We use the PREEMPT_RT patch set available at:

https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-sta...

That patch set is slowly being upstreamed into the mainline kernel. We regularly encounter incompatibilities with drivers for the Raspberry Pi SoC and upstream our fixes and feature work as well, e.g.:

https://git.kernel.org/linus/f7da7782aba9

https://git.kernel.org/linus/3c7b30f704b6

I must admit I wasn't aware of LinuxCNC, thanks for the pointer. This would seem to be a really nice use case for Revolution Pi.