100% CPU load with device scanning enabled

[Kees Meijs]

Hi list,

We're in the process of replacing Quagga with BIRD but stumble upon a
little problem.

When device scanning is on (obviously default) our testing machine
completely fills up a CPU core. The culprit isn't BIRD itself but an
Open vSwitch daemon.

After disabling the device protocol and restarting BIRD, everything goes
back to it's quiet state.

BIRD (1.6.3-2) and Open vSwitch (2.6.2~pre+git20161223-3) both were
installed as Debian stable packages.

The configuration is as simple as:

> # This is a minimal configuration file, which allows the bird daemon
> to start
> # but will not cause anything else to happen.
> #
> # Please refer to the documentation in the bird-doc package or BIRD User's
> # Guide on http://bird.network.cz/ for more information on configuring
> BIRD and
> # adding routing protocols.
>
> # Change this into your BIRD router ID. It's a world-wide unique
> identification
> # of your router, usually one of router's IPv4 addresses.
> router id 1.2.3.4;
>
> # The Device protocol is not a real routing protocol. It doesn't
> generate any
> # routes and it only serves as a module for getting information about
> network
> # interfaces from the kernel.
> protocol device {
> }
>
> # The Kernel protocol is not a real routing protocol. Instead of
> communicating
> # with other routers in the network, it performs synchronization of BIRD's
> # routing tables with the OS kernel.
> protocol kernel {
>     metric 64;    # Use explicit kernel route metric to avoid collisions
>             # with non-BIRD routes in the kernel routing table
>     import none;
>     export all;    # Actually insert routes into the kernel routing table
> }
>
> protocol bgp test {
>     description "BGP test";
>     local as REDACTED;
>     neighbor 1.2.3.4 as REDACTED;
>     direct;
>     next hop self;
>     deterministic med on;
>     export none;
>     import all;
> }

Meanwhile log messages such as below arise:

> bird: Kernel dropped some netlink messages, will resync on next scan.

For a test I deleted all existing Open vSwitch bridges and the load
dropped again. After adding an empty new bridge, the load spikes again
in an instant.

This is unexpected behaviour. Maybe it's an implementation problem in
Open vSwitch or maybe in BIRD. Anyway, it should happen I guess.

Any clues?

Thanks in advance!

Regards,
Kees