Re: [PATCH v2 2/2] gro: optimise redundant parsing of packets

[Eric Dumazet]

On Wed, Feb 22, 2023 at 4:13 PM Richard Gobert <richardbgobert@xxxxxxxxx> wrote:
>
> Currently the IPv6 extension headers are parsed twice: first in
> ipv6_gro_receive, and then again in ipv6_gro_complete.
>
> By using the new ->transport_proto field, and also storing the size of the
> network header, we can avoid parsing extension headers a second time in
> ipv6_gro_complete (which saves multiple memory dereferences and conditional
> checks inside ipv6_exthdrs_len for a varying amount of extension headers in IPv6
> packets).
>
> The implementation had to handle both inner and outer layers in case of
> encapsulation (as they can't use the same field).
>
> Performance tests for TCP stream over IPv6 with a varying amount of extension
> headers demonstrate throughput improvement of ~0.7%.
>
> In addition, I fixed a potential existing problem:
>  - The call to skb_set_inner_network_header at the beginning of
>    ipv6_gro_complete calculates inner_network_header based on skb->data by
>    calling skb_set_inner_network_header, and setting it to point to the beginning
>    of the ip header.
>  - If a packet is going to be handled by BIG TCP, the following code block is
>    going to shift the packet header, and skb->data is going to be changed as
>    well.
>
> When the two flows are combined, inner_network_header will point to the wrong
> place.

net-next is closed.

If you think a fix is needed, please send a stand-alone and minimal
patch so that we can discuss its merit.

Note :

BIG TCP only supports native IPv6, not encapsulated traffic,
so we should not bother with inner_network_header yet.