Re: [PATCH net-next V2 0/2] veth: qdisc backpressure and qdisc check refactor

[Jakub Sitnicki <jakub@cloudflare.com>]

On Tue, Apr 08, 2025 at 05:31 PM +02, Jesper Dangaard Brouer wrote:
> This series addresses TX drops observed in production when using veth
> devices with threaded NAPI, and refactors a common qdisc check into a
> shared helper.
>
> In threaded NAPI mode, packet drops can occur when the ptr_ring backing
> the veth peer fills up. This is typically due to a combination of
> scheduling delays and the consumer (NAPI thread) being slower than the
> producer. When the ring overflows, packets are dropped in veth_xmit().
>
> Patch 1 introduces a backpressure mechanism: when the ring is full, the
> driver returns NETDEV_TX_BUSY, signaling the qdisc layer to requeue the
> packet. This allows Active Queue Management (AQM) - such as fq or sfq -
> to spread traffic more fairly across flows and reduce damage from
> elephant flows.
>
> To minimize invasiveness, this backpressure behavior is only enabled when
> a qdisc is attached. If no qdisc is present, the driver retains its
> original behavior (dropping packets on a full ring), avoiding behavior
> changes for configurations without a qdisc.
>
> Detecting the presence of a "real" qdisc relies on a check that is
> already duplicated across multiple drivers (e.g., veth, vrf). Patch-2
> consolidates this logic into a new helper, qdisc_txq_is_noop(), to avoid
> duplication and clarify intent.
>
> ---
>
> Jesper Dangaard Brouer (2):
>       veth: apply qdisc backpressure on full ptr_ring to reduce TX drops
>       net: sched: generalize check for no-op qdisc
>
>
>  drivers/net/veth.c        | 49 ++++++++++++++++++++++++++++++++-------
>  drivers/net/vrf.c         |  3 +--
>  include/net/sch_generic.h |  7 +++++-
>  3 files changed, 48 insertions(+), 11 deletions(-)

This setup scenario is currently not covered by the veth selftest [1].
Would be great to extend it so the code gets exercised by the CI.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/netdev/net-next.git/tree/tools/testing/selftests/net/veth.sh