Re: [PATCH net] veth: fix queue index used to wake the peer txq in veth_poll
From: Jesper Dangaard Brouer <hawk@kernel.org>
Date: 2026-08-06 13:45:10
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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 <redacted>
Tested-by: Jesper Dangaard Brouer <hawk@kernel.org>Acked-by: Jesper Dangaard Brouer <hawk@kernel.org>
quoted hunk ↗ jump to hunk
--- 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;