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>
---
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);
/*
--
2.39.2
