veth: Do not record rx queue hint in veth_xmit

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 edbea92 ("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 edbea92 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: edbea92 ("veth: Store queue_mapping independently of XDP prog presence")
Fixes: 638264d ("veth: Support per queue XDP ring")
Reported-by: Laurent Bernaille <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Cc: Maciej Fijalkowski <[email protected]>
Cc: Toshiaki Makita <[email protected]>
Cc: Eric Dumazet <[email protected]>
Cc: Paolo Abeni <[email protected]>
Cc: John Fastabend <[email protected]>
Cc: Willem de Bruijn <[email protected]>
Acked-by: John Fastabend <[email protected]>
Reviewed-by: Eric Dumazet <[email protected]>
Acked-by: Toshiaki Makita <[email protected]>
Signed-off-by: David S. Miller <[email protected]>

Author: borkmann

drivers/net/veth.c

@@ -335,7 +335,6 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
 		 */
 		use_napi = rcu_access_pointer(rq->napi) &&
 			   veth_skb_is_eligible_for_gro(dev, rcv, skb);
-		skb_record_rx_queue(skb, rxq);
 	}
 
 	skb_tx_timestamp(skb);