Re: [patch V3] percpu_counter: scalability works

From: Shaohua Li
Date: Sun May 15 2011 - 20:59:11 EST

On Sat, 2011-05-14 at 06:03 +0800, Eric Dumazet wrote:
> Shaohua Li reported a scalability problem with many threads doing
> mmap()/munmap() calls. vm_committed_as percpu_counter is hitting its
> spinlock very hard, if size of mmaped zones are bigger than
> percpu_counter batch. We could tune this batch value but better have a
> more scalable percpu_counter infrastructure.
>
> Shaohua provided some patches to speedup __percpu_counter_add(), by
> removing the need to use a spinlock and use an atomic64_t fbc->count
> instead.
>
> Problem of these patches were a possible big deviation seen by
> __percpu_counter_sum()
>
> Idea of this patch is to extend Shaohua idea :
>
> We consider _sum() being slow path. We dont try to make it fast [ but
> this implementation should be better since we remove the spinlock that
> used to serialize _sum() / _add() invocations ]
>
> Add a fbc->sum_cnt, so that _add() can detect a _sum() is in flight, and
> directly add to a new atomic64_t field named "fbc->slowcount" (and not
> touch its percpu s32 variable so that _sum() can get more accurate
> percpu_counter 'Value')
>
> Use an out of line structure to make "struct percpu_count" mostly read
> This structure uses its own cache line to reduce false sharing.
>
> Each time one _add() thread overflows its percpu s32 variable, do an
> increment of a sequence, so that _sum() can detect at least one cpu
> messed the fbc->count and reset its s32 variable.
> _sum() can restart its loop, but since sum_cnt is non null, we have
> guarantee that the _sum() loop wont be restarted ad infinitum.
>
> _sum() is accurate and not blocking anymore _add() [ It's slowing it a
> bit of course since all _add() will touch shared fbc->slowcount ]
>
> On my 2x4x2 cpu (Intel(R) Xeon(R) CPU E5540 @ 2.53GHz) machine, and
> 64bit kernel, the following bench :
>
> loop (10000000 times) {
> p = mmap(128M, ANONYMOUS);
> munmap(p, 128M);
> }
>
> 16 processes started
>
> Before patch:
> real 2m14.509s
> user 0m13.780s
> sys 35m24.170s
>
> After patch:
> real 0m34.055s
> user 0m17.910s
> sys 8m1.680s
>
> Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
> CC: Shaohua Li <shaohua.li@xxxxxxxxx>
> CC: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> CC: Christoph Lameter <cl@xxxxxxxxx>
> CC: Tejun Heo <tj@xxxxxxxxxx>
> CC: Nick Piggin <npiggin@xxxxxxxxx>
> ---
> V3: remove irq masking in __percpu_counter_add()
> initialize fbc->sum_cnt in __percpu_counter_init
>
> include/linux/percpu_counter.h | 26 +++++++---
> lib/percpu_counter.c | 79 ++++++++++++++++++++-----------
> 2 files changed, 72 insertions(+), 33 deletions(-)
>
> diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
> index 46f6ba5..4aac7f5 100644
> --- a/include/linux/percpu_counter.h
> +++ b/include/linux/percpu_counter.h
> @@ -15,13 +15,25 @@
>
> #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;
> + atomic64_t slowcount;
> +
> + /* 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 +72,9 @@ 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) + atomic64_read(&pcrw->slowcount);
> }
>
> /*
> @@ -70,7 +84,7 @@ 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..ff486b2 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 +59,33 @@ 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);
> + atomic64_set(&pcrw->slowcount, 0);
> }
> 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->slowcount);
> + return;
> + }
>
> preempt_disable();
> count = __this_cpu_read(*fbc->counters) + amount;
> if (count >= batch || count <= -batch) {
> - spin_lock(&fbc->lock);
> - fbc->count += count;
> + atomic64_add(count, &pcrw->count);
so if _sum starts and ends here, _sum can still get deviation.

I had a patch which uses the idea which I described in last email and
should remove the deviation, see below. it will delay _add if _sum is
running, which sounds scaring. but since _sum is called quite rare, this
isn't a big problem. we can further convert add_start below to a percpu
count to reduce cache line bounce.

---
include/linux/percpu_counter.h | 18 ++++-------------
lib/percpu_counter.c | 43 +++++++++++++++++++++++++----------------
2 files changed, 32 insertions(+), 29 deletions(-)

Index: linux/include/linux/percpu_counter.h
===================================================================
--- linux.orig/include/linux/percpu_counter.h 2011-05-13 11:13:25.000000000 +0800
+++ linux/include/linux/percpu_counter.h 2011-05-13 16:22:41.000000000 +0800
@@ -16,8 +16,9 @@
#ifdef CONFIG_SMP

struct percpu_counter {
+ atomic_t sum_start, add_start;
+ atomic64_t count;
spinlock_t lock;
- s64 count;
#ifdef CONFIG_HOTPLUG_CPU
struct list_head list; /* All percpu_counters are on a list */
#endif
@@ -26,16 +27,7 @@ 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);
@@ -60,7 +52,7 @@ static inline s64 percpu_counter_sum(str

static inline s64 percpu_counter_read(struct percpu_counter *fbc)
{
- return fbc->count;
+ return atomic64_read(&fbc->count);
}

/*
@@ -70,7 +62,7 @@ static inline s64 percpu_counter_read(st
*/
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)
Index: linux/lib/percpu_counter.c
===================================================================
--- linux.orig/lib/percpu_counter.c 2011-05-13 10:29:04.000000000 +0800
+++ linux/lib/percpu_counter.c 2011-05-13 16:22:03.000000000 +0800
@@ -59,13 +59,11 @@ void percpu_counter_set(struct percpu_co
{
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);

@@ -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);
}
@@ -97,22 +105,28 @@ s64 __percpu_counter_sum(struct percpu_c
int cpu;

spin_lock(&fbc->lock);
- ret = fbc->count;
+ atomic_inc_return(&fbc->sum_start);
+ while (atomic_read(&fbc->add_start) != 0)
+ cpu_relax();
+
+ ret = atomic64_read(&fbc->count);
for_each_online_cpu(cpu) {
s32 *pcount = per_cpu_ptr(fbc->counters, cpu);
ret += *pcount;
}
+
+ atomic_dec(&fbc->sum_start);
spin_unlock(&fbc->lock);
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);
+ atomic_set(&fbc->sum_start, 0);
+ atomic_set(&fbc->add_start, 0);
fbc->counters = alloc_percpu(s32);
if (!fbc->counters)
return -ENOMEM;
@@ -127,7 +141,7 @@ 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 +185,10 @@ 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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