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

[Pete Heist]

> On Jan 5, 2019, at 2:35 PM, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
> 
> Pete Heist <pete at heistp.net> writes:
> 
>>> On Jan 5, 2019, at 2:10 PM, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
>>> 
>>> Hmm, that's odd. Could you try adding this debugging line in
>>> adjust_parent_qlen(), right before the sch->q.qlen += n line:
>>> 
>>> 		net_info_ratelimited("Adjusting parent qdisc %d with pkt += %d, len += %d",
>>> 				     parentid, n, len);
>>> 
>>> And see if you actually get any of those lines in your dmesg?
>> 
>> I do see the messages twice, then not after that in the rest of the
>> output...
> 
> Right. Looking at the HFSC code some more, I think the bug is actually
> caused by another, but related, interaction between HFSC and CAKE.
> 
> Specifically, this line:
> 
> https://elixir.bootlin.com/linux/v3.16.7/source/net/sched/sch_hfsc.c#L1605
> 
> where HFSC checks whether the child queue len is 1, which it interprets
> as the event that activates that queue. However, because CAKE splits the
> packet, this check will fail, and the HFSC class will not be activated.
> This also explains why you only see the bug with HFSC, and not with HTB
> (although I do think that we still need to update the hierarchy).
> 
> The good news it that it is a fairly simple to fix in HFSC. The bad news
> is that it's something that's hard to work around from the out-of-tree
> CAKE...

Aha, well, I wonder if we’ll see this problem with other qdiscs- maybe cbq, if I ever get a chance to try it (not hurrying yet). Ideally this interaction between qdiscs would be clarified somewhere, at some point. :)

Thanks a lot for doing the discovery though! We may not have hfsc+cake with GSO splitting on older kernels very soon, but what should we do with this? There’s nobody in MAINTAINERS for hfsc, so we may not get much of a response to any bug submissions...

Pete