Eric Dumazet [off-list ref] wrote:

On Wed, 2016-06-01 at 01:00 +0200, Florian Westphal wrote:
Generally speaking, a non work conserving qdisc could return NULL if it
decides to drop all packet(s) that were sitting in the queue.

Say we add a 'max sojourn time' on skbs, as yet another anti-bloat
features, in a Codel variant/extension.

HFSC does check this and eventually complains ( qdisc_warn_nonwc()),
but all its precise computations are probably wrong.

Right, this is already a problem when using hfsc with silly fq_codel config
(we can end up with underflow on sch->q.qlen, i.e. hfsc
ends up thinking it has 0xffffffff packets in class while fq_codel has 0).

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If you want to make HFSC generic, you would need a lot more changes.

Could you elaborate?

HFSC is based on packet lengths and realtime/deadline constraints.

With lazy packet lengths (if the attached children qdisc reorder
packets), this all becomes fuzzy. Which might be OK or not.

We are not helping users by silently making such decisions.
Maybe they prefer to be warned so that they can fix their setup.

Seems that limits us to pfifo or prio+pfifo only...

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I'm not interested in e.g. tbf leaves (makes no sense to me), but I
think it should play nice with netem, sfq, codel, fq_codel, fq, ...

I don't see any problems with fq_codel + hfsc unless I deliberately
misconfigure fq_codel (setting extremely low mem limit, e.g. 32kbyte).

That is pure luck, then. I would not be surprised that HTB + nonwc qdisc
would actually crash in some cases.

Ugh.  I think that using codel, sfq, fq_codel etc. should just work
when used with htb or hfsc.

Using tbf and friends should at least not crash kernel (it would
be nice if we could complain and/or reject at config time though).

Thanks for your input, I see this isn't easy to resolve.