Received-SPF: pass (google.com: domain of linux-wireless-owner@vger.kernel.org designates 2620:137:e000::1:18 as permitted sender) client-ip=2620:137:e000::1:18;
From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@kernel.org>
To:     Felix Fietkau <nbd@nbd.name>, linux-wireless@vger.kernel.org
Cc:     johannes@sipsolutions.net
Subject: Re: [PATCH 2/2] mac80211: rework the airtime fairness implementation
In-Reply-To: <20220603164307.92189-2-nbd@nbd.name>
References: <20220603164307.92189-1-nbd@nbd.name>
 <20220603164307.92189-2-nbd@nbd.name>
Date:   Sun, 05 Jun 2022 19:18:16 +0200
Message-ID: <87a6arowpz.fsf@toke.dk>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Felix Fietkau <nbd@nbd.name> writes:

> The current ATF implementation has a number of issues which have shown up
> during testing. Since it does not take into account the AQL budget of
> pending packets, the implementation might queue up large amounts of packets
> for a single txq until airtime gets reported after tx completion.
> The same then happens to the next txq afterwards. While the end result could
> still be considered fair, the bursty behavior introduces a large amount of
> latency.
> The current code also tries to avoid frequent re-sorting of txq entries in
> order to avoid having to re-balance the rbtree often.
>
> In order to fix these issues, introduce skip lists as a data structure, which
> offer similar lookup/insert/delete times as rbtree, but avoids the need for
> rebalacing by being probabilistic.
> Use this to keep tx entries sorted by virtual time + pending AQL budget and
> re-sort after each ieee80211_return_txq call.
>
> Since multiple txqs share a single air_time struct with a virtual time value,
> switch the active_txqs list to queue up air_time structs instead of queues.
> This helps avoid imbalance between shared txqs by servicing them round robin.
>
> ieee80211_next_txq now only dequeues the first element of active_txqs. To
> make that work for non-AQL or non-ATF drivers as well, add estimated tx
> airtime directly to air_info virtual time if either AQL or ATF is not
> supported.
>
> Signed-off-by: Felix Fietkau <nbd@nbd.name>

Thank you for working on this! I generally approve of this, with a few
comments and questions below:

> ---
>  include/linux/skiplist.h   | 250 +++++++++++++++++++++++++++++++++++++
>  net/mac80211/cfg.c         |   5 -
>  net/mac80211/ieee80211_i.h |  27 +++-
>  net/mac80211/main.c        |   2 +-
>  net/mac80211/sta_info.c    |   3 +-
>  net/mac80211/sta_info.h    |   6 +-
>  net/mac80211/tx.c          | 244 +++++++++++++++++-------------------
>  7 files changed, 396 insertions(+), 141 deletions(-)
>  create mode 100644 include/linux/skiplist.h
>
> diff --git a/include/linux/skiplist.h b/include/linux/skiplist.h
> new file mode 100644
> index 000000000000..2312ed87cfe9
> --- /dev/null
> +++ b/include/linux/skiplist.h
> @@ -0,0 +1,250 @@
> +/* SPDX-License-Identifier: GPL-2.0-or-later */
> +/*
> + * A skip list is a probabilistic alternative to balanced trees. Unlike the
> + * red-black tree, it does not require rebalancing.
> + *
> + * This implementation uses only unidirectional next pointers and is optimized
> + * for use in a priority queue where elements are mostly deleted from the front
> + * of the queue.
> + *
> + * When storing up to 2^n elements in a n-level skiplist. lookup and deletion
> + * for the first element happens in O(1) time, other than that, insertion and
> + * deletion takes O(log n) time, assuming that the number of elements for an
> + * n-level list does not exceed 2^n.
> + *
> + * Usage:
> + * DECLARE_SKIPLIST_TYPE(foo, 5) will define the data types for a 5-level list:
> + * struct foo_list: the list data type
> + * struct foo_node: the node data for an element in the list
> + *
> + * DECLARE_SKIPLIST_IMPL(foo, foo_cmp_fn)

I'm a little bit on the fence as to whether the macro-based definitions
is a bit too magical. It's not how any of the other list structures in
the kernel is done. OTOH, it's nice to have usage-specific structure
names. So yeah, on the fence... Hopefully others have a stronger opinion :)

[...]

> diff --git a/net/mac80211/cfg.c b/net/mac80211/cfg.c
> index f7896f257e1b..90968636ddfb 100644
> --- a/net/mac80211/cfg.c
> +++ b/net/mac80211/cfg.c
> @@ -1580,7 +1580,6 @@ static void sta_apply_airtime_params(struct ieee80211_local *local,
>  	for (ac = 0; ac < IEEE80211_NUM_ACS; ac++) {
>  		struct airtime_sched_info *air_sched = &local->airtime[ac];
>  		struct airtime_info *air_info = &sta->airtime[ac];
> -		struct txq_info *txqi;
>  		u8 tid;
>  
>  		spin_lock_bh(&air_sched->lock);
> @@ -1592,10 +1591,6 @@ static void sta_apply_airtime_params(struct ieee80211_local *local,
>  
>  			airtime_weight_set(air_info, params->airtime_weight);
>  
> -			txqi = to_txq_info(sta->sta.txq[tid]);
> -			if (RB_EMPTY_NODE(&txqi->schedule_order))
> -				continue;
> -
>  			ieee80211_update_airtime_weight(local, air_sched,
>  							0, true);
>  		}
> diff --git a/net/mac80211/ieee80211_i.h b/net/mac80211/ieee80211_i.h
> index 5ca4c2c6aa3c..1a6f9d119bdc 100644
> --- a/net/mac80211/ieee80211_i.h
> +++ b/net/mac80211/ieee80211_i.h
> @@ -25,7 +25,8 @@
>  #include <linux/leds.h>
>  #include <linux/idr.h>
>  #include <linux/rhashtable.h>
> -#include <linux/rbtree.h>
> +#include <linux/prandom.h>
> +#include <linux/skiplist.h>
>  #include <net/ieee80211_radiotap.h>
>  #include <net/cfg80211.h>
>  #include <net/mac80211.h>
> @@ -855,6 +856,7 @@ enum txq_info_flags {
>  	IEEE80211_TXQ_AMPDU,
>  	IEEE80211_TXQ_NO_AMSDU,
>  	IEEE80211_TXQ_STOP_NETIF_TX,
> +	IEEE80211_TXQ_FORCE_ACTIVE,
>  };
>  
>  /**
> @@ -871,7 +873,6 @@ struct txq_info {
>  	struct fq_tin tin;
>  	struct codel_vars def_cvars;
>  	struct codel_stats cstats;
> -	struct rb_node schedule_order;
>  
>  	struct sk_buff_head frags;
>  	unsigned long flags;
> @@ -1191,8 +1192,7 @@ enum mac80211_scan_state {
>   *
>   * @lock: spinlock that protects all the fields in this struct
>   * @active_txqs: rbtree of currently backlogged queues, sorted by virtual time
> - * @schedule_pos: the current position maintained while a driver walks the tree
> - *                with ieee80211_next_txq()
> + * @schedule_pos: last used airtime_info node while a driver walks the tree
>   * @active_list: list of struct airtime_info structs that were active within
>   *               the last AIRTIME_ACTIVE_DURATION (100 ms), used to compute
>   *               weight_sum
> @@ -1213,8 +1213,8 @@ enum mac80211_scan_state {
>   */
>  struct airtime_sched_info {
>  	spinlock_t lock;
> -	struct rb_root_cached active_txqs;
> -	struct rb_node *schedule_pos;
> +	struct airtime_sched_list active_txqs;
> +	struct airtime_sched_node *schedule_pos;
>  	struct list_head active_list;
>  	u64 last_weight_update;
>  	u64 last_schedule_activity;
> @@ -1674,6 +1674,20 @@ static inline struct airtime_info *to_airtime_info(struct ieee80211_txq *txq)
>  	return &sdata->airtime[txq->ac];
>  }
>  
> +static inline int
> +airtime_sched_cmp(struct airtime_sched_list *list,
> +		  struct airtime_sched_node *n1, struct airtime_sched_node *n2)
> +{
> +	struct airtime_info *a1, *a2;
> +
> +	a1 = container_of(n1, struct airtime_info, schedule_order);
> +	a2 = container_of(n2, struct airtime_info, schedule_order);
> +
> +	return a1->v_t_cur - a2->v_t_cur;
> +}
> +
> +DECLARE_SKIPLIST_IMPL(airtime_sched, airtime_sched_cmp);
> +
>  /* To avoid divisions in the fast path, we keep pre-computed reciprocals for
>   * airtime weight calculations. There are two different weights to keep track
>   * of: The per-station weight and the sum of weights per phy.
> @@ -1761,6 +1775,7 @@ static inline void init_airtime_info(struct airtime_info *air_info,
>  	air_info->aql_limit_high = air_sched->aql_txq_limit_high;
>  	airtime_weight_set(air_info, IEEE80211_DEFAULT_AIRTIME_WEIGHT);
>  	INIT_LIST_HEAD(&air_info->list);
> +	airtime_sched_node_init(&air_info->schedule_order);
>  }
>  
>  static inline int ieee80211_bssid_match(const u8 *raddr, const u8 *addr)
> diff --git a/net/mac80211/main.c b/net/mac80211/main.c
> index 5a385d4146b9..7d855167cafb 100644
> --- a/net/mac80211/main.c
> +++ b/net/mac80211/main.c
> @@ -709,7 +709,7 @@ struct ieee80211_hw *ieee80211_alloc_hw_nm(size_t priv_data_len,
>  	for (i = 0; i < IEEE80211_NUM_ACS; i++) {
>  		struct airtime_sched_info *air_sched = &local->airtime[i];
>  
> -		air_sched->active_txqs = RB_ROOT_CACHED;
> +		airtime_sched_list_init(&air_sched->active_txqs);
>  		INIT_LIST_HEAD(&air_sched->active_list);
>  		spin_lock_init(&air_sched->lock);
>  		air_sched->aql_txq_limit_low = IEEE80211_DEFAULT_AQL_TXQ_LIMIT_L;
> diff --git a/net/mac80211/sta_info.c b/net/mac80211/sta_info.c
> index 2f1cf9d4e68d..9481a6ab1405 100644
> --- a/net/mac80211/sta_info.c
> +++ b/net/mac80211/sta_info.c
> @@ -1914,8 +1914,7 @@ void ieee80211_register_airtime(struct ieee80211_txq *txq,
>  	air_sched = &local->airtime[txq->ac];
>  	air_info = to_airtime_info(txq);
>  
> -	if (local->airtime_flags & AIRTIME_USE_TX)
> -		airtime += tx_airtime;
> +	airtime += tx_airtime;
>  	if (local->airtime_flags & AIRTIME_USE_RX)
>  		airtime += rx_airtime;

If we no longer honour AIRTIME_USE_TX, maybe we should make sure it
never gets unset (so as not to confuse users who think they've turned
off ATF when they haven't)?

> diff --git a/net/mac80211/sta_info.h b/net/mac80211/sta_info.h
> index 35c390bedfba..9b8774e889f5 100644
> --- a/net/mac80211/sta_info.h
> +++ b/net/mac80211/sta_info.h
> @@ -135,11 +135,14 @@ enum ieee80211_agg_stop_reason {
>  #define AIRTIME_USE_TX		BIT(0)
>  #define AIRTIME_USE_RX		BIT(1)
>  
> +DECLARE_SKIPLIST_TYPE(airtime_sched, 5);
>  
>  struct airtime_info {
> +	struct airtime_sched_node schedule_order;
> +	struct ieee80211_txq *txq[3];
>  	u64 rx_airtime;
>  	u64 tx_airtime;
> -	u64 v_t;
> +	u64 v_t, v_t_cur;
>  	u64 last_scheduled;
>  	struct list_head list;
>  	atomic_t aql_tx_pending; /* Estimated airtime for frames pending */
> @@ -147,6 +150,7 @@ struct airtime_info {
>  	u32 aql_limit_high;
>  	u32 weight_reciprocal;
>  	u16 weight;
> +	u8 txq_idx;
>  };
>  
>  void ieee80211_sta_update_pending_airtime(struct ieee80211_local *local,
> diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
> index 0e4efc08c762..3a81829d2a7a 100644
> --- a/net/mac80211/tx.c
> +++ b/net/mac80211/tx.c
> @@ -19,6 +19,7 @@
>  #include <linux/rcupdate.h>
>  #include <linux/export.h>
>  #include <linux/timekeeping.h>
> +#include <linux/prandom.h>
>  #include <net/net_namespace.h>
>  #include <net/ieee80211_radiotap.h>
>  #include <net/cfg80211.h>
> @@ -1477,11 +1478,12 @@ void ieee80211_txq_init(struct ieee80211_sub_if_data *sdata,
>  			struct sta_info *sta,
>  			struct txq_info *txqi, int tid)
>  {
> +	struct airtime_info *air_info;
> +
>  	fq_tin_init(&txqi->tin);
>  	codel_vars_init(&txqi->def_cvars);
>  	codel_stats_init(&txqi->cstats);
>  	__skb_queue_head_init(&txqi->frags);
> -	RB_CLEAR_NODE(&txqi->schedule_order);
>  
>  	txqi->txq.vif = &sdata->vif;
>  
> @@ -1490,7 +1492,7 @@ void ieee80211_txq_init(struct ieee80211_sub_if_data *sdata,
>  		txqi->txq.tid = 0;
>  		txqi->txq.ac = IEEE80211_AC_BE;
>  
> -		return;
> +		goto out;
>  	}
>  
>  	if (tid == IEEE80211_NUM_TIDS) {
> @@ -1512,6 +1514,12 @@ void ieee80211_txq_init(struct ieee80211_sub_if_data *sdata,
>  	txqi->txq.sta = &sta->sta;
>  	txqi->txq.tid = tid;
>  	sta->sta.txq[tid] = &txqi->txq;
> +
> +out:
> +	air_info = to_airtime_info(&txqi->txq);
> +	air_info->txq[air_info->txq_idx++] = &txqi->txq;
> +	if (air_info->txq_idx == ARRAY_SIZE(air_info->txq))
> +		air_info->txq_idx--;
>  }
>  
>  void ieee80211_txq_purge(struct ieee80211_local *local,
> @@ -3632,6 +3640,8 @@ struct sk_buff *ieee80211_tx_dequeue(struct ieee80211_hw *hw,
>  	struct ieee80211_tx_data tx;
>  	ieee80211_tx_result r;
>  	struct ieee80211_vif *vif = txq->vif;
> +	u32 airtime;
> +	bool ampdu;
>  
>  	WARN_ON_ONCE(softirq_count() == 0);
>  
> @@ -3790,21 +3800,26 @@ struct sk_buff *ieee80211_tx_dequeue(struct ieee80211_hw *hw,
>  encap_out:
>  	IEEE80211_SKB_CB(skb)->control.vif = vif;
>  
> -	if (vif &&
> -	    wiphy_ext_feature_isset(local->hw.wiphy, NL80211_EXT_FEATURE_AQL)) {
> -		bool ampdu = txq->ac != IEEE80211_AC_VO;
> -		u32 airtime;
> -
> -		airtime = ieee80211_calc_expected_tx_airtime(hw, vif, txq->sta,
> -							     skb->len, ampdu);
> -		if (airtime) {
> -			airtime = ieee80211_info_set_tx_time_est(info, airtime);
> -			ieee80211_sta_update_pending_airtime(local, tx.sta,
> -							     txq->ac,
> -							     airtime,
> -							     false);
> -		}
> -	}
> +	if (!vif)
> +		return skb;
> +
> +	ampdu = txq->ac != IEEE80211_AC_VO;
> +	airtime = ieee80211_calc_expected_tx_airtime(hw, vif, txq->sta,
> +						     skb->len, ampdu);
> +	if (!airtime)
> +		return skb;
> +
> +	if (!wiphy_ext_feature_isset(local->hw.wiphy, NL80211_EXT_FEATURE_AQL) ||
> +	    !wiphy_ext_feature_isset(local->hw.wiphy,
> +				     NL80211_EXT_FEATURE_AIRTIME_FAIRNESS))
> +		ieee80211_register_airtime(txq, airtime, 0);

So this implies that we always schedule in airtime order (i.e., enforce
fairness) for any driver that can get a meaningful value returned from
ieee80211_calc_expected_tx_airtime(), right? That's probably OK, but
just want to make sure we've thought through all the implications of
this.

A comment here explaining why this is done would be useful; it's a bit
counter-intuitive when just looking at the code. Your comment in the
commit message implies that scheduling doesn't work correctly if this is
not done, but then what happens if airtime is 0 and we bail out above?

> +	if (!wiphy_ext_feature_isset(local->hw.wiphy, NL80211_EXT_FEATURE_AQL))
> +		return skb;
> +
> +	airtime = ieee80211_info_set_tx_time_est(info, airtime);
> +	ieee80211_sta_update_pending_airtime(local, tx.sta, txq->ac,
> +					     airtime, false);
>  
>  	return skb;
>  
> @@ -3815,85 +3830,95 @@ struct sk_buff *ieee80211_tx_dequeue(struct ieee80211_hw *hw,
>  }
>  EXPORT_SYMBOL(ieee80211_tx_dequeue);
>  
> +static void
> +airtime_info_next_txq_idx(struct airtime_info *air_info)
> +{
> +	air_info->txq_idx++;
> +	if (air_info->txq_idx >= ARRAY_SIZE(air_info->txq) ||
> +	    !air_info->txq[air_info->txq_idx])
> +		air_info->txq_idx = 0;
> +}

The void function is a bit cumbersome in use IMO. How about making it
return the txq pointer directly? You can still save the current txq_idx
at the beginning of the loop below to detect cycles.

>  struct ieee80211_txq *ieee80211_next_txq(struct ieee80211_hw *hw, u8 ac)
>  {
>  	struct ieee80211_local *local = hw_to_local(hw);
>  	struct airtime_sched_info *air_sched;
>  	u64 now = ktime_get_coarse_boottime_ns();
> -	struct ieee80211_txq *ret = NULL;
> +	struct airtime_sched_node *node = NULL;
> +	struct ieee80211_txq *txq;
>  	struct airtime_info *air_info;
>  	struct txq_info *txqi = NULL;
> -	struct rb_node *node;
> -	bool first = false;
> +	u8 txq_idx;
>  
>  	air_sched = &local->airtime[ac];
>  	spin_lock_bh(&air_sched->lock);
>  
> -	node = air_sched->schedule_pos;
> -
>  begin:
> -	if (!node) {
> -		node = rb_first_cached(&air_sched->active_txqs);
> -		first = true;
> -	} else {
> -		node = rb_next(node);
> -	}
> +	txq = NULL;
> +	if (airtime_sched_peek(&air_sched->active_txqs) ==
> +	    air_sched->schedule_pos)
> +		goto out;
>  
> +	node = airtime_sched_dequeue(&air_sched->active_txqs);
>  	if (!node)
>  		goto out;
>  
> -	txqi = container_of(node, struct txq_info, schedule_order);
> -	air_info = to_airtime_info(&txqi->txq);
> +	air_info = container_of(node, struct airtime_info, schedule_order);
>  
> -	if (air_info->v_t > air_sched->v_t &&
> -	    (!first || !airtime_catchup_v_t(air_sched, air_info->v_t, now)))
> -		goto out;
> -
> -	if (!ieee80211_txq_airtime_check(hw, &txqi->txq)) {
> -		first = false;
> +	airtime_info_next_txq_idx(air_info);
> +	txq_idx = air_info->txq_idx;
> +	txq = air_info->txq[txq_idx];
> +	if (!txq || !ieee80211_txq_airtime_check(hw, txq))
>  		goto begin;

Can txq actually be NULL here? airtime_info_next_txq_idx() has a NULL
check inside it as well...

> +	while (1) {
> +		txqi = to_txq_info(txq);
> +		if (test_and_clear_bit(IEEE80211_TXQ_FORCE_ACTIVE, &txqi->flags))
> +			break;
> +
> +		if (txq_has_queue(txq))
> +			break;
> +
> +		airtime_info_next_txq_idx(air_info);
> +		txq = air_info->txq[air_info->txq_idx];
> +		if (txq_idx == air_info->txq_idx)
> +			goto begin;

How can this happen in normal operation? This implies that a TXQ was
scheduled without a backlog; isn't that a bug that we should warn on?

> +	}
> +
> +	if (air_info->v_t_cur > air_sched->v_t) {
> +		if (node == airtime_sched_peek(&air_sched->active_txqs))
> +			airtime_catchup_v_t(air_sched, air_info->v_t_cur, now);

We obtained node with a dequeue above, so presumably it's no longer at
the head of the queue? So how can this check ever be true?

>  	}
>  
>  	air_sched->schedule_pos = node;
>  	air_sched->last_schedule_activity = now;
> -	ret = &txqi->txq;
>  out:
>  	spin_unlock_bh(&air_sched->lock);
> -	return ret;
> +	return txq;
>  }
>  EXPORT_SYMBOL(ieee80211_next_txq);
>  
> -static void __ieee80211_insert_txq(struct rb_root_cached *root,
> +static void __ieee80211_insert_txq(struct ieee80211_local *local,
> +				   struct airtime_sched_info *air_sched,
>  				   struct txq_info *txqi)
>  {
> -	struct rb_node **new = &root->rb_root.rb_node;
> -	struct airtime_info *old_air, *new_air;
> -	struct rb_node *parent = NULL;
> -	struct txq_info *__txqi;
> -	bool leftmost = true;
> -
> -	while (*new) {
> -		parent = *new;
> -		__txqi = rb_entry(parent, struct txq_info, schedule_order);
> -		old_air = to_airtime_info(&__txqi->txq);
> -		new_air = to_airtime_info(&txqi->txq);
> -
> -		if (new_air->v_t <= old_air->v_t) {
> -			new = &parent->rb_left;
> -		} else {
> -			new = &parent->rb_right;
> -			leftmost = false;
> -		}
> +	struct airtime_info *air_info = to_airtime_info(&txqi->txq);
> +	u32 aql_time = 0;
> +
> +	if (wiphy_ext_feature_isset(local->hw.wiphy, NL80211_EXT_FEATURE_AQL)) {
> +	    aql_time = atomic_read(&air_info->aql_tx_pending);
> +	    aql_time *= air_info->weight_reciprocal;
> +	    aql_time >>= IEEE80211_RECIPROCAL_SHIFT_STA - IEEE80211_WEIGHT_SHIFT;
>  	}
>  
> -	rb_link_node(&txqi->schedule_order, parent, new);
> -	rb_insert_color_cached(&txqi->schedule_order, root, leftmost);
> +	airtime_sched_delete(&air_sched->active_txqs, &air_info->schedule_order);
> +	air_info->v_t_cur = air_info->v_t + aql_time;
> +	airtime_sched_insert(&air_sched->active_txqs, &air_info->schedule_order);
>  }
>  
>  void ieee80211_resort_txq(struct ieee80211_hw *hw,
>  			  struct ieee80211_txq *txq)
>  {
> -	struct airtime_info *air_info = to_airtime_info(txq);
>  	struct ieee80211_local *local = hw_to_local(hw);
>  	struct txq_info *txqi = to_txq_info(txq);
>  	struct airtime_sched_info *air_sched;
> @@ -3901,41 +3926,7 @@ void ieee80211_resort_txq(struct ieee80211_hw *hw,
>  	air_sched = &local->airtime[txq->ac];
>  
>  	lockdep_assert_held(&air_sched->lock);
> -
> -	if (!RB_EMPTY_NODE(&txqi->schedule_order)) {
> -		struct airtime_info *a_prev = NULL, *a_next = NULL;
> -		struct txq_info *t_prev, *t_next;
> -		struct rb_node *n_prev, *n_next;
> -
> -		/* Erasing a node can cause an expensive rebalancing operation,
> -		 * so we check the previous and next nodes first and only remove
> -		 * and re-insert if the current node is not already in the
> -		 * correct position.
> -		 */
> -		if ((n_prev = rb_prev(&txqi->schedule_order)) != NULL) {
> -			t_prev = container_of(n_prev, struct txq_info,
> -					      schedule_order);
> -			a_prev = to_airtime_info(&t_prev->txq);
> -		}
> -
> -		if ((n_next = rb_next(&txqi->schedule_order)) != NULL) {
> -			t_next = container_of(n_next, struct txq_info,
> -					      schedule_order);
> -			a_next = to_airtime_info(&t_next->txq);
> -		}
> -
> -		if ((!a_prev || a_prev->v_t <= air_info->v_t) &&
> -		    (!a_next || a_next->v_t > air_info->v_t))
> -			return;
> -
> -		if (air_sched->schedule_pos == &txqi->schedule_order)
> -			air_sched->schedule_pos = n_prev;
> -
> -		rb_erase_cached(&txqi->schedule_order,
> -				&air_sched->active_txqs);
> -		RB_CLEAR_NODE(&txqi->schedule_order);
> -		__ieee80211_insert_txq(&air_sched->active_txqs, txqi);
> -	}
> +	__ieee80211_insert_txq(local, air_sched, txqi);
>  }
>  
>  void ieee80211_update_airtime_weight(struct ieee80211_local *local,
> @@ -3984,7 +3975,7 @@ void ieee80211_schedule_txq(struct ieee80211_hw *hw,
>  	was_active = airtime_is_active(air_info, now);
>  	airtime_set_active(air_sched, air_info, now);
>  
> -	if (!RB_EMPTY_NODE(&txqi->schedule_order))
> +	if (airtime_sched_is_queued(&air_info->schedule_order))
>  		goto out;
>  
>  	/* If the station has been inactive for a while, catch up its v_t so it
> @@ -3996,7 +3987,7 @@ void ieee80211_schedule_txq(struct ieee80211_hw *hw,
>  		air_info->v_t = air_sched->v_t;
>  
>  	ieee80211_update_airtime_weight(local, air_sched, now, !was_active);
> -	__ieee80211_insert_txq(&air_sched->active_txqs, txqi);
> +	__ieee80211_insert_txq(local, air_sched, txqi);
>  
>  out:
>  	spin_unlock_bh(&air_sched->lock);
> @@ -4022,19 +4013,10 @@ static void __ieee80211_unschedule_txq(struct ieee80211_hw *hw,
>  		ieee80211_update_airtime_weight(local, air_sched, 0, true);
>  	}
>  
> -	if (RB_EMPTY_NODE(&txqi->schedule_order))
> -		return;
> -
> -	if (air_sched->schedule_pos == &txqi->schedule_order)
> -		air_sched->schedule_pos = rb_prev(&txqi->schedule_order);
> -
> +	airtime_sched_delete(&air_sched->active_txqs, &air_info->schedule_order);
>  	if (!purge)
>  		airtime_set_active(air_sched, air_info,
>  				   ktime_get_coarse_boottime_ns());

Now that there's no early return, this could be made an 'else' branch of
the if (purge) block above...

> -
> -	rb_erase_cached(&txqi->schedule_order,
> -			&air_sched->active_txqs);
> -	RB_CLEAR_NODE(&txqi->schedule_order);
>  }
>  
>  void ieee80211_unschedule_txq(struct ieee80211_hw *hw,
> @@ -4054,14 +4036,24 @@ void ieee80211_return_txq(struct ieee80211_hw *hw,
>  {
>  	struct ieee80211_local *local = hw_to_local(hw);
>  	struct txq_info *txqi = to_txq_info(txq);
> +	struct airtime_sched_info *air_sched;
> +	struct airtime_info *air_info;
>  
> -	spin_lock_bh(&local->airtime[txq->ac].lock);
> +	air_sched = &local->airtime[txq->ac];
> +	air_info = to_airtime_info(&txqi->txq);
>  
> -	if (!RB_EMPTY_NODE(&txqi->schedule_order) && !force &&
> -	    !txq_has_queue(txq))
> -		__ieee80211_unschedule_txq(hw, txq, false);
> +	if (force)
> +		set_bit(IEEE80211_TXQ_FORCE_ACTIVE, &txqi->flags);

This sets the bit even if the AQL check fails below; is that intentional?
>  
> -	spin_unlock_bh(&local->airtime[txq->ac].lock);
> +	spin_lock_bh(&air_sched->lock);
> +	if (!ieee80211_txq_airtime_check(hw, &txqi->txq))
> +	    airtime_sched_delete(&air_sched->active_txqs,
> +				 &air_info->schedule_order);

why delete here instead of unschedule?

> +	else if (txq_has_queue(txq) || force)
> +		__ieee80211_insert_txq(local, air_sched, txqi);
> +	else
> +		__ieee80211_unschedule_txq(hw, txq, false);
> +	spin_unlock_bh(&air_sched->lock);
>  }
>  EXPORT_SYMBOL(ieee80211_return_txq);
>  
> @@ -4100,46 +4092,48 @@ EXPORT_SYMBOL(ieee80211_txq_airtime_check);
>  bool ieee80211_txq_may_transmit(struct ieee80211_hw *hw,
>  				struct ieee80211_txq *txq)
>  {
> -	struct txq_info *first_txqi = NULL, *txqi = to_txq_info(txq);
> +	struct txq_info *txqi = to_txq_info(txq);
>  	struct ieee80211_local *local = hw_to_local(hw);
>  	struct airtime_sched_info *air_sched;
> +	struct airtime_sched_node *node = NULL;
>  	struct airtime_info *air_info;
> -	struct rb_node *node = NULL;
>  	bool ret = false;
> +	u32 aql_slack;
>  	u64 now;
>  
> -
>  	if (!ieee80211_txq_airtime_check(hw, txq))
>  		return false;
>  
>  	air_sched = &local->airtime[txq->ac];
>  	spin_lock_bh(&air_sched->lock);
>  
> -	if (RB_EMPTY_NODE(&txqi->schedule_order))
> -		goto out;
> -
>  	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.
>  	 */
> -	node = rb_first_cached(&air_sched->active_txqs);
> +	node = airtime_sched_peek(&air_sched->active_txqs);
>  	if (node) {
> -		first_txqi = container_of(node, struct txq_info,
> -					  schedule_order);
> -		air_info = to_airtime_info(&first_txqi->txq);
> +		air_info = container_of(node, struct airtime_info,
> +					schedule_order);
>  
>  		if (air_sched->v_t < air_info->v_t)
>  			airtime_catchup_v_t(air_sched, air_info->v_t, now);
>  	}
>  
>  	air_info = to_airtime_info(&txqi->txq);
> -	if (air_info->v_t <= air_sched->v_t) {
> +	aql_slack = air_info->aql_limit_low;
> +	aql_slack *= air_info->weight_reciprocal;
> +	aql_slack >>= IEEE80211_RECIPROCAL_SHIFT_STA - IEEE80211_WEIGHT_SHIFT;
> +	/*
> +	 * add extra slack of aql_limit_low in order to avoid queue
> +	 * starvation when bypassing normal scheduling order
> +	 */
> +	if (air_info->v_t <= air_sched->v_t + aql_slack) {

I get the intention behind this, and think it's (probably) reasonable.
But testing it with an actual ath10k device in push/pull mode would be
good :)

>  		air_sched->last_schedule_activity = now;
>  		ret = true;
>  	}
>  
> -out:
>  	spin_unlock_bh(&air_sched->lock);
>  	return ret;
>  }
> @@ -4150,9 +4144,7 @@ void ieee80211_txq_schedule_start(struct ieee80211_hw *hw, u8 ac)
>  	struct ieee80211_local *local = hw_to_local(hw);
>  	struct airtime_sched_info *air_sched = &local->airtime[ac];
>  
> -	spin_lock_bh(&air_sched->lock);
>  	air_sched->schedule_pos = NULL;
> -	spin_unlock_bh(&air_sched->lock);
>  }
>  EXPORT_SYMBOL(ieee80211_txq_schedule_start);
>  
> -- 
> 2.36.1