Subject: RE: [PATCH net-next] veth: Do not record rx queue hint in veth_xmit

From: John Fastabend <[email-masked]>
Date: Wed, 05 Jan 2022 19:22:49 -0800

Daniel Borkmann wrote:
> 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.

Agree on all the above. Fix LGTM thanks!

Acked-by: John Fastabend <[email-masked]>

> 
> Fixes: edbea9220251 ("veth: Store queue_mapping independently of XDP prog presence")
> Fixes: 638264dc9022 ("veth: Support per queue XDP ring")
> Reported-by: Laurent Bernaille <[email-masked]>
> Signed-off-by: Daniel Borkmann <[email-masked]>
> Cc: Maciej Fijalkowski <[email-masked]>
> Cc: Toshiaki Makita <[email-masked]>
> Cc: Eric Dumazet <[email-masked]>
> Cc: Paolo Abeni <[email-masked]>
> Cc: John Fastabend <[email-masked]>
> Cc: Willem de Bruijn <[email-masked]>
> ---
>  drivers/net/veth.c | 1 -
>  1 file changed, 1 deletion(-)
> 
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index d21dd25f429e..354a963075c5 100644
> --- a/drivers/net/veth.c
> +++ b/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);
> -- 
> 2.21.0
>