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Date:   Wed, 11 Dec 2019 22:18:21 +0100
From:   Johannes Berg <johannes@...solutions.net>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Jens Axboe <axboe@...nel.dk>,
        Emmanuel Grumbach <emmanuel.grumbach@...el.com>,
        Luca Coelho <luciano.coelho@...el.com>
Cc:     "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>
Subject: Re: iwlwifi warnings in 5.5-rc1

On Wed, 2019-12-11 at 15:47 +0100, Toke Høiland-Jørgensen wrote:

> > Say you have some queues - some (Q1-Qn) got a LOT of traffic, and
> > another (Q0) just has some interactive traffic.
> > 
> > You could then end up in a situation where you have 24ms queued up on
> > Q1-Qn (with n high enough to not have hit the per-queue AQL limit),
> > right?
> > 
> > Say also the last frame on Q0 was dequeued by the hardware, but the
> > tx_dequeue() got NULL because of the AQL limit having been eaten up by
> > all the packets on Q1-Qn.
> > 
> > Now you'll no longer get a new dequeue attempt on Q0 (it was already
> > empty last time, so no hardware reclaim to trigger new dequeues), and a
> > new dequeue on the *other* queues will not do anything for this queue.
> 
> Oh, right, I see; yeah, that could probably happen. I guess we could
> either kick all available queues whenever the global limit goes from
> "above" to "below"; or we could remove the "return NULL" logic from
> tx_dequeue() and rely on next_txq() to throttle. I think the latter is
> probably simpler, but I'm a little worried that the throttling will
> become too lax (because the driver can keep dequeueing in the same
> scheduling round)...

I honestly have no idea what's better ... :) You're the expert, I'm just
poking holes into it ;-)

johannes

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