percpu_counter: FBC_BATCH should be a variable

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, and get rid of NR_CPUS.

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.

We rename FBC_BATCH to percpu_counter_batch since its not a constant
anymore.

Signed-off-by: Eric Dumazet <dada1@cosmosbay.com>
Acked-by: David S. Miller <davem@davemloft.net>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 14 insertions, 4 deletions

diff --git a/lib/percpu_counter.c b/lib/percpu_counter.c
index b255b939bc1b..a60bd8046095 100644
--- a/lib/percpu_counter.c
+++ b/lib/percpu_counter.c
@@ -9,10 +9,8 @@
 #include <linux/cpu.h>
 #include <linux/module.h>
 
-#ifdef CONFIG_HOTPLUG_CPU
 static LIST_HEAD(percpu_counters);
 static DEFINE_MUTEX(percpu_counters_lock);
-#endif
 
 void percpu_counter_set(struct percpu_counter *fbc, s64 amount)
 {
@@ -111,13 +109,24 @@ void percpu_counter_destroy(struct percpu_counter *fbc)
 }
 EXPORT_SYMBOL(percpu_counter_destroy);
 
-#ifdef CONFIG_HOTPLUG_CPU
+int percpu_counter_batch __read_mostly = 32;
+EXPORT_SYMBOL(percpu_counter_batch);
+
+static void compute_batch_value(void)
+{
+        int nr = num_online_cpus();
+
+        percpu_counter_batch = max(32, nr*2);
+}
+
 static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
                                         unsigned long action, void *hcpu)
 {
+#ifdef CONFIG_HOTPLUG_CPU
         unsigned int cpu;
         struct percpu_counter *fbc;
 
+        compute_batch_value();
         if (action != CPU_DEAD)
                 return NOTIFY_OK;
 
@@ -134,13 +143,14 @@ static int __cpuinit percpu_counter_hotcpu_callback(struct notifier_block *nb,
                 spin_unlock_irqrestore(&fbc->lock, flags);
         }
         mutex_unlock(&percpu_counters_lock);
+#endif
         return NOTIFY_OK;
 }
 
 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