Re: [Make-wifi-fast] [PATCH v3 3/6] mac80211: Add airtime accounting and scheduling to TXQs

[Felix Fietkau <nbd@nbd.name>]

On 2018-11-14 18:40, Toke Høiland-Jørgensen wrote:
>> This part doesn't really make much sense to me, but maybe I'm
>> misunderstanding how the code works.
>> Let's assume we have a driver like ath9k or mt76, which tries to keep a
>> number of aggregates in the hardware queue, and the hardware queue is
>> currently empty.
>> If the current txq entry is kept at the head of the schedule list,
>> wouldn't the code just pull from that one over and over again, until
>> enough packets are transmitted by the hardware and their tx status
>> processed?
>> It seems to me that while fairness is still preserved in the long run,
>> this could lead to rather bursty scheduling, which may not be
>> particularly latency friendly.
> 
> Yes, it'll be a bit more bursty when the hardware queue is completely
> empty. However, when a TX completion comes back, that will adjust the
> deficit of that sta and cause it to be rotated on the next dequeue. This
> obviously relies on the fact that the lower-level hardware queue is
> sufficiently shallow to not add a lot of latency. But we want that to be
> the case anyway. In practice, it works quite well for ath9k, but not so
> well for ath10k because it has a large buffer in firmware.
> 
> If we requeue the TXQ at the end of the list, a station that is taking
> up too much airtime will fail to be throttled properly, so the
> queue-at-head is kinda needed to ensure fairness...
Thanks for the explanation, that makes sense to me. I have an idea on
how to mitigate the burstiness within the driver. I'll write it down in
pseudocode, please let me know if you think that'll work.

do {
	struct ieee80211_txq *pending_txq[4];
	int n_pending_txq = 0;
	int i;

	if (hwq->pending < 4)
		break;

	nframes = 0;

	ieee80211_txq_schedule_start(hw, ac)
	do {
		bool requeue = false;

		struct ieee80211_txq *txq;

		txq = ieee80211_next_txq(hw, ac);
		if (!txq)
			break;

		nframes += schedule_txq(txq, &requeue);
		if (requeue)
			pending_txq[n_pending_txq++] = txq;

	} while (n_pending_txq < ARRAY_SIZE(pending_txq));

	for (i = n_pending_txq; i > 0; i--)
		ieee80211_return_txq(hw, pending_txq[i - 1]);

	ieee80211_txq_schedule_end(hw, ac)
} while (nframes);

- Felix