mac80211: add standard deviation to Minstrel stats

This patch adds the statistical descriptor "standard deviation"
to better describe the current properties of Minstrel and
Minstrel-HTs success probability distribution. The standard
deviation (SD) is calculated as exponential weighted moving
standard deviation (EWMSD) and its current value is added as
new column in all rc_stats (in debugfs).

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Acked-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

1 files changed, 14 insertions, 5 deletions

diff --git a/net/mac80211/rc80211_minstrel.c b/net/mac80211/rc80211_minstrel.c
index c4a3477812ee..247552a7f6c2 100644
--- a/net/mac80211/rc80211_minstrel.c
+++ b/net/mac80211/rc80211_minstrel.c
@@ -153,7 +153,7 @@ minstrel_update_rates(struct minstrel_priv *mp, struct minstrel_sta_info *mi)
 }
 
 /*
-* Recalculate success probabilities and counters for a given rate using EWMA
+* Recalculate statistics and counters of a given rate
 */
 void
 minstrel_calc_rate_stats(struct minstrel_rate_stats *mrs)
@@ -161,11 +161,20 @@ minstrel_calc_rate_stats(struct minstrel_rate_stats *mrs)
         if (unlikely(mrs->attempts > 0)) {
                 mrs->sample_skipped = 0;
                 mrs->cur_prob = MINSTREL_FRAC(mrs->success, mrs->attempts);
-                if (unlikely(!mrs->att_hist))
+                if (unlikely(!mrs->att_hist)) {
                         mrs->prob_ewma = mrs->cur_prob;
-                else
+                } else {
+                        /* update exponential weighted moving variance */
+                        mrs->prob_ewmsd = minstrel_ewmsd(mrs->prob_ewmsd,
+                                                         mrs->cur_prob,
+                                                         mrs->prob_ewma,
+                                                         EWMA_LEVEL);
+
+                        /*update exponential weighted moving avarage */
                         mrs->prob_ewma = minstrel_ewma(mrs->prob_ewma,
-                                                     mrs->cur_prob, EWMA_LEVEL);
+                                                       mrs->cur_prob,
+                                                       EWMA_LEVEL);
+                }
                 mrs->att_hist += mrs->attempts;
                 mrs->succ_hist += mrs->success;
         } else {
@@ -193,7 +202,7 @@ minstrel_update_stats(struct minstrel_priv *mp, struct minstrel_sta_info *mi)
                 struct minstrel_rate_stats *mrs = &mi->r[i].stats;
                 struct minstrel_rate_stats *tmp_mrs = &mi->r[tmp_prob_rate].stats;
 
-                /* Update success probabilities per rate */
+                /* Update statistics of success probability per rate */
                 minstrel_calc_rate_stats(mrs);
 
                 /* Sample less often below the 10% chance of success.