Re: [Make-wifi-fast] [PATCH RFC v3 1/4] mac80211: Add TXQ scheduling API

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

Johannes Berg <johannes@sipsolutions.net> writes:

> On Mon, 2018-09-10 at 15:18 +0200, Toke Høiland-Jørgensen wrote:
>
>> If we have the start_schedule() / end_schedule() pair anyway, the latter
>> could notify any TXQs that became eligible during the scheduling round.
>
> Do we even need end_schedule()? It's hard to pass multiple things to a
> single call (do you build a list?), so having
>
> 	start_schedule(), get_txq(), return_txq()
>
> would be sufficient?

Well, start_schedule() / end_schedule() would be needed if we are going
to add locking in mac80211?

>> Also, instead of having the three different API functions
>> (next_txq()/may_tx()/schedule_txq()), we could  have get_txq(txq)/put_txq(txq)
>> which would always need to be paired; but the argument to get_txq()
>> could be optional, and if the driver passes NULL it means "give me the
>> next available TXQ".
>
> I can't say I like this. It makes the meaning totally different:
>
>  * with NULL: use the internal scheduler to determine which one is good
>    to use next
>  * non-NULL: essentially equivalent to may_tx()

Yeah, it'll be two completely different uses for the same function; but
there wouldn't be two different APIs to keep track of. I'm fine with
keeping them as separately named functions. :)

>> So for ath9k it would be:
>> 
>> 
>> start_schedule(ac);
>> while ((txq = get_txq(NULL)) {
>>   queue_aggregate(txq);
>>   put_txq(txq);
>> }
>> end_schedule(ac);
>> 
>> And for ath10k/iwlwifi it would be:
>> 
>> on_hw_notify(txq) {
>>  start_schedule(ac);
>>  if (txq = get_txq(txq)) {
>>    queue_packets(txq);
>>    put_txq(txq);
>>  }
>>  end_schedule(ac);
>> }
>> 
>> 
>> I think that would be simpler, API-wise?
>
> I can't say I see much point in overloading get_txq() that way. You'd
> never use it the same way.
>
> Also, would you really start_schedule(ac) in the hw-managed case?

If we decide mac80211 needs to do locking to prevent two threads from
scheduling the same ac, that would also be needed for the hw-managed
case?

> It seems like not? Basically it seems to me that in the hw-managed
> case all you need is may_tx()? And in fact, once you opt in you don't
> even really need *that* since mac80211 can just return NULL from
> get_skb()?

Yeah, we could just throttle in get_skb(); the separate call was to
avoid the overhead of the check for every packet. Typically, you'll pick
a TXQ, then dequeue multiple packets from it in succession; with a
separate call to may_tx(), you only do the check once, not for every
packet...

-Toke