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

[Paolo Abeni <pabeni@redhat.com>]

hello,

On Fri, 2021-04-09 at 16:57 +0200, Toke Høiland-Jørgensen wrote:
> Paolo Abeni <pabeni@redhat.com> 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 <pabeni@redhat.com>
> > ---
> >  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?

Thank you for the feedback.

If XDP is enabled, TSO is forced of on 'dev'
and veth_skb_is_eligible_for_gro() returns true, so napi/GRO is always
used - there is no functional change when XDP is enabled.

Please let me know if the above is more clear, thanks!

Paolo