Subject: Re: [PATCH RFC net-next 0/3] Multi-CPU DSA support

From: Marek Behun <[email-masked]>
Date: Mon, 12 Apr 2021 21:30:45 +0200

On Mon, 12 Apr 2021 14:46:11 +0200
Tobias Waldekranz <[email-masked]> wrote:

> I agree. Unless you only have a few really wideband flows, a LAG will
> typically do a great job with balancing. This will happen without the
> user having to do any configuration at all. It would also perform well
> in "router-on-a-stick"-setups where the incoming and outgoing port is
> the same.

TLDR: The problem with LAGs how they are currently implemented is that
for Turris Omnia, basically in 1/16 of configurations the traffic would
go via one CPU port anyway.

One potencial problem that I see with using LAGs for aggregating CPU
ports on mv88e6xxx is how these switches determine the port for a
packet: only the src and dst MAC address is used for the hash that
chooses the port.

The most common scenario for Turris Omnia, for example, where we have 2
CPU ports and 5 user ports, is that into these 5 user ports the user
plugs 5 simple devices (no switches, so only one peer MAC address for
port). So we have only 5 pairs of src + dst MAC addresses. If we simply
fill the LAG table as it is done now, then there is 2 * 0.5^5 = 1/16
chance that all packets would go through one CPU port.

In order to have real load balancing in this scenario, we would either
have to recompute the LAG mask table depending on the MAC addresses, or
rewrite the LAG mask table somewhat randomly periodically. (This could
be in theory offloaded onto the Z80 internal CPU for some of the
switches of the mv88e6xxx family, but not for Omnia.)

Marek