I'd usually suggest just Rebooting and praying no critical data is lost, but considering your system is in the middle of updating potentially critical files - don't do it!
Don't login via SSH, use LISH instead to save a few io_tokens. Start killing whatever process you can safely kill, and try to bring the beast down gently.
Make every command you run count because I've had simple commands like "ps aux" take minutes on a system that had hit the limiter.
...or just let it chug away on the upgrade for the next couple weeks and probably annoy everyone else on your Linode.
