Re: [RFC PATCH net-next] veth: apply qdisc backpressure on full ptr_ring to reduce TX drops

[David Ahern <dsahern@kernel.org>]

On 4/8/25 5:23 AM, Toke Høiland-Jørgensen wrote:
> David Ahern <dsahern@kernel.org> writes:
> 
>> On 4/7/25 3:15 AM, Toke Høiland-Jørgensen wrote:
>>>> +static inline bool txq_has_qdisc(struct netdev_queue *txq)
>>>> +{
>>>> +	struct Qdisc *q;
>>>> +
>>>> +	q = rcu_dereference(txq->qdisc);
>>>> +	if (q->enqueue)
>>>> +		return true;
>>>> +	else
>>>> +		return false;
>>>> +}
>>>
>>> This seems like a pretty ugly layering violation, inspecting the qdisc
>>> like this in the driver?
>>
>> vrf driver has something very similar - been there since March 2017.
> 
> Doesn't make it any less ugly, though ;)

in my eyes, it is a thing of beauty.

> 
> And AFAICT, vrf is doing more with the information; basically picking a
> whole different TX path? Can you elaborate on the reasoning for this (do
> people actually install qdiscs on VRF devices in practice)?

It is not common AFAIK, but it is possible. I wanted to avoid the
overhead for what I think is a rare configuration.