On Fri, Oct 17, 2008 at 04:12:03PM +0200, Patrick McHardy wrote:
...

Some general thoughts ...

We've never had any systematic checks for useful and non-useful
combination of qdiscs, which is causing a lot of these complications.
Think of all the multiq work that was required to make it work
properly with non-work-conserving qdiscs - while at the same time,
using a non-work-conserving qdisc (which require a global view)
defeats basically all of the benefits.

So it would be really useful to come up with a systematic definition
of valid combinations instead of trying handling lots of purely
theoretical case that don't make sense. One more example - all the
qdiscs implement ->drop(), yet its only needed by CBQ and it doesn't
make any sense at all to use lets say HFSC as child of CBQ.

About this specific case - yes, it would break compatibility for
users using f.i. TBF as child of netem. But if you look at the
netem_enqueue() function, it in fact assumes that the inner qdisc
is a tfifo, so we'd be breaking an already broken case. We can
of course be nice and warn about it for a few releases, but I believe
there is some real potential for simplification that makes it
worth it.

I'm not sure this is all right: at least until there is not too much
problems with some code (like with requeuing). We probably should try
before this official ways like feature-removal-schedule.txt, and/or
maybe some CONFIG_XXX_DEPRECATED things. But I don't persist with this.

BTW, I'm not sure if I'm expected to redo any patches in this thread.
(Probably some things like this teql_peek() could be removed with
->requeue() killing.)

Jarek P.