mac80211: Use Airtime-based Queue Limits (AQL) on packet dequeue

The previous commit added the ability to throttle stations when they queue
too much airtime in the hardware. This commit enables the functionality by
calculating the expected airtime usage of each packet that is dequeued from
the TXQs in mac80211, and accounting that as pending airtime.

The estimated airtime for each skb is stored in the tx_info, so we can
subtract the same amount from the running total when the skb is freed or
recycled. The throttling mechanism relies on this accounting to be
accurate (i.e., that we are not freeing skbs without subtracting any
airtime they were accounted for), so we put the subtraction into
ieee80211_report_used_skb(). As an optimisation, we also subtract the
airtime on regular TX completion, zeroing out the value stored in the
packet afterwards, to avoid having to do an expensive lookup of the station
from the packet data on every packet.

This patch does *not* include any mechanism to wake a throttled TXQ again,
on the assumption that this will happen anyway as a side effect of whatever
freed the skb (most commonly a TX completion).

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

include/net/mac80211.h

···
 	};
 };
 
+static inline u16
+ieee80211_info_set_tx_time_est(struct ieee80211_tx_info *info, u16 tx_time_est)
+{
+	/* We only have 10 bits in tx_time_est, so store airtime
+	 * in increments of 4us and clamp the maximum to 2**12-1
+	 */
+	info->tx_time_est = min_t(u16, tx_time_est, 4095) >> 2;
+	return info->tx_time_est << 2;
+}
+
+static inline u16
+ieee80211_info_get_tx_time_est(struct ieee80211_tx_info *info)
+{
+	return info->tx_time_est << 2;
+}
+
 /**
  * struct ieee80211_tx_status - extended tx status info for rate control
  *

net/mac80211/status.c

···
 				      struct sk_buff *skb, bool dropped)
 {
 	struct ieee80211_tx_info *info = IEEE80211_SKB_CB(skb);
+	u16 tx_time_est = ieee80211_info_get_tx_time_est(info);
 	struct ieee80211_hdr *hdr = (void *)skb->data;
 	bool acked = info->flags & IEEE80211_TX_STAT_ACK;
 
 	if (dropped)
 		acked = false;
+
+	if (tx_time_est) {
+		struct sta_info *sta;
+
+		rcu_read_lock();
+
+		sta = sta_info_get_by_addrs(local, hdr->addr1, hdr->addr2);
+		ieee80211_sta_update_pending_airtime(local, sta,
+						     skb_get_queue_mapping(skb),
+						     tx_time_est,
+						     true);
+		rcu_read_unlock();
+	}
 
 	if (info->flags & IEEE80211_TX_INTFL_MLME_CONN_TX) {
 		struct ieee80211_sub_if_data *sdata;
···
 	struct ieee80211_bar *bar;
 	int shift = 0;
 	int tid = IEEE80211_NUM_TIDS;
+	u16 tx_time_est;
 
 	rates_idx = ieee80211_tx_get_rates(hw, info, &retry_count);
 
···
 					    NL80211_EXT_FEATURE_AIRTIME_FAIRNESS))
 			ieee80211_sta_register_airtime(&sta->sta, tid,
 						       info->status.tx_time, 0);
+
+		if ((tx_time_est = ieee80211_info_get_tx_time_est(info)) > 0) {
+			/* Do this here to avoid the expensive lookup of the sta
+			 * in ieee80211_report_used_skb().
+			 */
+			ieee80211_sta_update_pending_airtime(local, sta,
+							     skb_get_queue_mapping(skb),
+							     tx_time_est,
+							     true);
+			ieee80211_info_set_tx_time_est(info, 0);
+		}
 
 		if (ieee80211_hw_check(&local->hw, REPORTS_TX_ACK_STATUS)) {
 			if (info->flags & IEEE80211_TX_STAT_ACK) {

net/mac80211/tx.c

···
 
 	WARN_ON_ONCE(softirq_count() == 0);
 
+	if (!ieee80211_txq_airtime_check(hw, txq))
+		return NULL;
+
 begin:
 	spin_lock_bh(&fq->lock);
 
···
 	}
 
 	IEEE80211_SKB_CB(skb)->control.vif = vif;
+
+	if (local->airtime_flags & AIRTIME_USE_AQL) {
+		u32 airtime;
+
+		airtime = ieee80211_calc_expected_tx_airtime(hw, vif, txq->sta,
+							     skb->len);
+		if (airtime) {
+			airtime = ieee80211_info_set_tx_time_est(info, airtime);
+			ieee80211_sta_update_pending_airtime(local, tx.sta,
+							     txq->ac,
+							     airtime,
+							     false);
+		}
+	}
+
 	return skb;
 
 out: