I agree that Linux with RT_PREEMPT cannot be hard real-time in the sense that it cannot formally guarantee meeting a deadline due to CPU firmware interrupts. H

I agree that Linux with RT_PREEMPT cannot be hard real-time in the sense that it cannot formally guarantee meeting a deadline due to CPU firmware interrupts. However in many applications (e.g. robotics) where missing a single deadline will not result in loss of life it works perfectly fine. The unmaskable interrupts on x86 can be a problem but long terms benchmarking can help with possible edge cases. Other Linux-based solutions such as Xenomai & RTAI which provide even better performance deserve a mention.