Re: [PATCH 1/2] mac80211: implement fair queuing per txq

From: Avery Pennarun <apenwarr@gmail.com>
To: Michal Kazior <michal.kazior@tieto.com>, Tim Shepard <shep@alum.mit.edu>
Cc: linux-wireless <linux-wireless@vger.kernel.org>,
	Johannes Berg <johannes@sipsolutions.net>,
	Dave Taht <dave.taht@gmail.com>,
	make-wifi-fast@lists.bufferbloat.net,
	codel@lists.bufferbloat.net
Subject: Re: [PATCH 1/2] mac80211: implement fair queuing per txq
Date: Fri, 8 Apr 2016 00:37:23 -0400	[thread overview]
Message-ID: <CAHqTa-0yEFYAFpSKyko1ET33GsN==_D8TpBeq6AtwLj4XMTW0A@mail.gmail.com> (raw)
In-Reply-To: <1458898052-20601-2-git-send-email-michal.kazior@tieto.com>

On Fri, Mar 25, 2016 at 5:27 AM, Michal Kazior <michal.kazior@tieto.com> wrote:
> mac80211's software queues were designed to work
> very closely with device tx queues. They are
> required to make use of 802.11 packet aggregation
> easily and efficiently.
>
> However the logic imposed a per-AC queue limit.
> With the limit too small mac80211 wasn't be able
> to guarantee fairness across TIDs nor stations
> because single burst to a slow station could
> monopolize queues and reach per-AC limit
> preventing traffic from other stations being
> queued into mac80211's software queues. Having the
> limit too large would make smart qdiscs, e.g.
> fq_codel, a lot less efficient as they are
> designed on the premise that they are very close
> to the actualy device tx queues.

As usual, I'm way behind on everything, but I have been testing this
patch series in the background (no clear results to report yet) and
wanted to comment at a very high level.  I think you are actually
doing several stages of improvements all at once here:

[0. Baseline: one big queue going into the driver]
1. Switch ath10k to mac80211 per-station queues.
2. Change per-station queues to use NO_QUEUE qdisc and *not* ever stop
the kernel netdev queue (since there no longer is one).
3. Actively manage per-station queues with fq_codel.
4. DQL-like control system for managing hardware queues.

Just to clarify what I mean by #2, if I understand correctly, before
this patch, the driver+mac80211 keeps track of the total number of
packets in all the mac80211 queues.  When the total exceeds a fixed
amount (or when one of the per-station queues gets full?) mac80211
tells the kernel to stop sending in new packets, so they sit around in
the qdisc instead.  The problem with this behaviour is we probably
have a lot of packets for one station, and not many packets for other
stations, even if the netdev qdisc has plenty of packets still waiting
for those other stations.  When you then go to drain the mac80211
queues in a round-robin fashion, only the fullest queue (corresponding
to the busiest stream to the fastest station) can get optimal results.
The driver can then either send out from the fullest queue (unfair but
fast) or round robin using the non-full queues (fair but non-optimal
speed).

Upon implementing #2, we would essentially never tell the kernel to
stop sending packets; instead, it just always forwards them to
mac80211, which needs to learn how to drop them instead of providing
backpressure.  This moves the entire qdisc functionality into
mac80211, hence the use of NO_QUEUE.

It's then obvious that if you just did the obvious thing (tail drop),
you'll end up with high latency, so you added fq_codel to the mix.

However, as people on this thread have noticed, fq_codel is
complicated.  I'd like to be able to evaluate the performance impact
of each of the above steps separately.  In particular, my theory is
that if we implement #2 with just a simple FIFO queue per station,
then if we have two stations competing (one slow, one fast), and
dequeue aggregates using round robin, then we should get all of:

a) Full airtime utilization and max-length aggregates
and
b) High latency only on busy stations, but near-zero latency on idle
stations (because of round-robin servicing of the per-station queues).

Using just a tail drop implementation, it should be very easy for me
to test that (a) and (b) are true.  It should also be strictly equal
(one station) or better (multiple stations) than using mac80211 soft
queues with the pfifo_fast qdisc.  If that isn't what happens, then
we'll know something went wrong with that part of the code, and we can
debug that before moving on to a wifi-aware fq_codel.

So my request: do you mind splitting your patch into two patches, one
that implements just NO_QUEUE and per-station fifo tail drop, with a
second patch that converts the tail drop to fq_codel?

Another advantage of the split is that we could then test NO_QUEUE +
tail_drop + DQL.  Again, that should be strictly better than the
NO_QUEUE + tail_drop + fixed_driver_queue.  Then it might be easier to
debug the (much more fiddly) fq_codel on top.

Thoughts?

Thanks,

Avery