mac80211: avoid sampling rates that are too slow, improves throughput

Signed-off-by: Felix Fietkau <nbd@nbd.name>

package/kernel/mac80211/patches/377-mac80211-minstrel-do-not-sample-rates-3-times-slower.patch

+From: Felix Fietkau <nbd@nbd.name>
+Date: Sat, 3 Mar 2018 18:48:58 +0100
+Subject: [PATCH] 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>
+---
+
+--- a/net/mac80211/rc80211_minstrel_ht.c
++++ b/net/mac80211/rc80211_minstrel_ht.c
+@@ -1004,10 +1004,13 @@ minstrel_get_sample_rate(struct minstrel
+ 		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
+ 
+ 	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 ||