Paolo Abeni <pab...@redhat.com> writes:
> hello,
>
> On Fri, 2021-04-09 at 16:57 +0200, Toke Høiland-Jørgensen wrote:
>> Paolo Abeni <pab...@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 <pab...@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.
Ah, right, so it says right there in the comment; sorry for missing
that! :)
-Toke
