wifi: mac80211: only accumulate airtime deficit for active clients

When a client does not generate any local tx activity, accumulating airtime
deficit for the round-robin scheduler can be harmful. If this goes on for too
long, the deficit could grow quite large, which might cause unreasonable
initial latency once the client becomes active
Signed-off-by: Felix Fietkau <nbd@nbd.name>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Link: https://lore.kernel.org/r/20220625212411.36675-7-nbd@nbd.nameSigned-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/sta_info.c

@@ -2046,6 +2046,7 @@ void ieee80211_sta_register_airtime(struct ieee80211_sta *pubsta, u8 tid,
 	struct ieee80211_local *local = sta->sdata->local;
 	u8 ac = ieee80211_ac_from_tid(tid);
 	u32 airtime = 0;
+	u32 diff;
 
 	if (sta->local->airtime_flags & AIRTIME_USE_TX)
 		airtime += tx_airtime;
@@ -2055,7 +2056,11 @@ void ieee80211_sta_register_airtime(struct ieee80211_sta *pubsta, u8 tid,
 	spin_lock_bh(&local->active_txq_lock[ac]);
 	sta->airtime[ac].tx_airtime += tx_airtime;
 	sta->airtime[ac].rx_airtime += rx_airtime;
+
+	diff = (u32)jiffies - sta->airtime[ac].last_active;
+	if (diff <= AIRTIME_ACTIVE_DURATION)
 		sta->airtime[ac].deficit -= airtime;
+
 	spin_unlock_bh(&local->active_txq_lock[ac]);
 }
 EXPORT_SYMBOL(ieee80211_sta_register_airtime);