[PATCH v3] percpu_counter: add a cmpxchg-based _add_batch variant

[Mateusz Guzik]

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.

Signed-off-by: Mateusz Guzik <mjguzik@xxxxxxxxx>
Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
---

v3:
- add a missing word to the new comment

v2:
- dodge preemption
- use this_cpu_try_cmpxchg
- keep the old variant depending on CONFIG_HAVE_CMPXCHG_LOCAL

 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..c3140276bb36 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 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);
 
 /*
-- 
2.39.2