On 5/20/24 9:14 AM, Mateusz Guzik wrote:
> This was "percpu_counter: reimplement _add_batch with __this_cpu_cmpxchg".
>
> I chatted with vbabka a little bit and he pointed me at mod_zone_state,
> which does the same thing I needed except dodges preemption -- turns out
> cmpxchg with a gs-prefixed argument is safe here.
>
> ================ cut here ================
>
> Interrupt disable/enable trips are quite expensive on x86-64 compared to
> a mere cmpxchg (note: no lock prefix!) and percpu counters are used
> quite often.
>
> With this change I get a bump of 1% ops/s for negative path lookups,
> plugged into will-it-scale:
>
> void testcase(unsigned long long *iterations, unsigned long nr)
> {
> while (1) {
> int fd = open("/tmp/nonexistent", O_RDONLY);
> assert(fd == -1);
>
> (*iterations)++;
> }
> }
>
> The win would be higher if it was not for other slowdowns, but one has
> to start somewhere.
>
> v2:
> - dodge preemption
> - use this_cpu_try_cmpxchg
> - keep the old variant depending on CONFIG_HAVE_CMPXCHG_LOCAL
>
> Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
I tried a stupid microbenchmark just doing percpu_counter_inc() in a loop
and this cut the time by almost 50%.
As we discussed, should be also possible to make the fastpath inlined as the
next step, to avoid the function calls that are stupid expensive with cpu
mitigations.
> ---
> lib/percpu_counter.c | 44 +++++++++++++++++++++++++++++++++++++++-----
> 1 file changed, 39 insertions(+), 5 deletions(-)
>
> diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
> index 44dd133594d4..80ec2ffc981a 100644
> --- a/lib/percpu_counter.c
> +++ b/lib/percpu_counter.c
> @@ -73,17 +73,50 @@ void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
> EXPORT_SYMBOL(percpu_counter_set);
>
> /*
> - * local_irq_save() is needed to make the function irq safe:
> - * - The slow path would be ok as protected by an irq-safe spinlock.
> - * - this_cpu_add would be ok as it is irq-safe by definition.
> - * But:
> - * The decision slow path/fast path and the actual update must be atomic, too.
> + * Add to a counter while respecting batch size.
> + *
> + * There are 2 implementations, both dealing with the following problem:
> + *
> + * The decision slow path/fast path and the actual update must be atomic.
> * Otherwise a call in process context could check the current values and
> * decide that the fast path can be used. If now an interrupt occurs before
> * the this_cpu_add(), and the interrupt updates this_cpu(*fbc->counters),
> * then the this_cpu_add() that is executed after the interrupt has completed
> * can produce values larger than "batch" or even overflows.
> */
> +#ifdef CONFIG_HAVE_CMPXCHG_LOCAL
> +/*
> + * Safety against interrupts is achieved in 2 ways:
> + * 1. the fast path uses local cmpxchg (note: no lock prefix)
> + * 2. the slow path operates with interrupts disabled
> + */
> +void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
> +{
> + s64 count;
> + unsigned long flags;
> +
> + count = this_cpu_read(*fbc->counters);
> + do {
> + if (unlikely(abs(count + amount)) >= batch) {
> + raw_spin_lock_irqsave(&fbc->lock, flags);
> + /*
> + * Note: by now might have migrated to another CPU or
> + * the value might have changed.
> + */
> + count = __this_cpu_read(*fbc->counters);
> + fbc->count += count + amount;
> + __this_cpu_sub(*fbc->counters, count);
> + raw_spin_unlock_irqrestore(&fbc->lock, flags);
> + return;
> + }
> + } while (!this_cpu_try_cmpxchg(*fbc->counters, &count, count + amount));
> +}
> +#else
> +/*
> + * local_irq_save() is used to make the function irq safe:
> + * - The slow path would be ok as protected by an irq-safe spinlock.
> + * - this_cpu_add would be ok as it is irq-safe by definition.
> + */
> void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
> {
> s64 count;
> @@ -101,6 +134,7 @@ void percpu_counter_add_batch(struct percpu_counter *fbc, s64 amount, s32 batch)
> }
> local_irq_restore(flags);
> }
> +#endif
> EXPORT_SYMBOL(percpu_counter_add_batch);
>
> /*
