[PATCH] percpu_counter: FBC_BATCH might be too big

[Eric Dumazet <dada1@cosmosbay.com>]

For NR_CPUS >= 16 values, FBC_BATCH is 2*NR_CPUS

Considering more and more distros are using high NR_CPUS values,
it makes sense to use a more sensible value for FBC_BATCH.

A sensible value is 2*num_online_cpus(), with a minimum value of 32
(This minimum value helps branch prediction in __percpu_counter_add())

We already have a hotcpu notifier, so we can adjust FBC_BATCH dynamically.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
---
 include/linux/percpu_counter.h |    6 +-----
 lib/percpu_counter.c           |   14 +++++++++++++-
 2 files changed, 14 insertions(+), 6 deletions(-)

diff --git a/include/linux/percpu_counter.h b/include/linux/percpu_counter.h
index 9007ccd..c42a184 100644
--- a/include/linux/percpu_counter.h
+++ b/include/linux/percpu_counter.h
@@ -24,11 +24,7 @@ struct percpu_counter {
 	s32 *counters;
 };
 
-#if NR_CPUS >= 16
-#define FBC_BATCH	(NR_CPUS*2)
-#else
-#define FBC_BATCH	(NR_CPUS*4)
-#endif
+extern int FBC_BATCH;
 
 int percpu_counter_init(struct percpu_counter *fbc, s64 amount);
 int percpu_counter_init_irq(struct percpu_counter *fbc, s64 amount);
diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index a866389..2fcf943 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -114,6 +114,16 @@ void percpu_counter_destroy(struct percpu_counter *fbc)
 }
 EXPORT_SYMBOL(percpu_counter_destroy);
 
+int FBC_BATCH __read_mostly = 32;
+EXPORT_SYMBOL(FBC_BATCH);
+
+static void compute_batch_value(void)
+{
+	int nr = num_online_cpus();
+
+	FBC_BATCH = max(32, nr*2);
+}
+
 #ifdef CONFIG_HOTPLUG_CPU
 static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
 					unsigned long action, void *hcpu)
@@ -121,6 +131,7 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
 	unsigned int cpu;
 	struct percpu_counter *fbc;
 
+	compute_batch_value();
 	if (action != CPU_DEAD)
 		return NOTIFY_OK;
 
@@ -139,11 +150,12 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
 	mutex_unlock(&percpu_counters_lock);
 	return NOTIFY_OK;
 }
+#endif
 
 static int __init percpu_counter_startup(void)
 {
+	compute_batch_value();
 	hotcpu_notifier(percpu_counter_hotcpu_callback, 0);
 	return 0;
 }
 module_init(percpu_counter_startup);
-#endif