RT Linux is a small, simple, real time OS that runs Linux as a low priority task. Changes to the Linux kernel are minimal. Real Time tasks can be scheduled precisely. On a P120 system a task can be scheduled with under 20 uS precision. RT Linux uses kernel modules and Linux can dynamically load and unload them just like normal kernel modules.
Users can write their own schedulers as loadable kernel modules.
RT Linux can communicate to linux user processes through FIFOs or shared memory. RT Linux is considered to be hard real time. KURT is a modification to the linux kernel which adds microsecond timing and three modes to the kernel. Real Time Mode where only designated real time processes are allowed to run. In either Focused or Mixed Real Time Modes the real time processes can still access any of the system services of the linux kernel.