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>

1 files changed, 8 insertions, 0 deletions

diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
index d48c657191cd..b6c177f7141c 100644
--- a/include/net/sch_generic.h
+++ b/include/net/sch_generic.h
@@ -803,6 +803,14 @@ static inline bool qdisc_tx_changing(const struct net_device *dev)
 	return false;
 }
 
+/* "noqueue" qdisc identified by not having any enqueue, see noqueue_init() */
+static inline bool qdisc_txq_has_no_queue(const struct netdev_queue *txq)
+{
+	struct Qdisc *qdisc = rcu_access_pointer(txq->qdisc);
+
+	return qdisc->enqueue == NULL;
+}
+
 /* Is the device using the noop qdisc on all queues?  */
 static inline bool qdisc_tx_is_noop(const struct net_device *dev)
 {