The approach of making general-purpose OS kernels realtime seems like hard work to me. Surely what we want is a realtime microkernel, on which the conventional

The approach of making general-purpose OS kernels realtime seems like hard work to me. Surely what we want is a realtime microkernel, on which the conventional operating system can run as a (large!) task. As long as there is a way for processes running under the kernel to start realtime tasks directly on the microkernel, you can support all your realtime use cases, without having to make the whole operating realtime.