veth: apply qdisc backpressure on full ptr_ring to reduce TX drops

In production, we're seeing TX drops on veth devices when the ptr_ring
fills up. This can occur when NAPI mode is enabled, though it's
relatively rare. However, with threaded NAPI - which we use in
production - the drops become significantly more frequent.

The underlying issue is that with threaded NAPI, the consumer often runs
on a different CPU than the producer. This increases the likelihood of
the ring filling up before the consumer gets scheduled, especially under
load, leading to drops in veth_xmit() (ndo_start_xmit()).

This patch introduces backpressure by returning NETDEV_TX_BUSY when the
ring is full, signaling the qdisc layer to requeue the packet. The txq
(netdev queue) is stopped in this condition and restarted once
veth_poll() drains entries from the ring, ensuring coordination between
NAPI and qdisc.

Backpressure is only enabled when a qdisc is attached. Without a qdisc,
the driver retains its original behavior - dropping packets immediately
when the ring is full. This avoids unexpected behavior changes in setups
without a configured qdisc.

With a qdisc in place (e.g. fq, sfq) this allows Active Queue Management
(AQM) to fairly schedule packets across flows and reduce collateral
damage from elephant flows.

A known limitation of this approach is that the full ring sits in front
of the qdisc layer, effectively forming a FIFO buffer that introduces
base latency. While AQM still improves fairness and mitigates flow
dominance, the latency impact is measurable.

In hardware drivers, this issue is typically addressed using BQL (Byte
Queue Limits), which tracks in-flight bytes needed based on physical link
rate. However, for virtual drivers like veth, there is no fixed bandwidth
constraint - the bottleneck is CPU availability and the scheduler's ability
to run the NAPI thread. It is unclear how effective BQL would be in this
context.

This patch serves as a first step toward addressing TX drops. Future work
may explore adapting a BQL-like mechanism to better suit virtual devices
like veth.

Reported-by: Yan Zhai <yan@cloudflare.com>
Signed-off-by: Jesper Dangaard Brouer <hawk@kernel.org>
Reviewed-by: Toshiaki Makita <toshiaki.makita1@gmail.com>
Link: https://patch.msgid.link/174559294022.827981.1282809941662942189.stgit@firesoul
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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