Re: [PATCH net] veth: fix queue index used to wake the peer txq in veth_poll

[Jesper Dangaard Brouer <hawk@kernel.org>]

On 06/08/2026 13.43, Jonas Köppeler wrote:
> veth_poll() derives the index of the peer TX queue to wake from
> rq->xdp_rxq.queue_index. That field is only initialized by
> xdp_rxq_info_reg() in veth_enable_xdp_range(), which runs only when an
> XDP program is attached. On the plain GRO/NAPI path
> (veth_napi_enable_range()) xdp_rxq_info_reg() is never called, so
> queue_index stays 0 for every queue, as priv->rq is zero-allocated. The
> same happens once an XDP program is detached while GRO stays enabled,
> since xdp_rxq_info_unreg() clears the struct again.
> 
> So in a multi-queue setup with GRO enabled and no XDP program attached,
> every NAPI instance looks at the peer's TX queue 0. If veth_xmit() stops
> peer TX queue 1 because the ptr_ring is full (NETDEV_TX_BUSY), nothing
> ever wakes it again: the poller draining queue 1 wakes queue 0 instead.
> veth implements no ndo_tx_timeout, so the netdev watchdog does not kick
> in either, and the queue stays stopped indefinitely.
> 
> Derive the index from the position of the rq within priv->rq instead,
> which is correct regardless of whether XDP was ever enabled.
> 
> Scripts to reproduce the stall are available at
> https://github.com/netoptimizer/veth-backpressure-performance-testing

I've modified both the reproducer[12] and selftests[13] to detect this.
Previously reproducer only had a 50% chance to catch this, and we didn't 
notice, code have been changed to catch this 100% of the time.  The 
selftests script never hit the bug as it always used one queue, this 
have been changed to always use two queues, but force/steer traffic to 
always hit queue index 1 via XPS config (as we want to see the queue 
build up and ptr_ring HoL queuing effects in the latency measurements).


- [12] 
https://github.com/netoptimizer/veth-backpressure-performance-testing/pull/12
- [13] 
https://github.com/netoptimizer/veth-backpressure-performance-testing/pull/13

> Fixes: dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops")
> Signed-off-by: Jonas Köppeler <j.koeppeler@tu-berlin.de>
> Tested-by: Jesper Dangaard Brouer <hawk@kernel.org>

Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>

> ---
>   drivers/net/veth.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index 00e34afd858e..e927ef3f47b3 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -961,7 +961,7 @@ static int veth_poll(struct napi_struct *napi, int budget)
>   	struct veth_rq *rq =
>   		container_of(napi, struct veth_rq, xdp_napi);
>   	struct veth_priv *priv = netdev_priv(rq->dev);
> -	int queue_idx = rq->xdp_rxq.queue_index;
> +	int queue_idx = rq - priv->rq;

It took me a while to convince myself that this is correct via checking
the data-structures and how they get allocated.

This is a stable fix so it is good to keep this change as small as possible.

I do feel like we should add a queue_idx to struct veth_rq, and init
this in function veth_alloc_queues(). This could be a followup to net-
next IMHO.


>   	struct netdev_queue *peer_txq;
>   	struct veth_stats stats = {};
>   	struct net_device *peer_dev;