Message-ID: <87pl8z9s4y.fsf@toke.dk>
Date: Sun, 30 Nov 2025 21:34:21 +0100
From: Toke Høiland-Jørgensen <toke@...e.dk>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, Willem de Bruijn
 <willemdebruijn.kernel@...il.com>, Willem de Bruijn
 <willemdebruijn.kernel@...il.com>, Willem de Bruijn
 <willemdebruijn.kernel@...il.com>, Jamal Hadi Salim <jhs@...atatu.com>,
 Cong Wang <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet
 <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
 <pabeni@...hat.com>, Simon Horman <horms@...nel.org>
Cc: Jonas Köppeler <j.koeppeler@...berlin.de>,
 cake@...ts.bufferbloat.net,
 netdev@...r.kernel.org
Subject: Re: [PATCH net-next v2 2/4] net/sched: sch_cake: Add cake_mq qdisc
 for using cake on mq devices

Willem de Bruijn <willemdebruijn.kernel@...il.com> writes:

> Toke Høiland-Jørgensen wrote:
>> Willem de Bruijn <willemdebruijn.kernel@...il.com> writes:
>> 
>> > Toke Høiland-Jørgensen wrote:
>> >> Willem de Bruijn <willemdebruijn.kernel@...il.com> writes:
>> >> 
>> >> > Toke Høiland-Jørgensen wrote:
>> >> >> Willem de Bruijn <willemdebruijn.kernel@...il.com> writes:
>> >> >> 
>> >> >> > Toke Høiland-Jørgensen wrote:
>> >> >> >> Add a cake_mq qdisc which installs cake instances on each hardware
>> >> >> >> queue on a multi-queue device.
>> >> >> >> 
>> >> >> >> This is just a copy of sch_mq that installs cake instead of the default
>> >> >> >> qdisc on each queue. Subsequent commits will add sharing of the config
>> >> >> >> between cake instances, as well as a multi-queue aware shaper algorithm.
>> >> >> >> 
>> >> >> >> Reviewed-by: Jamal Hadi Salim <jhs@...atatu.com>
>> >> >> >> Signed-off-by: Toke Høiland-Jørgensen <toke@...hat.com>
>> >> >> >> ---
>> >> >> >>  net/sched/sch_cake.c | 214 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>> >> >> >>  1 file changed, 213 insertions(+), 1 deletion(-)
>> >> >> >
>> >> >> > Is this code duplication unavoidable?
>> >> >> >
>> >> >> > Could the same be achieved by either
>> >> >> >
>> >> >> > extending the original sch_mq to have a variant that calls the
>> >> >> > custom cake_mq_change.
>> >> >> >
>> >> >> > Or avoid hanging the shared state off of parent mq entirely. Have the
>> >> >> > cake instances share it directly. E.g., where all but the instance on
>> >> >> > netdev_get_tx_queue(dev, 0) are opened in a special "shared" mode (a
>> >> >> > bit like SO_REUSEPORT sockets) and lookup the state from that
>> >> >> > instance.
>> >> >> 
>> >> >> We actually started out with something like that, but ended up with the
>> >> >> current variant for primarily UAPI reasons: Having the mq variant be a
>> >> >> separate named qdisc is simple and easy to understand ('cake' gets you
>> >> >> single-queue, 'cake_mq' gets you multi-queue).
>> >> >> 
>> >> >> I think having that variant live with the cake code makes sense. I
>> >> >> suppose we could reuse a couple of the mq callbacks by exporting them
>> >> >> and calling them from the cake code and avoid some duplication that way.
>> >> >> I can follow up with a patch to consolidate those if you think it is
>> >> >> worth it to do so?
>> >> >
>> >> > Since most functions are identical, I do think reusing them is
>> >> > preferable over duplicating them.
>> >> 
>> >> Sure, that's fair. Seems relatively straight forward too:
>> >> 
>> >> https://git.kernel.org/pub/scm/linux/kernel/git/toke/linux.git/commit/?h=mq-cake-sub-qdisc&id=fdb6891cc584a22d4823d771a602f9f1ee56eeae
>> >
>> > Great. That's good enough for me.
>> 
>> Cool. I folded it into the series, and it does make the patch a lot
>> simpler, so thank you for the suggestion!
>> 
>> >> > I'm not fully convinced that mq_cake + cake is preferable over
>> >> > mq + cake (my second suggestion). We also do not have a separate
>> >> > mq_fq, say. But mine is just one opinion from the peanut gallery.
>> >> 
>> >> Right, I do see what you mean; as I said we did consider this initially,
>> >> but went with this implementation from a configuration simplicity
>> >> consideration. 
>> >
>> > Then admins have only to install one qdisc, rather than what we do for
>> > FQ today which is one MQ + a loop over the FQs.
>> >
>> > I don't know if we have to coddle admins like that.
>> 
>> I don't really view it as coddling, but as making it as easy as possible
>> to take advantage of the mq variant in the most common configuration.
>> The primary use case for cake is shaping on the whole link (on home
>> routers, in particular), and the mq extension came about to address the
>> common bottleneck here where the cake shaper can't keep up with link
>> speeds on a single CPU. So I think it's worthwhile to make it as easy as
>> possible to consume that seems worthwhile, in a way that retains
>> compatibility with the existing tools that work on top of cake, such as
>> the autorate scripts:
>> 
>> https://github.com/sqm-autorate/sqm-autorate
>> 
>> >> If we were to implement this as an option when running
>> >> under the existing mq, we'd have to add an option to cake itself to opt
>> >> in to this behaviour, and then deal with the various combinations of
>> >> sub-qdiscs being added and removed (including mixing cake and non-cake
>> >> sub-qdiscs of the same mq). And userspace would have to setup the mq,
>> >> then manually add the cake instances with the shared flag underneath it.
>> >
>> > One question is whether the kernel needs to protect admins from doing
>> > the unexpected thing, which would be mixing mq children of different
>> > type when using shared cake state between children.
>> >
>> > Honestly, I don't think so. But it could be done. For instance by
>> > adding an mq option that requires all children to be of the same kind,
>> > or even by silently setting this if the first child added is a cake
>> > instance with shared option set.
>> >
>> > As for shared state, in cake_init the qdisc could check that the dev
>> > root is mq and it is a direct child of this qdisc, and then scan the
>> > mq children for the existence of a cake child. If one exists, take a
>> > ref on a shared state struct. If not, create the struct. Again, like
>> > SO_REUSEPORT.
>> >
>> > All easier said than implemented, of course, but seems doable?
>> 
>> Yeah, I do think it's doable; just needs a bit of thought around the
>> lifetime management of the shared config struct as sub-qdiscs come and
>> go.
>> 
>> I am not necessarily opposed to supporting this mode, including the case
>> where there's a mix of qdiscs on different HW queues. However, I view it
>> as an extension of the base use case, as described above. Now that we're
>> reusing the mq code, cake_mq becomes quite a lightweight addition, so we
>> could potentially support both? I.e., the cake_mq qdisc would be the
>> straight-forward way to load cake across a device, but we could add
>> support for sharing cake state across (a subset of) regular mq as well?
>> WDYT?
>
> I'd only plan to do it once, do it right.
>
> mq_cake has the advantage of being simpler to configure.
>
> The alternative that it allows more configurations. But we don't
> immediately see real use cases for those.
>
> Your call, assuming no one else chimes in.

Right. Well, given the somewhat speculative nature of the combination
use cases, I am still leaning towards the cake_mq version. Will submit a
v3 with the shared sch_mq code.

Thank you for looking at the code and chiming in!

-Toke

Powered by blists - more mailing lists