On Thu, 2024-06-20 at 15:19 -0700, Yan Zhai wrote:
> Software GRO is currently controlled by a single switch, i.e.
>
> ethtool -K dev gro on|off
>
> However, this is not always desired. When GRO is enabled, even if the
> kernel cannot GRO certain traffic, it has to run through the GRO receive
> handlers with no benefit.
>
> There are also scenarios that turning off GRO is a requirement. For
> example, our production environment has a scenario that a TC egress hook
> may add multiple encapsulation headers to forwarded skbs for load
> balancing and isolation purpose. The encapsulation is implemented via
> BPF. But the problem arises then: there is no way to properly offload a
> double-encapsulated packet, since skb only has network_header and
> inner_network_header to track one layer of encapsulation, but not two.
> On the other hand, not all the traffic through this device needs double
> encapsulation. But we have to turn off GRO completely for any ingress
> device as a result.
>
> Introduce a bit on skb so that GRO engine can be notified to skip GRO on
> this skb, rather than having to be 0-or-1 for all traffic.
>
> Signed-off-by: Yan Zhai <yan@xxxxxxxxxxxxxx>
> ---
> include/linux/netdevice.h | 9 +++++++--
> include/linux/skbuff.h | 10 ++++++++++
> net/Kconfig | 10 ++++++++++
> net/core/gro.c | 2 +-
> net/core/gro_cells.c | 2 +-
> net/core/skbuff.c | 4 ++++
> 6 files changed, 33 insertions(+), 4 deletions(-)
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index c83b390191d4..2ca0870b1221 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -2415,11 +2415,16 @@ struct net_device {
> ((dev)->devlink_port = (port)); \
> })
>
> -static inline bool netif_elide_gro(const struct net_device *dev)
> +static inline bool netif_elide_gro(const struct sk_buff *skb)
> {
> - if (!(dev->features & NETIF_F_GRO) || dev->xdp_prog)
> + if (!(skb->dev->features & NETIF_F_GRO) || skb->dev->xdp_prog)
> return true;
> +
> +#ifdef CONFIG_SKB_GRO_CONTROL
> + return skb->gro_disabled;
> +#else
> return false;
> +#endif
This will generate OoO if the gro_disabled is flipped in the middle of
a stream.
Assuming the above is fine for your use case (I think it's _not_ in
general), you could get the same result without an additional costly
bit in sk_buff.
Let xdp_frame_fixup_skb_offloading() return a bool - e.g. 'true' when
gro should be avoided - and let the NIC driver call netif_receive_skb()
instead of the gro rx hook for such packet.
All in all the approach implemented in this series does not look worthy
to me.
Thanks,
Paolo
