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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Fri, 09 Apr 2021 17:18:01 +0200

Paolo Abeni <[email-masked]> writes:

> hello,
>
> On Fri, 2021-04-09 at 16:57 +0200, Toke Høiland-Jørgensen wrote:
>> 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?
>
> 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.

Ah, right, so it says right there in the comment; sorry for missing
that! :)

-Toke