It's worth repeating that threading in io-pkt is much more restrictive than with io-net. The io-pkt drivers shouldn't create threads or set up direct timer calls unless absolutely necessary, to allow io-pkt to control execution for optimal performance.
A complete discussion of io-pkt threads is outside the scope of this document (see "Threading model" in the Overview chapter of the Core Networking Stack User's Guide), but suffice to say that unless you really, really need to create threads — which you can — it's far easier not to.