Messages in this thread Patch in this message | | | Subject | Re: [patch V3] percpu_counter: scalability works | From | Shaohua Li <> | Date | Mon, 16 May 2011 14:37:08 +0800 |
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On Mon, 2011-05-16 at 14:11 +0800, Eric Dumazet wrote: > 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. I don't agree here. The original implementation also just has quite small window we have deviation, the window only exists between the two lines: atomic64_add(count, &fbc->count); __this_cpu_write(*fbc->counters, 0); if you think we should ignore it, we'd better not use any protection here.
> > @@ -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. as I wrote in the email, the atomic and cacheline issue can be resolved with a per_cpu data, I just didn't post the patch. I post it this time, please see below. There is no cache line bounce anymore.
> 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); we still have the deviation issue if _sum starts and ends here. this doesn't change anything.
> 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); smp_wmb() or atomic64_add_return() here to guarantee the changes are seen before sequence++;
> + pcrw->sequence++; sequence++ can introduce cache line bouncing.
add_start causes a lot of cache bouncing because it's updated by all cpus. We can actually make it a percpu variable. This will completely reduce the cache bouncing. With the patch and last patch, I get about 7x faster running the workload that last patch described. Only with last patch, the workload is only about 4x faster. This doesn't slow down _sum because we removed lock for _sum. I did a stress test. 23 CPU run _add, one cpu runs _sum. In _add fast path (don't hold) lock, _sum runs a little slow (about 20% slower). In _add slow path (hold lock), _sum runs much faster (about 9x faster);
Signed-off-by: Shaohua Li <shaohua.li@intel.com> --- include/linux/percpu_counter.h | 3 ++- lib/percpu_counter.c | 22 ++++++++++++++++------ 2 files changed, 18 insertions(+), 7 deletions(-)
Index: linux/include/linux/percpu_counter.h =================================================================== --- linux.orig/include/linux/percpu_counter.h 2011-05-16 10:26:05.000000000 +0800 +++ linux/include/linux/percpu_counter.h 2011-05-16 10:27:48.000000000 +0800 @@ -16,12 +16,13 @@ #ifdef CONFIG_SMP struct percpu_counter { - atomic_t sum_start, add_start; + atomic_t sum_start; atomic64_t count; #ifdef CONFIG_HOTPLUG_CPU struct list_head list; /* All percpu_counters are on a list */ #endif s32 __percpu *counters; + char __percpu *add_starts; }; extern int percpu_counter_batch; Index: linux/lib/percpu_counter.c =================================================================== --- linux.orig/lib/percpu_counter.c 2011-05-16 10:26:58.000000000 +0800 +++ linux/lib/percpu_counter.c 2011-05-16 10:46:12.000000000 +0800 @@ -75,10 +75,12 @@ void __percpu_counter_add(struct percpu_ count = __this_cpu_read(*fbc->counters) + amount; if (count >= batch || count <= -batch) { while (1) { - atomic_inc_return(&fbc->add_start); + __this_cpu_write(*fbc->add_starts, 1); + /* Guarantee add_starts is seen by _sum */ + smp_wmb(); if (atomic_read(&fbc->sum_start) == 0) break; - atomic_dec(&fbc->add_start); + __this_cpu_write(*fbc->add_starts, 0); while (atomic_read(&fbc->sum_start) != 0) cpu_relax(); } @@ -86,7 +88,7 @@ void __percpu_counter_add(struct percpu_ atomic64_add(count, &fbc->count); __this_cpu_write(*fbc->counters, 0); - atomic_dec(&fbc->add_start); + __this_cpu_write(*fbc->add_starts, 0); } else { __this_cpu_write(*fbc->counters, count); } @@ -104,8 +106,10 @@ s64 __percpu_counter_sum(struct percpu_c int cpu; atomic_inc_return(&fbc->sum_start); - while (atomic_read(&fbc->add_start) != 0) - cpu_relax(); + for_each_online_cpu(cpu) { + while (*per_cpu_ptr(fbc->add_starts, cpu) != 0) + cpu_relax(); + } ret = atomic64_read(&fbc->count); for_each_online_cpu(cpu) { @@ -122,10 +126,15 @@ int percpu_counter_init(struct percpu_co { 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; + fbc->add_starts = alloc_percpu(char); + if (!fbc->add_starts) { + free_percpu(fbc->counters); + return -ENOMEM; + } + debug_percpu_counter_activate(fbc); @@ -152,6 +161,7 @@ void percpu_counter_destroy(struct percp mutex_unlock(&percpu_counters_lock); #endif free_percpu(fbc->counters); + free_percpu(fbc->add_starts); fbc->counters = NULL; } EXPORT_SYMBOL(percpu_counter_destroy);
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