Received-SPF: pass (google.com: best guess record for domain of linux-wireless-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= <toke@redhat.com>
To:     Johannes Berg <johannes@sipsolutions.net>,
        Jens Axboe <axboe@kernel.dk>,
        Emmanuel Grumbach <emmanuel.grumbach@intel.com>,
        Luca Coelho <luciano.coelho@intel.com>
Cc:     "linux-wireless\@vger.kernel.org" <linux-wireless@vger.kernel.org>,
        Networking <netdev@vger.kernel.org>
Subject: Re: iwlwifi warnings in 5.5-rc1
In-Reply-To: <b14519e81b6d2335bd0cb7dcf074f0d1a4eec707.camel@sipsolutions.net>
References: <ceb74ea2-6a1b-4cef-8749-db21a2ee4311@kernel.dk> <9727368004ceef03f72d259b0779c2cf401432e1.camel@sipsolutions.net> <878snjgs5l.fsf@toke.dk> <3420d73e667b01ec64bf0cc9da6232b41e862860.camel@sipsolutions.net> <875zingnzt.fsf@toke.dk> <bfab4987668990ea8d86a98f3e87c3fa31403745.camel@sipsolutions.net> <14bbfcc8408500704c46701251546e7ff65c6fd0.camel@sipsolutions.net> <87r21bez5g.fsf@toke.dk> <b14519e81b6d2335bd0cb7dcf074f0d1a4eec707.camel@sipsolutions.net>
Date:   Wed, 11 Dec 2019 15:47:08 +0100
Message-ID: <87k172gbrn.fsf@toke.dk>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Transfer-Encoding: quoted-printable
Sender: linux-wireless-owner@vger.kernel.org
Precedence: bulk

Johannes Berg <johannes@sipsolutions.net> writes:

> On Wed, 2019-12-11 at 15:04 +0100, Toke H=C3=B8iland-J=C3=B8rgensen wrote=
:
>> Johannes Berg <johannes@sipsolutions.net> writes:
>>=20
>> > Btw, there's *another* issue. You said in the commit log:
>> >=20
>> >     This patch does *not* include any mechanism to wake a throttled TX=
Q again,
>> >     on the assumption that this will happen anyway as a side effect of=
 whatever
>> >     freed the skb (most commonly a TX completion).
>> >=20
>> > 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 yo=
u
>> > 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 limi=
t,
>> > and prevented dequeuing, no?
>>=20
>> 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.

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)...

-Toke