Re: [PATCH net-next] net: dsa: Remove indirect function call for flow dissection

On Fri, 3 Jan 2020 at 22:50, Florian Fainelli [off-list ref] wrote:

The call path is the following on TX (e.g.: when you run a DHCP client),

Oh, it gets called on TX too, ok.
In that case, static proto_off information won't work for asymmetric
taggers such as ocelot which may have an independently configurable
prefix length on RX and TX.
I want to get rid of the RX tag prefix in ocelot though, but just saying.

I don't think your formula works for EDSA which has an EtherType, but

Why doesn't it work with edsa?

this would probably work for all tags we currently support except trailer.

proto = (__be16 *)(skb->data)[overhead / 2 - 1];

I wasn't suggesting to do this exact calculation in flow_dissector.c,
but rather to pre-populate proto_off with a value statically derived
from it on a piece of paper, with the trailer exception where it would
be -2 in bytes or -1 in shorts, but nonetheless a negative and valid
value.

I don't think anyone except Alexander did serious investigation this.
For now, what I am interested in is reducing the amount of technical
debt and expensive function calls.

Does the change bring any measurable improvement?

--
Florian

Regards,
-Vladimir