Re: [Make-wifi-fast] [PATCH v11 4/4] mac80211: Use Airtime-based Queue Limits (AQL) on packet dequeue

[Felix Fietkau <nbd@nbd.name>]

On 2020-02-27 11:07, Toke Høiland-Jørgensen wrote:
> Felix Fietkau <nbd@nbd.name> writes:
> 
>> On 2020-02-26 22:56, Toke Høiland-Jørgensen wrote:
>>> Felix Fietkau <nbd@nbd.name> writes:
>>>> - We need an API that allows the driver to change the pending airtime
>>>> values, e.g. subtract estimated tx time for a packet.
>>>> mt76 an ath9k can queue packets inside the driver that are not currently
>>>> in the hardware queues. Typically if the txqs have more data than what
>>>> gets put into the hardware queue, both drivers will pull an extra frame
>>>> and queue it in its private txq struct. This frame will get used on the
>>>> next txq scheduling round for that particular station.
>>>> If you have lots of stations doing traffic (or having driver buffered
>>>> frames in powersave mode), this could use up a sizable chunk of the AQL
>>>> budget.
>>> 
>>> I'm a bit more skeptical about this. If the driver buffers a bunch of
>>> packets that are not accounted that will hurt that station due to extra
>>> latency when it wakes up. For ath9k, this is the retry_q you're talking
>>> about, right? The number of packets queued on that is fairly limited,
>>> isn't it? What kind of powersave buffering is the driver doing, and why
>>> can't it leave the packets on the TXQ? That would allow them to be
>>> scheduled along with any new ones that might have arrived in the
>>> meantime, which would be a benefit for latency.
>> For mt76 there should be max. 1 frame in the retry queue, it's just a
>> frame that was pulled from the txq in a transmission attempt but that it
>> couldn't put in the hw queue because it didn't fit in the current
>> aggregate batch.
> 
> Wait, if it's only a single frame that is queued in the driver, how is
> this causing problems? We deliberately set the limit so there was a bit
> of slack above the size of an aggregate for things like this. Could you
> please describe in a bit more detail what symptoms you are seeing of
> this problem? :)
It would be a single frame per sta/txq. I don't know if it will cause
problems in practice, it's just a potential corner case that I found
during review. I'd imagine this would probably show up in some
benchmarks at least.
I'm not seeing any symptoms myself, but I also haven't run any intricate
tests yet.

- Felix