The changes introduced in commit dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops") have been found to cause a race condition in production environments. Under specific circumstances, observed exclusively on ARM64 (aarch64) systems with Ampere Altra Max CPUs, a transmit queue (TXQ) can become permanently stalled. This happens when the race condition leads to the TXQ entering the QUEUE_STATE_DRV_XOFF state without a corresponding queue wake-up, preventing the attached qdisc from dequeueing packets and causing the network link to halt. As a first step towards resolving this issue, this patch introduces a failsafe mechanism. It enables the net device watchdog by setting a timeout value and implements the .ndo_tx_timeout callback. If a TXQ stalls, the watchdog will trigger the veth_tx_timeout() function, which logs a warning and calls netif_tx_wake_queue() to unstall the queue and allow traffic to resume. The log message will look like this: veth42: NETDEV WATCHDOG: CPU: 34: transmit queue 0 timed out 5393 ms veth42: veth backpressure stalled(n:1) TXQ(0) re-enable This provides a necessary recovery mechanism while the underlying race condition is investigated further. Subsequent patches will address the root cause and add more robust state handling in ndo_open/ndo_stop. Fixes: dc82a33297fc ("veth: apply qdisc backpressure on full ptr_ring to reduce TX drops") Signed-off-by: Jesper Dangaard Brouer --- drivers/net/veth.c | 16 +++++++++++++++- 1 file changed, 15 insertions(+), 1 deletion(-) diff --git a/drivers/net/veth.c b/drivers/net/veth.c index a3046142cb8e..7b1a9805b270 100644 --- a/drivers/net/veth.c +++ b/drivers/net/veth.c @@ -959,8 +959,10 @@ static int veth_xdp_rcv(struct veth_rq *rq, int budget, rq->stats.vs.xdp_packets += done; u64_stats_update_end(&rq->stats.syncp); - if (peer_txq && unlikely(netif_tx_queue_stopped(peer_txq))) + if (peer_txq && unlikely(netif_tx_queue_stopped(peer_txq))) { + txq_trans_cond_update(peer_txq); netif_tx_wake_queue(peer_txq); + } return done; } @@ -1373,6 +1375,16 @@ static int veth_set_channels(struct net_device *dev, goto out; } +static void veth_tx_timeout(struct net_device *dev, unsigned int txqueue) +{ + struct netdev_queue *txq = netdev_get_tx_queue(dev, txqueue); + + netdev_err(dev, "veth backpressure stalled(n:%ld) TXQ(%u) re-enable\n", + atomic_long_read(&txq->trans_timeout), txqueue); + + netif_tx_wake_queue(txq); +} + static int veth_open(struct net_device *dev) { struct veth_priv *priv = netdev_priv(dev); @@ -1711,6 +1723,7 @@ static const struct net_device_ops veth_netdev_ops = { .ndo_bpf = veth_xdp, .ndo_xdp_xmit = veth_ndo_xdp_xmit, .ndo_get_peer_dev = veth_peer_dev, + .ndo_tx_timeout = veth_tx_timeout, }; static const struct xdp_metadata_ops veth_xdp_metadata_ops = { @@ -1749,6 +1762,7 @@ static void veth_setup(struct net_device *dev) dev->priv_destructor = veth_dev_free; dev->pcpu_stat_type = NETDEV_PCPU_STAT_TSTATS; dev->max_mtu = ETH_MAX_MTU; + dev->watchdog_timeo = msecs_to_jiffies(5000); dev->hw_features = VETH_FEATURES; dev->hw_enc_features = VETH_FEATURES;