORT_SYMBOL(__percpu_counter_sum);
@@ -110,19 +126,28 @@ EXPORT_SYMBOL(__percpu_counter_sum);
int __percpu_counter_init(struct percpu_counter *fbc, s64 amount,
struct lock_class_key *key)
{
- spin_lock_init(&fbc->lock);
- lockdep_set_class(&fbc->lock, key);
- fbc->count = amount;
+ struct percpu_counter_rw *pcrw;
+
+ pcrw = kzalloc(sizeof(*pcrw), GFP_KERNEL);
+ if (!pcrw)
+ return -ENOMEM;
+ atomic64_set(&pcrw->count, amount);
+
fbc->counters = alloc_percpu(s32);
- if (!fbc->counters)
+ if (!fbc->counters) {
+ kfree(pcrw);
return -ENOMEM;
+ }
+ fbc->pcrw = pcrw;
+ atomic_set(&fbc->sum_cnt, 0);
debug_percpu_counter_activate(fbc);
#ifdef CONFIG_HOTPLUG_CPU
- INIT_LIST_HEAD(&fbc->list);
+ INIT_LIST_HEAD(&pcrw->list);
+ pcrw->fbc = fbc;
mutex_lock(&percpu_counters_lock);
- list_add(&fbc->list, &percpu_counters);
+ list_add(&pcrw->list, &percpu_counters);
mutex_unlock(&percpu_counters_lock);
#endif
return 0;
@@ -138,11 +163,13 @@ void percpu_counter_destroy(struct percpu_counter *fbc)
#ifdef CONFIG_HOTPLUG_CPU
mutex_lock(&percpu_counters_lock);
- list_del(&fbc->list);
+ list_del(&fbc->pcrw->list);
mutex_unlock(&percpu_counters_lock);
#endif
free_percpu(fbc->counters);
fbc->counters = NULL;
+ kfree(fbc->pcrw);
+ fbc->pcrw = NULL;
}
EXPORT_SYMBOL(percpu_counter_destroy);
@@ -161,7 +188,7 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
{
#ifdef CONFIG_HOTPLUG_CPU
unsigned int cpu;
- struct percpu_counter *fbc;
+ struct percpu_counter_rw *pcrw;
compute_batch_value();
if (action != CPU_DEAD)
@@ -169,15 +196,12 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
cpu = (unsigned long)hcpu;
mutex_lock(&percpu_counters_lock);
- list_for_each_entry(fbc, &percpu_counters, list) {
+ list_for_each_entry(pcrw, &percpu_counters, list) {
s32 *pcount;
- unsigned long flags;
- spin_lock_irqsave(&fbc->lock, flags);
- pcount = per_cpu_ptr(fbc->counters, cpu);
- fbc->count += *pcount;
+ pcount = per_cpu_ptr(pcrw->fbc->counters, cpu);
+ atomic64_add(*pcount, &pcrw->count);
*pcount = 0;
- spin_unlock_irqrestore(&fbc->lock, flags);
}
mutex_unlock(&percpu_counters_lock);
#endif
--
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