Re: [Make-wifi-fast] [PATCH v2 4/4] mac80211: Use Airtime-based Queue Limits (AQL) on packet dequeue

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Sebastian Moeller <moeller0@gmx.de> writes:

>> On Oct 17, 2019, at 11:44, Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>> 
>> Kan Yan <kyan@google.com> writes:
>> 
>>> Hi Toke,
>>> 
>>> Thanks for getting this done! I will give it a try in the next few
>>> days.  A few comments:
>>> 
>>>> 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.
>>> 
>>> Looks like ath10k driver zero out the info->status before calling
>>> ieee80211_tx_status(...):
>>> int ath10k_txrx_tx_unref(struct ath10k_htt *htt,
>>>                         const struct htt_tx_done *tx_done)
>>> {
>>> ...
>>>        info = IEEE80211_SKB_CB(msdu);
>>>        memset(&info->status, 0, sizeof(info->status));
>>> ...
>>>        ieee80211_tx_status(htt->ar->hw, msdu);
>>> }
>> 
>> Ah, bugger; I was afraid we'd run into this. A quick grep indicates that
>> it's only ath10k and iwl that do this, though, so it's probably
>> manageable to just fix this. I think the simplest solution is just to
>> restore the field after clearing, no?
>> 
>>> We need either restore the info->status.tx_time_est or calling
>>> ieee80211_sta_update_pending_airtime() in ath10k before tx_time_est
>>> get erased.
>>> 
>>>> +       if (local->airtime_flags & AIRTIME_USE_AQL) {
>>>> +               airtime = ieee80211_calc_expected_tx_airtime(hw, vif, txq->sta,
>>>> +                                                            skb->len + 38);
>>> 
>>> I think it is better to put the "+  38" that takes care of the header
>>> overhead inside ieee80211_calc_expected_tx_airtime().
>> 
>> Hmm, no strong opinion about this; but yeah, since we have a dedicated
>> function for this use I guess there's no harm in adding it there :)
>> 
>
> Silly question, is this Overhead guaranteed to be 38 Bytes for all
> eternity? Otherwise a variable or a preprocessor constant might be
> more future proof?

Well, yeah, as long as we're sending Ethernet packets. Which is kinda
baked into the WiFi standard :)

-Toke