Merge branch 'veth-qdisc-backpressure-and-qdisc-check-refactor'

Jesper Dangaard Brouer says:

====================
veth: qdisc backpressure and qdisc check refactor

This patch series addresses TX drops seen on veth devices under load,
particularly when using threaded NAPI, which is our setup in production.

The root cause is that the NAPI consumer often runs on a different CPU
than the producer. Combined with scheduling delays or simply slower
consumption, this increases the chance that the ptr_ring fills up before
packets are drained, resulting in drops from veth_xmit() (ndo_start_xmit()).

To make this easier to reproduce, we’ve created a script that sets up a
test scenario using network namespaces. The script inserts 1000 iptables
rules in the consumer namespace to slow down packet processing and
amplify the issue. Reproducer script:

https://github.com/xdp-project/xdp-project/blob/main/areas/core/veth_setup01_NAPI_TX_drops.sh

This series first introduces a helper to detect no-queue qdiscs and then
uses it in the veth driver to conditionally apply qdisc-level
backpressure when a real qdisc is attached. The behavior is off by
default and opt-in, ensuring minimal impact and easy activation.

v6: https://lore.kernel.org/174549933665.608169.392044991754158047.stgit@firesoul
v5: https://lore.kernel.org/174489803410.355490.13216831426556849084.stgit@firesoul
v4 https://lore.kernel.org/174472463778.274639.12670590457453196991.stgit@firesoul
v3: https://lore.kernel.org/174464549885.20396.6987653753122223942.stgit@firesoul
v2: https://lore.kernel.org/174412623473.3702169.4235683143719614624.stgit@firesoul
RFC-v1: https://lore.kernel.org/174377814192.3376479.16481605648460889310.stgit@firesoul
====================

Link: https://patch.msgid.link/174559288731.827981.8748257839971869213.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>