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Date: Tue, 17 May 2011 06:56:52 +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 mardi 17 mai 2011 à 08:55 +0800, Shaohua Li a écrit :
> I'm still interesting in improving percpu_counter itself. If we can
> improve it, why we don't? My patch doesn't slow down anything for all
> tests. Why didn't you ever look at it?
>
I did and said there were obvious problems in it.
1) 4096 cpus : size of one percpu_counter is 16Kbytes.
After your last patch -> 20 kbytes for no good reason.
2) Two separate alloc_percpu() -> two separate cache lines instead of
one.
But then, if one alloc_percpu() -> 32 kbytes per object.
3) Focus on percpu_counter() implementation instead of making an
analysis of callers.
I did a lot of rwlocks removal in network stack because they are not the
right synchronization primitive in many cases. I did not optimize
rwlocks. If rwlocks were even slower, I suspect other people would have
help me to convert things faster.
4) There is a possible way to solve your deviation case : add at _add()
beginning a short cut for crazy 'amount' values. Its a bit expensive on
32bit arches, so might be added in a new helper to let _add() be fast
for normal and gentle users.
if (unlikely(amount >= batch || amount <= -batch)) {
atomic64(amount, &fbc->count);
return;
}
Ie dont care to get the s32 value and change it to 0, just leave it
alone.
Thanks
------------------------------------------------------------------
About making percpu s32 'atomic', here is the patch to show the idea:
Each time we call __percpu_counter_sum(), fbc->counters[] values are
zeroed and fbc->count is the "precise" value of the counter. (deviation
comes close to 0)
So if we need to dynamically change one percpu counter batch value, we
can call __percpu_counter_sum() to minimize the 'deviation' close to 0,
no need to send an IPI to all cpus so that they perform the transfert
themselves.
1) vm_committed_as could use a big vm_committed_as_batch when
(sysctl_overcommit_memory == OVERCOMMIT_ALWAYS or OVERCOMMIT_GUESS)
2) We could switch vm_committed_as_batch to an adequate value if/when
sysctl_overcommit_memory is changed to OVERCOMMIT_NEVER (and redo the
batch computation is swap space is changed as well, or hugepages
reservations, or number of online cpus, or ...)
Note: this has a cost, because percpu_counter() fast path would have one
atomic operation instead of 0 in current implementation (cmpxchg() on
the percpu s32). Not sure current cpus would care for percpu data (no
contention)
include/linux/percpu_counter.h | 7 +---
lib/percpu_counter.c | 51 +++++++++++++++----------------
2 files changed, 29 insertions(+), 29 deletions(-)
diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 46f6ba5..d6b7831 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -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
@@ -60,7 +59,7 @@ static inline s64 percpu_counter_sum(struct percpu_counter *fbc)
static inline s64 percpu_counter_read(struct percpu_counter *fbc)
{
- return fbc->count;
+ return atomic64_read(&fbc->count);
}
/*
@@ -70,7 +69,7 @@ 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..745787e 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -59,29 +59,36 @@ void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
{
int cpu;
- 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(&fbc->count, amount);
}
EXPORT_SYMBOL(percpu_counter_set);
void __percpu_counter_add(struct percpu_counter *fbc, s64 amount, s32 batch)
{
s64 count;
+ s32 *ptr, old;
+
+ if (unlikely(amount >= batch || amount <= -batch)) {
+ atomic64_add(amount, &fbc->count);
+ return;
+ }
preempt_disable();
- count = __this_cpu_read(*fbc->counters) + amount;
+ ptr = this_cpu_ptr(fbc->counters);
+retry:
+ old = *ptr;
+ count = old + amount;
if (count >= batch || count <= -batch) {
- spin_lock(&fbc->lock);
- fbc->count += count;
- __this_cpu_write(*fbc->counters, 0);
- spin_unlock(&fbc->lock);
+ if (unlikely(cmpxchg(ptr, old, 0) != old))
+ goto retry;
+ atomic64_add(count, &fbc->count);
} else {
- __this_cpu_write(*fbc->counters, count);
+ if (unlikely(cmpxchg(ptr, old, count) != old))
+ goto retry;
}
preempt_enable();
}
@@ -93,26 +100,23 @@ EXPORT_SYMBOL(__percpu_counter_add);
*/
s64 __percpu_counter_sum(struct percpu_counter *fbc)
{
- s64 ret;
int cpu;
- spin_lock(&fbc->lock);
- ret = fbc->count;
for_each_online_cpu(cpu) {
- s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
- ret += *pcount;
+ s32 count, *pcount = per_cpu_ptr(fbc->counters, cpu);
+
+ count = xchg(pcount, 0);
+ if (count)
+ atomic64_add(count, &fbc->count);
}
- spin_unlock(&fbc->lock);
- return ret;
+ return atomic64_read(&fbc->count);
}
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;
+ atomic64_set(&fbc->count, amount);
fbc->counters = alloc_percpu(s32);
if (!fbc->counters)
return -ENOMEM;
@@ -170,14 +174,11 @@ 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) {
- s32 *pcount;
- unsigned long flags;
+ s32 count, *pcount;
- spin_lock_irqsave(&fbc->lock, flags);
pcount = per_cpu_ptr(fbc->counters, cpu);
- fbc->count += *pcount;
- *pcount = 0;
- spin_unlock_irqrestore(&fbc->lock, flags);
+ count = xchg(pcount, 0);
+ atomic64_add(count, &fbc->count);
}
mutex_unlock(&percpu_counters_lock);
#endif
--
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