Subject: [PATCH] mac80211: minstrel_ht: rework rate downgrade code and max_prob rate selection
From: Felix Fietkau <[email-masked]>
Date: Sat, 6 Feb 2021 16:33:14 +0100

The current fallback code for fast rate switching on potentially failing rates
is triggering too often if there is some strong noise on the channel. This can
lead to wild fluctuations in the rate selection.
Additionally, switching down to max_prob_rate can create a significant gap down
in throughput, especially when using only 2 spatial streams, because max_prob_rate
is limited to using fewer streams than the max_tp rates.
In order to improve throughput without reducing reliability too much, use the
rate downgrade code for the max_prob_rate only, and allow the non-downgraded
max_prob_rate to use as many spatial streams as the max_tp rates

Signed-off-by: Felix Fietkau <[email-masked]>
---

--- a/net/mac80211/rc80211_minstrel_ht.c
+++ b/net/mac80211/rc80211_minstrel_ht.c
@@ -511,6 +511,14 @@ minstrel_ht_set_best_prob_rate(struct mi
 	int cur_tp_avg, cur_group, cur_idx;
 	int max_gpr_group, max_gpr_idx;
 	int max_gpr_tp_avg, max_gpr_prob;
+	int min_dur;
+
+	min_dur = max(minstrel_get_duration(mi->max_tp_rate[0]),
+		      minstrel_get_duration(mi->max_tp_rate[1]));
+
+	/* make the rate at least 18% slower than max tp rates */
+	if (minstrel_get_duration(index) <= min_dur * 19 / 16)
+		return;
 
 	cur_group = MI_RATE_GROUP(index);
 	cur_idx = MI_RATE_IDX(index);
@@ -532,11 +540,6 @@ minstrel_ht_set_best_prob_rate(struct mi
 	    !minstrel_ht_is_legacy_group(max_tp_group))
 		return;
 
-	/* skip rates faster than max tp rate with lower prob */
-	if (minstrel_get_duration(mi->max_tp_rate[0]) > minstrel_get_duration(index) &&
-	    mrs->prob_avg < max_tp_prob)
-		return;
-
 	max_gpr_group = MI_RATE_GROUP(mg->max_group_prob_rate);
 	max_gpr_idx = MI_RATE_IDX(mg->max_group_prob_rate);
 	max_gpr_prob = mi->groups[max_gpr_group].rates[max_gpr_idx].prob_avg;
@@ -594,40 +597,6 @@ minstrel_ht_assign_best_tp_rates(struct
 
 }
 
-/*
- * Try to increase robustness of max_prob rate by decrease number of
- * streams if possible.
- */
-static inline void
-minstrel_ht_prob_rate_reduce_streams(struct minstrel_ht_sta *mi)
-{
-	struct minstrel_mcs_group_data *mg;
-	int tmp_max_streams, group, tmp_idx, tmp_prob;
-	int tmp_tp = 0;
-
-	if (!mi->sta->ht_cap.ht_supported)
-		return;
-
-	group = MI_RATE_GROUP(mi->max_tp_rate[0]);
-	tmp_max_streams = minstrel_mcs_groups[group].streams;
-	for (group = 0; group < ARRAY_SIZE(minstrel_mcs_groups); group++) {
-		mg = &mi->groups[group];
-		if (!mi->supported[group] || group == MINSTREL_CCK_GROUP)
-			continue;
-
-		tmp_idx = MI_RATE_IDX(mg->max_group_prob_rate);
-		tmp_prob = mi->groups[group].rates[tmp_idx].prob_avg;
-
-		if (tmp_tp < minstrel_ht_get_tp_avg(mi, group, tmp_idx, tmp_prob) &&
-		   (minstrel_mcs_groups[group].streams < tmp_max_streams)) {
-				mi->max_prob_rate = mg->max_group_prob_rate;
-				tmp_tp = minstrel_ht_get_tp_avg(mi, group,
-								tmp_idx,
-								tmp_prob);
-		}
-	}
-}
-
 static u16
 __minstrel_ht_get_sample_rate(struct minstrel_ht_sta *mi,
 			      enum minstrel_sample_type type)
@@ -1111,8 +1080,6 @@ minstrel_ht_update_stats(struct minstrel
 
 	mi->max_prob_rate = tmp_max_prob_rate;
 
-	/* Try to increase robustness of max_prob_rate*/
-	minstrel_ht_prob_rate_reduce_streams(mi);
 	minstrel_ht_refill_sample_rates(mi);
 
 #ifdef CPTCFG_MAC80211_DEBUGFS
@@ -1157,7 +1124,7 @@ minstrel_ht_txstat_valid(struct minstrel
 }
 
 static void
-minstrel_downgrade_rate(struct minstrel_ht_sta *mi, u16 *idx, bool primary)
+minstrel_downgrade_prob_rate(struct minstrel_ht_sta *mi, u16 *idx)
 {
 	int group, orig_group;
 
@@ -1172,11 +1139,7 @@ minstrel_downgrade_rate(struct minstrel_
 		    minstrel_mcs_groups[orig_group].streams)
 			continue;
 
-		if (primary)
-			*idx = mi->groups[group].max_group_tp_rate[0];
-		else
-			*idx = mi->groups[group].max_group_tp_rate[1];
-		break;
+		*idx = mi->groups[group].max_group_prob_rate;
 	}
 }
 
@@ -1210,7 +1173,7 @@ minstrel_ht_tx_status(void *priv, struct
 	struct ieee80211_tx_info *info = st->info;
 	struct minstrel_ht_sta *mi = priv_sta;
 	struct ieee80211_tx_rate *ar = info->status.rates;
-	struct minstrel_rate_stats *rate, *rate2;
+	struct minstrel_rate_stats *rate;
 	struct minstrel_priv *mp = priv;
 	u32 update_interval = mp->update_interval;
 	bool last, update = false;
@@ -1256,18 +1219,13 @@ minstrel_ht_tx_status(void *priv, struct
 		/*
 		 * check for sudden death of spatial multiplexing,
 		 * downgrade to a lower number of streams if necessary.
+		 * only do this for the max_prob_rate to prevent spurious
+		 * rate fluctuations when the link changes suddenly
 		 */
-		rate = minstrel_get_ratestats(mi, mi->max_tp_rate[0]);
+		rate = minstrel_get_ratestats(mi, mi->max_prob_rate);
 		if (rate->attempts > 30 &&
 		    rate->success < rate->attempts / 4) {
-			minstrel_downgrade_rate(mi, &mi->max_tp_rate[0], true);
-			update = true;
-		}
-
-		rate2 = minstrel_get_ratestats(mi, mi->max_tp_rate[1]);
-		if (rate2->attempts > 30 &&
-		    rate2->success < rate2->attempts / 4) {
-			minstrel_downgrade_rate(mi, &mi->max_tp_rate[1], false);
+			minstrel_downgrade_prob_rate(mi, &mi->max_prob_rate);
 			update = true;
 		}
 	}