Subject: Re: [PATCH net-next 3/4] veth: refine napi usage

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 09 Apr 2021 16:57:47 +0200

Paolo Abeni <[email-masked]> writes:

> After the previous patch, when enabling GRO, locally generated
> TCP traffic experiences some measurable overhead, as it traverses
> the GRO engine without any chance of aggregation.
>
> This change refine the NAPI receive path admission test, to avoid
> unnecessary GRO overhead in most scenarios, when GRO is enabled
> on a veth peer.
>
> Only skbs that are eligible for aggregation enter the GRO layer,
> the others will go through the traditional receive path.
>
> Signed-off-by: Paolo Abeni <[email-masked]>
> ---
>  drivers/net/veth.c | 23 ++++++++++++++++++++++-
>  1 file changed, 22 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/net/veth.c b/drivers/net/veth.c
> index ca44e82d1edeb..85f90f33d437e 100644
> --- a/drivers/net/veth.c
> +++ b/drivers/net/veth.c
> @@ -282,6 +282,25 @@ static int veth_forward_skb(struct net_device *dev, struct sk_buff *skb,
>  		netif_rx(skb);
>  }
>  
> +/* return true if the specified skb has chances of GRO aggregation
> + * Don't strive for accuracy, but try to avoid GRO overhead in the most
> + * common scenarios.
> + * When XDP is enabled, all traffic is considered eligible, as the xmit
> + * device has TSO off.
> + * When TSO is enabled on the xmit device, we are likely interested only
> + * in UDP aggregation, explicitly check for that if the skb is suspected
> + * - the sock_wfree destructor is used by UDP, ICMP and XDP sockets -
> + * to belong to locally generated UDP traffic.
> + */
> +static bool veth_skb_is_eligible_for_gro(const struct net_device *dev,
> +					 const struct net_device *rcv,
> +					 const struct sk_buff *skb)
> +{
> +	return !(dev->features & NETIF_F_ALL_TSO) ||
> +		(skb->destructor == sock_wfree &&
> +		 rcv->features & (NETIF_F_GRO_FRAGLIST | NETIF_F_GRO_UDP_FWD));
> +}
> +
>  static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
>  {
>  	struct veth_priv *rcv_priv, *priv = netdev_priv(dev);
> @@ -305,8 +324,10 @@ static netdev_tx_t veth_xmit(struct sk_buff *skb, struct net_device *dev)
>  
>  		/* The napi pointer is available when an XDP program is
>  		 * attached or when GRO is enabled
> +		 * Don't bother with napi/GRO if the skb can't be aggregated
>  		 */
> -		use_napi = rcu_access_pointer(rq->napi);
> +		use_napi = rcu_access_pointer(rq->napi) &&
> +			   veth_skb_is_eligible_for_gro(dev, rcv, skb);
>  		skb_record_rx_queue(skb, rxq);
>  	}

You just changed the 'xdp_rcv' check to this use_napi, and now you're
conditioning it on GRO eligibility, so doesn't this break XDP if that
was the reason NAPI was turned on in the first place?

-Toke