mac80211: minstrel_ht: reduce the need to sample slower rates

In order to more gracefully be able to fall back to lower rates without too
much throughput fluctuations, initialize all untested rates below tested ones
to the maximum probabilty of higher rates.
Usually this leads to untested lower rates getting initialized with a
probability value of 100%, making them better candidates for fallback without
having to rely on random probing

Signed-off-by: Felix Fietkau <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Johannes Berg <[e-mail]>

net/mac80211/rc80211_minstrel_ht.c

···
 	unsigned int cur_prob;
 
 	if (unlikely(mrs->attempts > 0)) {
-		mrs->sample_skipped = 0;
 		cur_prob = MINSTREL_FRAC(mrs->success, mrs->attempts);
 		minstrel_filter_avg_add(&mrs->prob_avg,
 					&mrs->prob_avg_1, cur_prob);
 		mrs->att_hist += mrs->attempts;
 		mrs->succ_hist += mrs->success;
-	} else {
-		mrs->sample_skipped++;
 	}
 
 	mrs->last_success = mrs->success;
···
 		mi->ampdu_packets = 0;
 	}
 
-	mi->sample_slow = 0;
 	mi->sample_count = 0;
 
 	if (mi->supported[MINSTREL_CCK_GROUP])
···
 	/* Find best rate sets within all MCS groups*/
 	for (group = 0; group < ARRAY_SIZE(minstrel_mcs_groups); group++) {
 		u16 *tp_rate = tmp_mcs_tp_rate;
+		u16 last_prob = 0;
 
 		mg = &mi->groups[group];
 		if (!mi->supported[group])
···
 		if (group == MINSTREL_CCK_GROUP && ht_supported)
 			tp_rate = tmp_legacy_tp_rate;
 
-		for (i = 0; i < MCS_GROUP_RATES; i++) {
+		for (i = MCS_GROUP_RATES - 1; i >= 0; i--) {
 			if (!(mi->supported[group] & BIT(i)))
 				continue;
 
···
 			mrs = &mg->rates[i];
 			mrs->retry_updated = false;
 			minstrel_ht_calc_rate_stats(mp, mrs);
+
+			if (mrs->att_hist)
+				last_prob = max(last_prob, mrs->prob_avg);
+			else
+				mrs->prob_avg = max(last_prob, mrs->prob_avg);
 			cur_prob = mrs->prob_avg;
 
 			if (minstrel_ht_get_tp_avg(mi, group, i, cur_prob) == 0)
···
 	if (sample_dur >= minstrel_get_duration(tp_rate2) &&
 	    (cur_max_tp_streams - 1 <
 	     minstrel_mcs_groups[sample_group].streams ||
-	     sample_dur >= minstrel_get_duration(mi->max_prob_rate))) {
-		if (mrs->sample_skipped < 20)
+	     sample_dur >= minstrel_get_duration(mi->max_prob_rate)))
 			return -1;
 
-		if (mi->sample_slow++ > 2)
-			return -1;
-	}
 	mi->sample_tries--;
 
 	return sample_idx;

net/mac80211/rc80211_minstrel_ht.h

···
 	u8 retry_count;
 	u8 retry_count_rtscts;
 
-	u8 sample_skipped;
 	bool retry_updated;
 };
 
···
 	u8 sample_wait;
 	u8 sample_tries;
 	u8 sample_count;
-	u8 sample_slow;
 
 	enum minstrel_sample_mode sample_mode;
 	u16 sample_rate;