mac80211: minstrel: do not sample rates 3 times slower than max_prob_rate

These rates are highly unlikely to be used quickly, even if the link
deteriorates rapidly. This improves throughput in cases where CCK rates
are not reliable enough to be skipped entirely during sampling.
Sampling these rates regularly can cost a lot of airtime.
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/rc80211_minstrel_ht.c

@@ -1004,10 +1004,13 @@ minstrel_get_sample_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi)
 		return -1;
 
 	/*
-	 * Do not sample if the probability is already higher than 95%
-	 * to avoid wasting airtime.
+	 * Do not sample if the probability is already higher than 95%,
+	 * or if the rate is 3 times slower than the current max probability
+	 * rate, to avoid wasting airtime.
 	 */
-	if (mrs->prob_ewma > MINSTREL_FRAC(95, 100))
+	sample_dur = minstrel_get_duration(sample_idx);
+	if (mrs->prob_ewma > MINSTREL_FRAC(95, 100) ||
+	    minstrel_get_duration(mi->max_prob_rate) * 3 < sample_dur)
 		return -1;
 
 	/*
@@ -1017,7 +1020,6 @@ minstrel_get_sample_rate(struct minstrel_priv *mp, struct minstrel_ht_sta *mi)
 
 	cur_max_tp_streams = minstrel_mcs_groups[tp_rate1 /
 		MCS_GROUP_RATES].streams;
-	sample_dur = minstrel_get_duration(sample_idx);
 	if (sample_dur >= minstrel_get_duration(tp_rate2) &&
 	    (cur_max_tp_streams - 1 <
 	     minstrel_mcs_groups[sample_group].streams ||