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percpu_counter: add a cmpxchg-based _add_batch variant

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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.

Link: https://lkml.kernel.org/r/20240528204257.434817-1-mjguzik@gmail.com
Signed-off-by: Mateusz Guzik <mjguzik@gmail.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Acked-by: Dennis Zhou <dennis@kernel.org>
Cc: Hugh Dickins <hughd@google.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Vlastimil Babka <vbabka@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
This commit is contained in:
Mateusz Guzik 2024-05-28 22:42:57 +02:00 committed by Andrew Morton
parent 727759d748
commit 51d821654b
1 changed files with 39 additions and 5 deletions
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44
lib/percpu_counter.c
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@ -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 we 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);
/*