lib/average.c

   1 /*
   2  * lib/average.c
   3  *
   4  * This source code is licensed under the GNU General Public License,
   5  * Version 2.  See the file COPYING for more details.
   6  */
   7
   8 #include <linux/export.h>
   9 #include <linux/average.h>
  10 #include <linux/kernel.h>
  11 #include <linux/bug.h>
  12 #include <linux/log2.h>
  13
  14 /**
  15  * DOC: Exponentially Weighted Moving Average (EWMA)
  16  *
  17  * These are generic functions for calculating Exponentially Weighted Moving
  18  * Averages (EWMA). We keep a structure with the EWMA parameters and a scaled
  19  * up internal representation of the average value to prevent rounding errors.
  20  * The factor for scaling up and the exponential weight (or decay rate) have to
  21  * be specified thru the init fuction. The structure should not be accessed
  22  * directly but only thru the helper functions.
  23  */
  24
  25 /**
  26  * ewma_init() - Initialize EWMA parameters
  27  * @avg: Average structure
  28  * @factor: Factor to use for the scaled up internal value. The maximum value
  29  *      of averages can be ULONG_MAX/(factor*weight). For performance reasons
  30  *      factor has to be a power of 2.
  31  * @weight: Exponential weight, or decay rate. This defines how fast the
  32  *      influence of older values decreases. For performance reasons weight has
  33  *      to be a power of 2.
  34  *
  35  * Initialize the EWMA parameters for a given struct ewma @avg.
  36  */
  37 void ewma_init(struct ewma *avg, unsigned long factor, unsigned long weight)
  38 {
  39         WARN_ON(!is_power_of_2(weight) || !is_power_of_2(factor));
  40
  41         avg->weight = ilog2(weight);
  42         avg->factor = ilog2(factor);
  43         avg->internal = 0;
  44 }
  45 EXPORT_SYMBOL(ewma_init);
  46
  47 /**
  48  * ewma_add() - Exponentially weighted moving average (EWMA)
  49  * @avg: Average structure
  50  * @val: Current value
  51  *
  52  * Add a sample to the average.
  53  */
  54 struct ewma *ewma_add(struct ewma *avg, unsigned long val)
  55 {
  56         unsigned long internal = ACCESS_ONCE(avg->internal);
  57
  58         ACCESS_ONCE(avg->internal) = internal ?
  59                 (((internal << avg->weight) - internal) +
  60                         (val << avg->factor)) >> avg->weight :
  61                 (val << avg->factor);
  62         return avg;
  63 }
  64 EXPORT_SYMBOL(ewma_add);