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

[Sebastian Moeller]

Dear all,

I am most likely wrong, but did you have a look at https://bugs.openwrt.org/index.php?do=details&task_id=1136 yet?
Especially https://bugzilla.kernel.org/show_bug.cgi?id=109581 and https://www.spinics.net/lists/netdev/msg450655.html might be related to Pete's bug.
Then again, I might be wrong as the whole flurry of emails went past my head quickly.

Best Regards
	Sebastian


> On Jan 5, 2019, at 17:32, Toke Høiland-Jørgensen <toke at toke.dk> wrote:
> 
> 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
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