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

From: Vladimir Oltean
Date: Fri Jan 03 2020 - 16:29:03 EST

On Fri, 3 Jan 2020 at 22:50, Florian Fainelli <f.fainelli@xxxxxxxxx> 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

> 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