netdev - Re: iwlwifi warnings in 5.5-rc1

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Date:   Wed, 11 Dec 2019 15:12:31 +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 15:04 +0100, Toke Høiland-Jørgensen wrote:
> Johannes Berg <johannes@...solutions.net> writes:
> 
> > Btw, there's *another* issue. You said in the commit log:
> > 
> >     This patch does *not* include any mechanism to wake a throttled TXQ again,
> >     on the assumption that this will happen anyway as a side effect of whatever
> >     freed the skb (most commonly a TX completion).
> > 
> > Thinking about this some more, I'm not convinced that this assumption
> > holds. You could have been stopped due to the global limit, and now you
> > wake some queue but the TXQ is empty - now you should reschedule some
> > *other* TXQ since the global limit had kicked in, not the per-TXQ limit,
> > and prevented dequeuing, no?
> 
> Well if you hit the global limit that means you have 24ms worth of data
> queued in the hardware; those should be completed in turn, and enable
> more to be dequeued, no?

Yes, but on which queues?

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.

johannes