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. --- V5: - use rcu_dereference_check to signal that NAPI is a RCU section - whitespace fixes reported by checkpatch.pl - handle unlikely race - Link to V4 https://lore.kernel.org/all/174472463778.274639.12670590457453196991.stgit@firesoul/ V4: - Check against no-queue instead of no-op qdisc - Link to V3: https://lore.kernel.org/all/174464549885.20396.6987653753122223942.stgit@firesoul/ V3: - Reorder patches, generalize check for no-op qdisc as first patch - RFC: As testing show this is incorrect - rcu_dereference(priv->peer) in veth_xdp_rcv as this runs in NAPI context rcu_read_lock() is implicit. - Link to V2: https://lore.kernel.org/all/174412623473.3702169.4235683143719614624.stgit@firesoul/ V2: - Generalize check for no-op qdisc - Link to RFC-V1: https://lore.kernel.org/all/174377814192.3376479.16481605648460889310.stgit@firesoul/ Jesper Dangaard Brouer (2): net: sched: generalize check for no-queue qdisc on TX queue veth: apply qdisc backpressure on full ptr_ring to reduce TX drops drivers/net/veth.c | 55 ++++++++++++++++++++++++++++++++------- drivers/net/vrf.c | 4 +-- include/net/sch_generic.h | 8 ++++++ 3 files changed, 54 insertions(+), 13 deletions(-) --
