Re: [PATCH v3 2/2] gro: optimise redundant parsing of packets

[Eric Dumazet]

On Mon, Mar 13, 2023 at 9:46 AM 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). I've applied a similar
> optimisation to Ethernet.
>
> 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 future problem:

I would remove all this block.

We fix current problems, not future hypothetical ones.

>  - 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 - which might happen if encapsulation of BIG TCP will be
> supported in the future.
>
> The fix is to place the whole encapsulation branch after the BIG TCP code
> block. This way, if encapsulation of BIG TCP will be supported,
> inner_network_header will still be calculated with the correct value of
> skb->data.

We do not support encapsulated BIG TCP yet.
We will do this later, and whoever does it will make sure to also support GRO.

> Also, by arranging the code that way, the optimisation does not
> add an additional branch.
>
> Signed-off-by: Richard Gobert <richardbgobert@xxxxxxxxx>
> ---
>

Can you give us a good explanation of why extension headers are used exactly ?

I am not sure we want to add code to GRO for something that 99.99% of
us do not use.