Re: [PATCH v4 2/2] gro: optimise redundant parsing of packets

[Paolo Abeni]

On Mon, 2023-03-20 at 18:00 +0100, Richard Gobert 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%.

I'm surprised that the improvement is measurable: for large aggregate
packets a single ipv6_exthdrs_len() call is avoided out of tens calls
for the individual pkts. Additionally such figure is comparable to
noise level in my tests.

This adds a couple of additional branches for the common (no extensions
header) case. 

while patch 1/2 could be useful, patch 2/2 overall looks not worthy to
me.

I suggest to re-post for inclusion only patch 1, unless others have
strong different opinions.

Cheers,

Paolo