[Make-wifi-fast] [RFC] mac80211: Add airtime fairness accounting

Johannes Berg johannes at sipsolutions.net
Fri Oct 6 13:18:39 EDT 2017

On Fri, 2017-10-06 at 16:29 +0200, Toke Høiland-Jørgensen wrote:
> Johannes Berg <johannes at sipsolutions.net> writes:
> > On Fri, 2017-10-06 at 13:52 +0200, Toke Høiland-Jørgensen wrote:
> > > This adds accounting of each station's airtime usage to mac80211.
> > > On
> > > the RX side, airtime is calculated from the packet length and
> > > duration.
> > 
> > I think you should probably try to get something here from the
> > driver?
> > Doing the calculations is really awkward, because you have to take
> > into account aggregation, etc.
> Well, calculating the duration from the rate and size is what ath9k
> is currently doing, and that has all the information available to do
> so (I did verify that the airtime was almost identical on the TX and
> RX side for the same traffic).

It's still iffy - with aggregation you might have a better idea of the
total airtime, but not really see - in the higher layers - all the
padding that comes between A-MPDUs etc. I think many drivers could do
better by exposing the total airtime from the device/firmware, whereas
exposing _all_ the little details that you need to calculate it post-
facto will be really difficult, and make the calculation really hard.

> But yeah, I guess that is not necessarily the case for all drivers?
> Maybe having a field that the driver can fill in is better (that also
> has the nice property that a driver that doesn't supply any airtime
> information won't get the scheduling; which is probably desirable).

I think that'd be better, yes. Perhaps for some simple cases a helper
function can be provided, but I'd want to actually know about those
drivers before adding that.

> You're right that it does nothing for one-way UDP, of course. But not
> a lot of traffic is actually one-way (most is either TCP or something
> like Quic that also has a feedback loop). So in most of our tests we
> saw a pretty nice effect from TCP back-pressure.

Yeah that makes some sense. I'm a bit concerned about pathological
corner cases, you don't really know about the protocol used on top. Can
a client starve itself by sending things? And perhaps, if it has a dumb
higher layer protocol, it'll retransmit rather than back off, if it
doesn't get the higher layer ACK?

> > Then again, I'm not even sure yet why you care about the AC at all?
> > Should fairness really be something that's within an AC?
> No, ideally it shouldn't. This is mostly an artifact of how ath9k
> schedules TXQs independently for each AC. This means that if (say)
> three stations are back-logged on the BE queue, and one of those also
> has the occasional packet on the VO queue, if the deficit is shared
> between all levels, every time a VO packet arrives that station
> effectively gets its deficit reset to zero, meaning it will get too
> much airtime.
> Having a separate deficit for each AC level is a crude workaround for
> this, which is what we went with for the initial version in ath9k;
> and this patch is just a straight-forward port of that. But I guess
> this is a good opportunity to figure out how to solve this properly;
> I'll think about how to do that (ideas welcome!)...

So ... no, I don't understand. You have a TXQ per _TID_, so splitting
up this per _AC_ still doesn't make sense since you have two TXQs for
each AC?

But then again I'm not sure I've sufficiently wrapped my head around
the whole algorithm, so ...


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