RTAI and Xenomai can be considered real time Linux and both require application support. Applications don't just become real time, they must be programmed that way. Common Linux desktop application are not programmed this way.

RTLinux is more widely used than RTAI or Xenomai, but it's a completely different model, it's more like a hypervisor. Again, it doesn't help. While the OS provides the serivices, applications must be programmed specifically to use it.