[Cake] cake infinite loop(?) with hfsc on one-armed router

[Toke Høiland-Jørgensen]

Pete Heist <pete at heistp.net> writes:

> Ok, the lockup goes away if you use no-split-gso on the cake qdiscs for the default traffic (noted below in the drr and hfsc cases with "!!! must use no-split-gso here !!!"). Only I’d like my 600 μs back. :)
>
> This smells of a bug Toke fixed on Sep 12, 2018 in 42e87f12ea5c390bf5eeb658c942bc810046160a, but then reverted in the next commit because it was fixed upstream. However, if I re-apply that commit, it still doesn’t fix it.
>
> Perhaps there are more cases where skb_reset_mac_len(skb) needs to be called somewhere for VLAN support?
>
> I managed to capture some output from what happens to hfsc:
>
> [  683.864456] ------------[ cut here ]------------
> [  683.869116] WARNING: CPU: 1 PID: 11 at net/sched/sch_hfsc.c:1427
> 0xf9ced4ef()

So this seems to be this line:

WARN_ON(next_time == 0);

See https://elixir.bootlin.com/linux/v3.16.7/source/net/sched/sch_hfsc.c#L1427

Which seems to indicate that HFSC can't find the next class to schedule.
Not entirely sure why, nor why this only happens with CAKE as a qdisc.
But I don't think it's actually an infinite loop that's causing it...

-Toke