mac80211: optimize airtime fairness code to reduce cpu usage

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

package/kernel/mac80211/patches/subsys/311-mac80211-use-coarse-boottime-for-airtime-fairness-co.patch

+From: Felix Fietkau <nbd@nbd.name>
+Date: Tue, 14 Dec 2021 17:53:12 +0100
+Subject: [PATCH] mac80211: use coarse boottime for airtime fairness code
+
+The time values used by the airtime fairness code only need to be accurate
+enough to cover station activity detection.
+Using ktime_get_coarse_boottime_ns instead of ktime_get_boottime_ns will
+drop the accuracy down to jiffies intervals, but at the same time saves
+a lot of CPU cycles in a hot path
+
+Signed-off-by: Felix Fietkau <nbd@nbd.name>
+---
+
+--- a/net/mac80211/tx.c
++++ b/net/mac80211/tx.c
+@@ -3827,7 +3827,7 @@ struct ieee80211_txq *ieee80211_next_txq
+ {
+ 	struct ieee80211_local *local = hw_to_local(hw);
+ 	struct airtime_sched_info *air_sched;
+-	u64 now = ktime_get_boottime_ns();
++	u64 now = ktime_get_coarse_boottime_ns();
+ 	struct ieee80211_txq *ret = NULL;
+ 	struct airtime_info *air_info;
+ 	struct txq_info *txqi = NULL;
+@@ -3954,7 +3954,7 @@ void ieee80211_update_airtime_weight(str
+ 	u64 weight_sum = 0;
+ 
+ 	if (unlikely(!now))
+-		now = ktime_get_boottime_ns();
++		now = ktime_get_coarse_boottime_ns();
+ 
+ 	lockdep_assert_held(&air_sched->lock);
+ 
+@@ -3980,7 +3980,7 @@ void ieee80211_schedule_txq(struct ieee8
+ 	struct ieee80211_local *local = hw_to_local(hw);
+ 	struct txq_info *txqi = to_txq_info(txq);
+ 	struct airtime_sched_info *air_sched;
+-	u64 now = ktime_get_boottime_ns();
++	u64 now = ktime_get_coarse_boottime_ns();
+ 	struct airtime_info *air_info;
+ 	u8 ac = txq->ac;
+ 	bool was_active;
+@@ -4038,7 +4038,7 @@ static void __ieee80211_unschedule_txq(s
+ 
+ 	if (!purge)
+ 		airtime_set_active(air_sched, air_info,
+-				   ktime_get_boottime_ns());
++				   ktime_get_coarse_boottime_ns());
+ 
+ 	rb_erase_cached(&txqi->schedule_order,
+ 			&air_sched->active_txqs);
+@@ -4126,7 +4126,7 @@ bool ieee80211_txq_may_transmit(struct i
+ 	if (RB_EMPTY_NODE(&txqi->schedule_order))
+ 		goto out;
+ 
+-	now = ktime_get_boottime_ns();
++	now = ktime_get_coarse_boottime_ns();
+ 
+ 	/* Like in ieee80211_next_txq(), make sure the first station in the
+ 	 * scheduling order is eligible for transmission to avoid starvation.