Re: [PATCH v9 4/4] mac80211: Use Airtime-based Queue Limits (AQL) on packet dequeue

[Kan Yan <kyan@google.com>]

> +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;
> +}
> +
> +static inline u16
> +ieee80211_info_get_tx_time_est(struct ieee80211_tx_info *info)
> +{
> +       return info->tx_time_est << 2;
> +}
> +

set_tx_time_est() returns airtime in different units (4us) than
get_tx_time_est(), this will cause the pending_airtime out of whack.

Given the fact that AQL is only tested in very limited platforms,
should we set the default to disabled by removing this change in the
next update?

-       local->airtime_flags = AIRTIME_USE_TX | AIRTIME_USE_RX;
+
+       local->airtime_flags = AIRTIME_USE_TX |
+                              AIRTIME_USE_RX |
+                              AIRTIME_USE_AQL;
+       local->aql_threshold = IEEE80211_AQL_THRESHOLD;
+       atomic_set(&local->aql_total_pending_airtime, 0);


On Fri, Nov 15, 2019 at 5:20 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>
> From: Toke Høiland-Jørgensen <toke@redhat.com>
>
> 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 <toke@redhat.com>
> ---
>  include/net/mac80211.h |   16 ++++++++++++++++
>  net/mac80211/status.c  |   26 ++++++++++++++++++++++++++
>  net/mac80211/tx.c      |   18 ++++++++++++++++++
>  3 files changed, 60 insertions(+)
>
> diff --git a/include/net/mac80211.h b/include/net/mac80211.h
> index ba3f33cc41ea..dcb4a1f19829 100644
> --- a/include/net/mac80211.h
> +++ b/include/net/mac80211.h
> @@ -1060,6 +1060,22 @@ struct ieee80211_tx_info {
>         };
>  };
>
> +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;
> +}
> +
> +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
>   *
> diff --git a/net/mac80211/status.c b/net/mac80211/status.c
> index 0e51def35b8a..39da82b35be9 100644
> --- a/net/mac80211/status.c
> +++ b/net/mac80211/status.c
> @@ -670,12 +670,26 @@ static void ieee80211_report_used_skb(struct ieee80211_local *local,
>                                       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;
>
> @@ -877,6 +891,7 @@ static void __ieee80211_tx_status(struct ieee80211_hw *hw,
>         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);
>
> @@ -986,6 +1001,17 @@ static void __ieee80211_tx_status(struct ieee80211_hw *hw,
>                         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) {
>                                 if (sta->status_stats.lost_packets)
> diff --git a/net/mac80211/tx.c b/net/mac80211/tx.c
> index aafc67b562eb..2fb6571453e7 100644
> --- a/net/mac80211/tx.c
> +++ b/net/mac80211/tx.c
> @@ -3551,6 +3551,9 @@ struct sk_buff *ieee80211_tx_dequeue(struct ieee80211_hw *hw,
>
>         WARN_ON_ONCE(softirq_count() == 0);
>
> +       if (!ieee80211_txq_airtime_check(hw, txq))
> +               return NULL;
> +
>  begin:
>         spin_lock_bh(&fq->lock);
>
> @@ -3661,6 +3664,21 @@ struct sk_buff *ieee80211_tx_dequeue(struct ieee80211_hw *hw,
>         }
>
>         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:
>