netdev - Re: [RFC/RFT] mac80211: implement fq

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Date:	Mon, 7 Mar 2016 12:14:34 -0500
From:	Avery Pennarun <apenwarr@...il.com>
To:	Dave Taht <dave.taht@...il.com>
Cc:	Felix Fietkau <nbd@...nwrt.org>,
	Michal Kazior <michal.kazior@...to.com>,
	Tim Shepard <shep@...m.mit.edu>,
	linux-wireless <linux-wireless@...r.kernel.org>,
	Johannes Berg <johannes@...solutions.net>,
	Network Development <netdev@...r.kernel.org>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
	Andrew Mcgregor <andrewmcgr@...gle.com>,
	Toke Høiland-Jørgensen <toke@...e.dk>
Subject: Re: [RFC/RFT] mac80211: implement fq_codel for software queuing

On Mon, Mar 7, 2016 at 11:54 AM, Dave Taht <dave.taht@...il.com> wrote:
> If I can just get a coherent patch set that I can build, I will gladly
> join you on the testing ath9k in particular... can probably do ath10k,
> too - and do a bit of code review... this week. it's very exciting
> watching all this activity...
>
> but I confess that I've totally lost track of what set of trees and
> patchwork I should try to pull from. wireless-drivers-next? ath10k?
> wireless-next? net-next? toke and I have a ton of x86 platforms
> available to test on....
>
> Avery - which patches did you use??? on top of what?

The patch series I'm currently using can be found here:

  git fetch https://gfiber.googlesource.com/vendor/opensource/backports
ath9k_txq+fq_codel

That's again backports-20160122, which comes from linux-next as of
20160122.  You can either build backports against whatever kernel
you're using (probably easiest) or try to use that version of
linux-next, or rebase the patches onto your favourite kernel.

> In terms of "smoothing" codel...
>
> I emphatically do not think codel in it's current form is "ready" for
> wireless, at the very least the target should not be much lower than
> 20ms in your 2 station tests.  There is another bit in codel where the
> algo "turns off" with only a single MTU's worth of packets
> outstanding, which could get bumped to the ideal size of the
> aggregate. "ideal" kind of being a variable based on a ton of other
> factors...

Yeah, I figured that sort of thing would come up.  I'm feeling forward
progress just by finally seeing the buggy oscillations finally happen,
though. :)

> the underlying code needs to be striving successfully for per-station
> airtime fairness for this to work at all, and the driver/card
> interface nearly as tight as BQL is for the fq portion to behave
> sanely. I'd configure codel at a higher target and try to observe what
> is going on at the fq level til that got saner.

That seems like two good goals.  So Emmanuel's BQL-like thing seems
like we'll need it soon.

As for per-station airtime fairness, what's a good approximation of
that?  Perhaps round-robin between stations, one aggregate per turn,
where each aggregate has a maximum allowed latency?  I don't know how
the current code works, but it's probably almost like that, as long as
we only put one aggregate's worth of stuff into each hwq, which I
guess is what the BQL-like thing will do.

So if I understand correctly, what we need is, in the following order:
1) Disable fq_codel for now, and get BQL-like thing working in ath9k
(and ensure we're getting airtime fairness even without fq_codel);
2) Re-enable fq_codel and increase fq_codel's target up to 20ms for now;
3) Tweak fq_codel's "turn off" size to be larger (how important is this?)

Is that right?