netdev - Re: [RFC PATCH net-next] veth: apply qdisc backpressure on full ptr

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Message-ID: <87h62yx5gd.fsf@toke.dk>
Date: Tue, 08 Apr 2025 13:23:14 +0200
From: Toke Høiland-Jørgensen <toke@...hat.com>
To: David Ahern <dsahern@...nel.org>, Jesper Dangaard Brouer
 <hawk@...nel.org>, netdev@...r.kernel.org, Jakub Kicinski
 <kuba@...nel.org>
Cc: bpf@...r.kernel.org, tom@...bertland.com, Eric Dumazet
 <eric.dumazet@...il.com>, "David S. Miller" <davem@...emloft.net>, Paolo
 Abeni <pabeni@...hat.com>, kernel-team@...udflare.com
Subject: Re: [RFC PATCH net-next] veth: apply qdisc backpressure on full
 ptr_ring to reduce TX drops

David Ahern <dsahern@...nel.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 ;)

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)?

-Toke