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

[Toke Høiland-Jørgensen]

Pete Heist <pete at heistp.net> writes:

>> 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!

You're welcome, and thanks for you help :)

> 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...

$ ./scripts/get_maintainer.pl net/sched/sch_hfsc.c 
Jamal Hadi Salim <jhs at mojatatu.com> (maintainer:TC subsystem)
Cong Wang <xiyou.wangcong at gmail.com> (maintainer:TC subsystem)
Jiri Pirko <jiri at resnulli.us> (maintainer:TC subsystem)
"David S. Miller" <davem at davemloft.net> (maintainer:NETWORKING [GENERAL])
netdev at vger.kernel.org (open list:TC subsystem)

I'll submit a patch sometime next week, and also look into the qlen
adjustment for CAKE GSO splitting...

-Toke