mac80211: minstrel: store probability variance instead of standard deviation

This avoids the costly int_sqrt calls in the statistics update and moves
it to the debugfs code instead.
This also fixes an overflow in the previous standard deviation
calculation.
Signed-off-by: Thomas Huehn <thomas.huehn@evernet-eg.de>
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel.c

@@ -168,7 +168,7 @@ minstrel_calc_rate_stats(struct minstrel_rate_stats *mrs)
 			mrs->prob_ewma = cur_prob;
 		} else {
 			/* update exponential weighted moving variance */
-			mrs->prob_ewmsd = minstrel_ewmsd(mrs->prob_ewmsd,
+			mrs->prob_ewmv = minstrel_ewmv(mrs->prob_ewmv,
 							cur_prob,
 							mrs->prob_ewma,
 							EWMA_LEVEL);

net/mac80211/rc80211_minstrel.h

@@ -36,21 +36,16 @@ minstrel_ewma(int old, int new, int weight)
 }
 
 /*
- * Perform EWMSD (Exponentially Weighted Moving Standard Deviation) calculation
+ * Perform EWMV (Exponentially Weighted Moving Variance) calculation
  */
 static inline int
-minstrel_ewmsd(int old_ewmsd, int cur_prob, int prob_ewma, int weight)
+minstrel_ewmv(int old_ewmv, int cur_prob, int prob_ewma, int weight)
 {
-	int diff, incr, tmp_var;
+	int diff, incr;
 
-	/* calculate exponential weighted moving variance */
-	diff = MINSTREL_TRUNC((cur_prob - prob_ewma) * 1000000);
+	diff = cur_prob - prob_ewma;
 	incr = (EWMA_DIV - weight) * diff / EWMA_DIV;
-	tmp_var = old_ewmsd * old_ewmsd;
-	tmp_var = weight * (tmp_var + diff * incr / 1000000) / EWMA_DIV;
-
-	/* return standard deviation */
-	return (u16) int_sqrt(tmp_var);
+	return weight * (old_ewmv + MINSTREL_TRUNC(diff * incr)) / EWMA_DIV;
 }
 
 struct minstrel_rate_stats {
@@ -65,7 +60,7 @@ struct minstrel_rate_stats {
 	 *  prob_ewma - exponential weighted moving average of prob
 	 *  prob_ewmsd - exp. weighted moving standard deviation of prob */
 	unsigned int prob_ewma;
-	u16 prob_ewmsd;
+	u16 prob_ewmv;
 
 	/* maximum retry counts */
 	u8 retry_count;
@@ -151,6 +146,14 @@ struct minstrel_debugfs_info {
 	char buf[];
 };
 
+/* Get EWMSD (Exponentially Weighted Moving Standard Deviation) * 10 */
+static inline int
+minstrel_get_ewmsd10(struct minstrel_rate_stats *mrs)
+{
+	unsigned int ewmv = mrs->prob_ewmv;
+	return int_sqrt(MINSTREL_TRUNC(ewmv * 1000 * 1000));
+}
+
 extern const struct rate_control_ops mac80211_minstrel;
 void minstrel_add_sta_debugfs(void *priv, void *priv_sta, struct dentry *dir);
 void minstrel_remove_sta_debugfs(void *priv, void *priv_sta);

net/mac80211/rc80211_minstrel_debugfs.c

@@ -93,6 +93,7 @@ minstrel_stats_open(struct inode *inode, struct file *file)
 	for (i = 0; i < mi->n_rates; i++) {
 		struct minstrel_rate *mr = &mi->r[i];
 		struct minstrel_rate_stats *mrs = &mi->r[i].stats;
+		unsigned int prob_ewmsd;
 
 		*(p++) = (i == mi->max_tp_rate[0]) ? 'A' : ' ';
 		*(p++) = (i == mi->max_tp_rate[1]) ? 'B' : ' ';
@@ -108,6 +109,7 @@ minstrel_stats_open(struct inode *inode, struct file *file)
 		tp_max = minstrel_get_tp_avg(mr, MINSTREL_FRAC(100,100));
 		tp_avg = minstrel_get_tp_avg(mr, mrs->prob_ewma);
 		eprob = MINSTREL_TRUNC(mrs->prob_ewma * 1000);
+		prob_ewmsd = minstrel_get_ewmsd10(mrs);
 
 		p += sprintf(p, "%4u.%1u    %4u.%1u     %3u.%1u    %3u.%1u"
 				"     %3u   %3u %-3u   "
@@ -115,7 +117,7 @@ minstrel_stats_open(struct inode *inode, struct file *file)
 				tp_max / 10, tp_max % 10,
 				tp_avg / 10, tp_avg % 10,
 				eprob / 10, eprob % 10,
-				mrs->prob_ewmsd / 10, mrs->prob_ewmsd % 10,
+				prob_ewmsd / 10, prob_ewmsd % 10,
 				mrs->retry_count,
 				mrs->last_success,
 				mrs->last_attempts,
@@ -159,6 +161,7 @@ minstrel_stats_csv_open(struct inode *inode, struct file *file)
 	for (i = 0; i < mi->n_rates; i++) {
 		struct minstrel_rate *mr = &mi->r[i];
 		struct minstrel_rate_stats *mrs = &mi->r[i].stats;
+		unsigned int prob_ewmsd;
 
 		p += sprintf(p, "%s" ,((i == mi->max_tp_rate[0]) ? "A" : ""));
 		p += sprintf(p, "%s" ,((i == mi->max_tp_rate[1]) ? "B" : ""));
@@ -174,13 +177,14 @@ minstrel_stats_csv_open(struct inode *inode, struct file *file)
 		tp_max = minstrel_get_tp_avg(mr, MINSTREL_FRAC(100,100));
 		tp_avg = minstrel_get_tp_avg(mr, mrs->prob_ewma);
 		eprob = MINSTREL_TRUNC(mrs->prob_ewma * 1000);
+		prob_ewmsd = minstrel_get_ewmsd10(mrs);
 
 		p += sprintf(p, "%u.%u,%u.%u,%u.%u,%u.%u,%u,%u,%u,"
 				"%llu,%llu,%d,%d\n",
 				tp_max / 10, tp_max % 10,
 				tp_avg / 10, tp_avg % 10,
 				eprob / 10, eprob % 10,
-				mrs->prob_ewmsd / 10, mrs->prob_ewmsd % 10,
+				prob_ewmsd / 10, prob_ewmsd % 10,
 				mrs->retry_count,
 				mrs->last_success,
 				mrs->last_attempts,

net/mac80211/rc80211_minstrel_ht_debugfs.c

@@ -41,6 +41,7 @@ minstrel_ht_stats_dump(struct minstrel_ht_sta *mi, int i, char *p)
 		struct minstrel_rate_stats *mrs = &mi->groups[i].rates[j];
 		static const int bitrates[4] = { 10, 20, 55, 110 };
 		int idx = i * MCS_GROUP_RATES + j;
+		unsigned int prob_ewmsd;
 
 		if (!(mi->supported[i] & BIT(j)))
 			continue;
@@ -84,6 +85,7 @@ minstrel_ht_stats_dump(struct minstrel_ht_sta *mi, int i, char *p)
 		tp_max = minstrel_ht_get_tp_avg(mi, i, j, MINSTREL_FRAC(100, 100));
 		tp_avg = minstrel_ht_get_tp_avg(mi, i, j, mrs->prob_ewma);
 		eprob = MINSTREL_TRUNC(mrs->prob_ewma * 1000);
+		prob_ewmsd = minstrel_get_ewmsd10(mrs);
 
 		p += sprintf(p, "%4u.%1u    %4u.%1u     %3u.%1u    %3u.%1u"
 				"     %3u   %3u %-3u   "
@@ -91,7 +93,7 @@ minstrel_ht_stats_dump(struct minstrel_ht_sta *mi, int i, char *p)
 				tp_max / 10, tp_max % 10,
 				tp_avg / 10, tp_avg % 10,
 				eprob / 10, eprob % 10,
-				mrs->prob_ewmsd / 10, mrs->prob_ewmsd % 10,
+				prob_ewmsd / 10, prob_ewmsd % 10,
 				mrs->retry_count,
 				mrs->last_success,
 				mrs->last_attempts,
@@ -185,6 +187,7 @@ minstrel_ht_stats_csv_dump(struct minstrel_ht_sta *mi, int i, char *p)
 		struct minstrel_rate_stats *mrs = &mi->groups[i].rates[j];
 		static const int bitrates[4] = { 10, 20, 55, 110 };
 		int idx = i * MCS_GROUP_RATES + j;
+		unsigned int prob_ewmsd;
 
 		if (!(mi->supported[i] & BIT(j)))
 			continue;
@@ -225,13 +228,14 @@ minstrel_ht_stats_csv_dump(struct minstrel_ht_sta *mi, int i, char *p)
 		tp_max = minstrel_ht_get_tp_avg(mi, i, j, MINSTREL_FRAC(100, 100));
 		tp_avg = minstrel_ht_get_tp_avg(mi, i, j, mrs->prob_ewma);
 		eprob = MINSTREL_TRUNC(mrs->prob_ewma * 1000);
+		prob_ewmsd = minstrel_get_ewmsd10(mrs);
 
 		p += sprintf(p, "%u.%u,%u.%u,%u.%u,%u.%u,%u,%u,"
 				"%u,%llu,%llu,",
 				tp_max / 10, tp_max % 10,
 				tp_avg / 10, tp_avg % 10,
 				eprob / 10, eprob % 10,
-				mrs->prob_ewmsd / 10, mrs->prob_ewmsd % 10,
+				prob_ewmsd / 10, prob_ewmsd % 10,
 				mrs->retry_count,
 				mrs->last_success,
 				mrs->last_attempts,