Designed realtime with Linux is non-trivial.

I worked on HA (Highly Available) system with Linux inside Xilinx's Vertex Pro PPC. It is redundant system with multiple fault detections and switch over if any subsystem detected failure.

There was one 250 ms hard real time requirements: If I am a slave and don't detect the master 's UDP ping for 250 ms. I will assume the master has failed somehow, I will start action and take over control as master.

The sub-system did trigger from time to time while the master is alive and working perfectly OK.

Eventually I figured out that one of the system API was using > 250 ms time. (Forget which one now, that was > 10 years ago.) I have to profile very carefully and redesign the code to get around that API.