From: Daniel Borkmann daniel@iogearbox.net
commit 710ad98c363a66a0cd8526465426c5c5f8377ee0 upstream.
Laurent reported that they have seen a significant amount of TCP retransmissions at high throughput from applications residing in network namespaces talking to the outside world via veths. The drops were seen on the qdisc layer (fq_codel, as per systemd default) of the phys device such as ena or virtio_net due to all traffic hitting a _single_ TX queue _despite_ multi-queue device. (Note that the setup was _not_ using XDP on veths as the issue is generic.)
More specifically, after edbea9220251 ("veth: Store queue_mapping independently of XDP prog presence") which made it all the way back to v4.19.184+, skb_record_rx_queue() would set skb->queue_mapping to 1 (given 1 RX and 1 TX queue by default for veths) instead of leaving at 0.
This is eventually retained and callbacks like ena_select_queue() will also pick single queue via netdev_core_pick_tx()'s ndo_select_queue() once all the traffic is forwarded to that device via upper stack or other means. Similarly, for others not implementing ndo_select_queue() if XPS is disabled, netdev_pick_tx() might call into the skb_tx_hash() and check for prior skb_rx_queue_recorded() as well.
In general, it is a _bad_ idea for virtual devices like veth to mess around with queue selection [by default]. Given dev->real_num_tx_queues is by default 1, the skb->queue_mapping was left untouched, and so prior to edbea9220251 the netdev_core_pick_tx() could do its job upon __dev_queue_xmit() on the phys device.
Unbreak this and restore prior behavior by removing the skb_record_rx_queue() from veth_xmit() altogether.
If the veth peer has an XDP program attached, then it would return the first RX queue index in xdp_md->rx_queue_index (unless configured in non-default manner). However, this is still better than breaking the generic case.
Fixes: edbea9220251 ("veth: Store queue_mapping independently of XDP prog presence") Fixes: 638264dc9022 ("veth: Support per queue XDP ring") Reported-by: Laurent Bernaille laurent.bernaille@datadoghq.com Signed-off-by: Daniel Borkmann daniel@iogearbox.net Cc: Maciej Fijalkowski maciej.fijalkowski@intel.com Cc: Toshiaki Makita toshiaki.makita1@gmail.com Cc: Eric Dumazet eric.dumazet@gmail.com Cc: Paolo Abeni pabeni@redhat.com Cc: John Fastabend john.fastabend@gmail.com Cc: Willem de Bruijn willemb@google.com Acked-by: John Fastabend john.fastabend@gmail.com Reviewed-by: Eric Dumazet edumazet@google.com Acked-by: Toshiaki Makita toshiaki.makita1@gmail.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/net/veth.c | 1 - 1 file changed, 1 deletion(-)
--- a/drivers/net/veth.c +++ b/drivers/net/veth.c @@ -301,7 +301,6 @@ static netdev_tx_t veth_xmit(struct sk_b if (rxq < rcv->real_num_rx_queues) { rq = &rcv_priv->rq[rxq]; rcv_xdp = rcu_access_pointer(rq->xdp_prog); - skb_record_rx_queue(skb, rxq); }
skb_tx_timestamp(skb);