average: change to declare precision, not factor

Declaring the factor is counter-intuitive, and people are prone
to using small(-ish) values even when that makes no sense.

Change the DECLARE_EWMA() macro to take the fractional precision,
in bits, rather than a factor, and update all users.

While at it, add some more documentation.

Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

7 files changed, 47 insertions, 26 deletions

diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
index bf95016f442a..e9d7e2b70085 100644
--- a/drivers/net/virtio_net.c
+++ b/drivers/net/virtio_net.c
@@ -51,7 +51,7 @@ module_param(gso, bool, 0444);
  * at once, the weight is chosen so that the EWMA will be insensitive to short-
  * term, transient changes in packet size.
  */
-DECLARE_EWMA(pkt_len, 1, 64)
+DECLARE_EWMA(pkt_len, 0, 64)
 
 /* With mergeable buffers we align buffer address and use the low bits to
  * encode its true size. Buffer size is up to 1 page so we need to align to
diff --git a/drivers/net/wireless/ath/ath5k/ath5k.h b/drivers/net/wireless/ath/ath5k/ath5k.h
index 67fedb61fcc0..979800c6f57f 100644
--- a/drivers/net/wireless/ath/ath5k/ath5k.h
+++ b/drivers/net/wireless/ath/ath5k/ath5k.h
@@ -1252,7 +1252,7 @@ struct ath5k_statistics {
 #define ATH5K_TXQ_LEN_MAX       (ATH_TXBUF / 4)         /* bufs per queue */
 #define ATH5K_TXQ_LEN_LOW       (ATH5K_TXQ_LEN_MAX / 2) /* low mark */
 
-DECLARE_EWMA(beacon_rssi, 1024, 8)
+DECLARE_EWMA(beacon_rssi, 10, 8)
 
 /* Driver state associated with an instance of a device */
 struct ath5k_hw {
diff --git a/drivers/net/wireless/ralink/rt2x00/rt2x00.h b/drivers/net/wireless/ralink/rt2x00/rt2x00.h
index 26869b3bef45..340787894c69 100644
--- a/drivers/net/wireless/ralink/rt2x00/rt2x00.h
+++ b/drivers/net/wireless/ralink/rt2x00/rt2x00.h
@@ -257,7 +257,7 @@ struct link_qual {
         int tx_failed;
 };
 
-DECLARE_EWMA(rssi, 1024, 8)
+DECLARE_EWMA(rssi, 10, 8)
 
 /*
  * Antenna settings about the currently active link.
diff --git a/include/linux/average.h b/include/linux/average.h
index d04aa58280de..7ddaf340d2ac 100644
--- a/include/linux/average.h
+++ b/include/linux/average.h
@@ -1,45 +1,66 @@
 #ifndef _LINUX_AVERAGE_H
 #define _LINUX_AVERAGE_H
 
-/* Exponentially weighted moving average (EWMA) */
+/*
+ * Exponentially weighted moving average (EWMA)
+ *
+ * This implements a fixed-precision EWMA algorithm, with both the
+ * precision and fall-off coefficient determined at compile-time
+ * and built into the generated helper funtions.
+ *
+ * The first argument to the macro is the name that will be used
+ * for the struct and helper functions.
+ *
+ * The second argument, the precision, expresses how many bits are
+ * used for the fractional part of the fixed-precision values.
+ *
+ * The third argument, the weight reciprocal, determines how the
+ * new values will be weighed vs. the old state, new values will
+ * get weight 1/weight_rcp and old values 1-1/weight_rcp. Note
+ * that this parameter must be a power of two for efficiency.
+ */
 
-#define DECLARE_EWMA(name, _factor, _weight)                            \
+#define DECLARE_EWMA(name, _precision, _weight_rcp)                     \
         struct ewma_##name {                                            \
                 unsigned long internal;                                 \
         };                                                              \
         static inline void ewma_##name##_init(struct ewma_##name *e)    \
         {                                                               \
-                BUILD_BUG_ON(!__builtin_constant_p(_factor));           \
-                BUILD_BUG_ON(!__builtin_constant_p(_weight));           \
-                BUILD_BUG_ON_NOT_POWER_OF_2(_factor);                   \
-                BUILD_BUG_ON_NOT_POWER_OF_2(_weight);                   \
+                BUILD_BUG_ON(!__builtin_constant_p(_precision));        \
+                BUILD_BUG_ON(!__builtin_constant_p(_weight_rcp));       \
+                /*                                                      \
+                 * Even if you want to feed it just 0/1 you should have \
+                 * some bits for the non-fractional part...             \
+                 */                                                     \
+                BUILD_BUG_ON((_precision) > 30);                        \
+                BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp);               \
                 e->internal = 0;                                        \
         }                                                               \
         static inline unsigned long                                     \
         ewma_##name##_read(struct ewma_##name *e)                       \
         {                                                               \
-                BUILD_BUG_ON(!__builtin_constant_p(_factor));           \
-                BUILD_BUG_ON(!__builtin_constant_p(_weight));           \
-                BUILD_BUG_ON_NOT_POWER_OF_2(_factor);                   \
-                BUILD_BUG_ON_NOT_POWER_OF_2(_weight);                   \
-                return e->internal >> ilog2(_factor);                   \
+                BUILD_BUG_ON(!__builtin_constant_p(_precision));        \
+                BUILD_BUG_ON(!__builtin_constant_p(_weight_rcp));       \
+                BUILD_BUG_ON((_precision) > 30);                        \
+                BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp);               \
+                return e->internal >> (_precision);                     \
         }                                                               \
         static inline void ewma_##name##_add(struct ewma_##name *e,     \
                                              unsigned long val)         \
         {                                                               \
                 unsigned long internal = ACCESS_ONCE(e->internal);      \
-                unsigned long weight = ilog2(_weight);                  \
-                unsigned long factor = ilog2(_factor);                  \
+                unsigned long weight_rcp = ilog2(_weight_rcp);          \
+                unsigned long precision = _precision;                   \
                                                                         \
-                BUILD_BUG_ON(!__builtin_constant_p(_factor));           \
-                BUILD_BUG_ON(!__builtin_constant_p(_weight));           \
-                BUILD_BUG_ON_NOT_POWER_OF_2(_factor);                   \
-                BUILD_BUG_ON_NOT_POWER_OF_2(_weight);                   \
+                BUILD_BUG_ON(!__builtin_constant_p(_precision));        \
+                BUILD_BUG_ON(!__builtin_constant_p(_weight_rcp));       \
+                BUILD_BUG_ON((_precision) > 30);                        \
+                BUILD_BUG_ON_NOT_POWER_OF_2(_weight_rcp);               \
                                                                         \
                 ACCESS_ONCE(e->internal) = internal ?                   \
-                        (((internal << weight) - internal) +            \
-                                (val << factor)) >> weight :            \
-                        (val << factor);                                \
+                        (((internal << weight_rcp) - internal) +        \
+                                (val << precision)) >> weight_rcp :     \
+                        (val << precision);                             \
         }
 
 #endif /* _LINUX_AVERAGE_H */
diff --git a/net/batman-adv/types.h b/net/batman-adv/types.h
index 8f64a5c01345..66b25e410a41 100644
--- a/net/batman-adv/types.h
+++ b/net/batman-adv/types.h
@@ -402,7 +402,7 @@ struct batadv_gw_node {
         struct rcu_head rcu;
 };
 
-DECLARE_EWMA(throughput, 1024, 8)
+DECLARE_EWMA(throughput, 10, 8)
 
 /**
  * struct batadv_hardif_neigh_node_bat_v - B.A.T.M.A.N. V private neighbor
diff --git a/net/mac80211/ieee80211_i.h b/net/mac80211/ieee80211_i.h
index 159a1a733725..0e718437d080 100644
--- a/net/mac80211/ieee80211_i.h
+++ b/net/mac80211/ieee80211_i.h
@@ -428,7 +428,7 @@ struct ieee80211_sta_tx_tspec {
         bool downgraded;
 };
 
-DECLARE_EWMA(beacon_signal, 16, 4)
+DECLARE_EWMA(beacon_signal, 4, 4)
 
 struct ieee80211_if_managed {
         struct timer_list timer;
diff --git a/net/mac80211/sta_info.h b/net/mac80211/sta_info.h
index 15599c70a38f..e65cda34d2bc 100644
--- a/net/mac80211/sta_info.h
+++ b/net/mac80211/sta_info.h
@@ -372,7 +372,7 @@ struct mesh_sta {
         unsigned int fail_avg;
 };
 
-DECLARE_EWMA(signal, 1024, 8)
+DECLARE_EWMA(signal, 10, 8)
 
 struct ieee80211_sta_rx_stats {
         unsigned long packets;