Looks like Dragon runs VxWorks, see under Flight Software.

http://www.spacex.com/sites/spacex/files/pdf/DragonLabFactSh...

Can anyone who works at SpaceX chime in?

That article also says

> One of the areas they focus on is scheduler performance. They do not have hard realtime requirements, but do care about wakeup latencies, he said.

It sounds like their hard real time controls are on non-Linux OSs.