netdev - Re: [PATCH net] net/sched: ets: use old 'nbands' while purging unused classes

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Message-ID: <2ac9d393-a87b-4b55-87d6-0b76542e63c9@mojatatu.com>
Date: Mon, 11 Aug 2025 14:35:50 -0300
From: Victor Nogueira <victor@...atatu.com>
To: Davide Caratti <dcaratti@...hat.com>
Cc: Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang
 <xiyou.wangcong@...il.com>, Jiri Pirko <jiri@...nulli.us>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>,
 Simon Horman <horms@...nel.org>, Lion Ackermann <nnamrec@...il.com>,
 Petr Machata <petrm@...lanox.com>, netdev@...r.kernel.org,
 Ivan Vecera <ivecera@...hat.com>, Li Shuang <shuali@...hat.com>
Subject: Re: [PATCH net] net/sched: ets: use old 'nbands' while purging unused
 classes

On 8/11/25 13:09, Davide Caratti wrote:
> On Mon, Aug 11, 2025 at 10:52:08AM -0300, Victor Nogueira wrote:
>> On 8/11/25 06:53, Victor Nogueira wrote:
>>> On 8/11/25 04:49, Davide Caratti wrote:
>>>> Maybe it's better to extend sch_ets.sh from net/forwarding instead?
>>>> If so, I can follow-up on net-next with a patch that adds a new
>>>> test-case that includes the 3-lines in [1] - while this patch can go
>>>> as-is in 'net' (and eventually in stable). In alternative, I can
>>>> investigate on TDC adding "sch_plug" to the qdisc tree in a way
>>>> that DWRR never deplete, and the crash would then happen with
>>>> "verifyCmd".
>>>>
>>>> WDYT?
>>>
>>> That works for me as well.
>>
>> Sorry, should've been more specific.
>> I meant that the net/forwarding approach you suggested
>> seems ok. The tdc approach would be a lot of work and
>> I don't believe it's worth it.
> 
> I was more of the idea of avoiding a non-deterministic kselftest, because
> with mausezahn running in the background we have to be "lucky" enough to
> see the tc qdisc change command executed while the packet socket is
> still emitting packets. And the sch_plug approach I mentioned this morning
> looks doable: I just reproduced the NULL dereference on unpatched kernel
> using something like:
> 
>   # ip link add name ddd0 type dummy
>   # tc qdisc add dev ddd0 root handle 1: ets bands 4 strict 2 priomap 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3 3
>   # tc qdisc add dev ddd0 handle 10: parent 1:4 plug
>   # ip link set dev ddd0 up
>   # tc qdisc change dev ddd0 handle 10: plug limit 5
>   # mausezahn ddd0 -A 10.10.10.1 -B 10.10.10.2 -c 0 -a own -c 5 00:c1:a0:c1:a0:00 -t udp
>   # printf "press enter to crash..."
>   # read -r _
>   # tc qdisc change dev ddd0 handle 1: ets bands 2 strict 0
> 
> so, including "plug" children in the tree should make kselftest feasible either with 'net/forwarding'
> or with TDC + scapy plugin superpowers.

I see, so I think it would be better to use the 'net/forwarding' 
approach with
"plug" children mainly because it looks simpler.

> @Victor + @Jakub, can we apply this patch to 'net', so that regression is fixed ASAP, and then I post
> the kselftest in a separate submission for net-next?

 From my side, that's ok.

cheers,
Victor