Subject: [PATCH] mac80211: add a per-PHY AQL limit to improve fairness
From: Felix Fietkau <[email-masked]>
Date: Mon, 20 Jun 2022 21:26:34 +0200

In order to maintain fairness, the amount of queueing needs to be limited
beyond the simple per-station AQL budget, otherwise the driver can simply
repeatedly do scheduling rounds until all queues that have not used their
AQL budget become eligble.

To be conservative, use the high AQL limit for the first txq and add half
of the low AQL for each subsequent queue.

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

--- a/net/mac80211/ieee80211_i.h
+++ b/net/mac80211/ieee80211_i.h
@@ -1215,6 +1215,7 @@ struct ieee80211_local {
 	u32 aql_txq_limit_high[IEEE80211_NUM_ACS];
 	u32 aql_threshold;
 	atomic_t aql_total_pending_airtime;
+	atomic_t aql_ac_pending_airtime[IEEE80211_NUM_ACS];
 
 	const struct ieee80211_ops *ops;
 
--- a/net/mac80211/main.c
+++ b/net/mac80211/main.c
@@ -712,6 +712,7 @@ struct ieee80211_hw *ieee80211_alloc_hw_
 		local->aql_txq_limit_low[i] = IEEE80211_DEFAULT_AQL_TXQ_LIMIT_L;
 		local->aql_txq_limit_high[i] =
 			IEEE80211_DEFAULT_AQL_TXQ_LIMIT_H;
+		atomic_set(&local->aql_ac_pending_airtime[i], 0);
 	}
 
 	local->airtime_flags = AIRTIME_USE_TX | AIRTIME_USE_RX;
--- a/net/mac80211/sta_info.c
+++ b/net/mac80211/sta_info.c
@@ -1932,6 +1932,7 @@ void ieee80211_sta_update_pending_airtim
 				   &sta->airtime[ac].aql_tx_pending);
 
 		atomic_add(tx_airtime, &local->aql_total_pending_airtime);
+		atomic_add(tx_airtime, &local->aql_ac_pending_airtime[ac]);
 		return;
 	}
 
@@ -1943,14 +1944,17 @@ void ieee80211_sta_update_pending_airtim
 				       tx_pending, 0);
 	}
 
+	atomic_sub(tx_airtime, &local->aql_total_pending_airtime);
 	tx_pending = atomic_sub_return(tx_airtime,
-				       &local->aql_total_pending_airtime);
+				       &local->aql_ac_pending_airtime[ac]);
 	if (WARN_ONCE(tx_pending < 0,
 		      "Device %s AC %d pending airtime underflow: %u, %u",
 		      wiphy_name(local->hw.wiphy), ac, tx_pending,
-		      tx_airtime))
-		atomic_cmpxchg(&local->aql_total_pending_airtime,
+		      tx_airtime)) {
+		atomic_cmpxchg(&local->aql_ac_pending_airtime[ac],
 			       tx_pending, 0);
+		atomic_sub(tx_pending, &local->aql_total_pending_airtime);
+	}
 }
 
 int sta_info_move_state(struct sta_info *sta,
--- a/net/mac80211/tx.c
+++ b/net/mac80211/tx.c
@@ -3865,6 +3865,9 @@ struct ieee80211_txq *ieee80211_next_txq
 
 	spin_lock_bh(&local->active_txq_lock[ac]);
 
+	if (!local->schedule_round[ac])
+		goto out;
+
  begin:
 	txqi = list_first_entry_or_null(&local->active_txqs[ac],
 					struct txq_info,
@@ -3986,6 +3989,25 @@ bool ieee80211_txq_airtime_check(struct
 }
 EXPORT_SYMBOL(ieee80211_txq_airtime_check);
 
+static bool
+ieee80211_txq_schedule_airtime_check(struct ieee80211_local *local, u8 ac)
+{
+	unsigned int num_txq = 0;
+	struct txq_info *txq;
+	u32 aql_limit;
+
+	if (!wiphy_ext_feature_isset(local->hw.wiphy, NL80211_EXT_FEATURE_AQL))
+		return true;
+
+	list_for_each_entry(txq, &local->active_txqs[ac], schedule_order)
+		num_txq++;
+
+	aql_limit = (num_txq - 1) * local->aql_txq_limit_low[ac] / 2 +
+		    local->aql_txq_limit_high[ac];
+
+	return atomic_read(&local->aql_ac_pending_airtime[ac]) < aql_limit;
+}
+
 bool ieee80211_txq_may_transmit(struct ieee80211_hw *hw,
 				struct ieee80211_txq *txq)
 {
@@ -4002,6 +4024,9 @@ bool ieee80211_txq_may_transmit(struct i
 	if (list_empty(&txqi->schedule_order))
 		goto out;
 
+	if (!ieee80211_txq_schedule_airtime_check(local, ac))
+		goto out;
+
 	list_for_each_entry_safe(iter, tmp, &local->active_txqs[ac],
 				 schedule_order) {
 		if (iter == txqi)
@@ -4041,7 +4066,15 @@ void ieee80211_txq_schedule_start(struct
 	struct ieee80211_local *local = hw_to_local(hw);
 
 	spin_lock_bh(&local->active_txq_lock[ac]);
-	local->schedule_round[ac]++;
+
+	if (ieee80211_txq_schedule_airtime_check(local, ac)) {
+		local->schedule_round[ac]++;
+		if (!local->schedule_round[ac])
+			local->schedule_round[ac]++;
+	} else {
+		local->schedule_round[ac] = 0;
+	}
+
 	spin_unlock_bh(&local->active_txq_lock[ac]);
 }
 EXPORT_SYMBOL(ieee80211_txq_schedule_start);