lib: Improve EWMA efficiency by using bitshifts

Using bitshifts instead of division and multiplication should improve
performance. That requires weight and factor to be powers of two, but i think
this is something we can live with.

Thanks to Peter Zijlstra for the improved formula!

Signed-off-by: Bruno Randolf <br1@einfach.org>

--

v2:	use log2.h functions
Signed-off-by: John W. Linville <linville@tuxdriver.com>

1 files changed, 12 insertions, 8 deletions

diff --git a/lib/average.c b/lib/average.c
index f1d1b4660c42..5576c2841496 100644
--- a/lib/average.c
+++ b/lib/average.c
@@ -8,6 +8,7 @@
 #include <linux/module.h>
 #include <linux/average.h>
 #include <linux/bug.h>
+#include <linux/log2.h>
 
 /**
  * DOC: Exponentially Weighted Moving Average (EWMA)
@@ -24,18 +25,21 @@
  * ewma_init() - Initialize EWMA parameters
  * @avg: Average structure
  * @factor: Factor to use for the scaled up internal value. The maximum value
- *      of averages can be ULONG_MAX/(factor*weight).
+ *      of averages can be ULONG_MAX/(factor*weight). For performance reasons
+ *      factor has to be a power of 2.
  * @weight: Exponential weight, or decay rate. This defines how fast the
- *      influence of older values decreases. Has to be bigger than 1.
+ *      influence of older values decreases. For performance reasons weight has
+ *      to be a power of 2.
  *
  * Initialize the EWMA parameters for a given struct ewma @avg.
  */
 void ewma_init(struct ewma *avg, unsigned long factor, unsigned long weight)
 {
-        WARN_ON(weight <= 1 || factor == 0);
+        WARN_ON(!is_power_of_2(weight) || !is_power_of_2(factor));
+
+        avg->weight = ilog2(weight);
+        avg->factor = ilog2(factor);
         avg->internal = 0;
-        avg->weight = weight;
-        avg->factor = factor;
 }
 EXPORT_SYMBOL(ewma_init);
 
@@ -49,9 +53,9 @@ EXPORT_SYMBOL(ewma_init);
 struct ewma *ewma_add(struct ewma *avg, unsigned long val)
 {
         avg->internal = avg->internal  ?
-                (((avg->internal * (avg->weight - 1)) +
-                        (val * avg->factor)) / avg->weight) :
-                (val * avg->factor);
+                (((avg->internal << avg->weight) - avg->internal) +
+                        (val << avg->factor)) >> avg->weight :
+                (val << avg->factor);
         return avg;
 }
 EXPORT_SYMBOL(ewma_add);