The irony is that when you employ a Real TimeOperationSystem, the response to interrupts get less real. This is one of least undferstood facts re multitasking/RTOS.
While a RTOS supposedly free you from some mundane tasks, in '51 applications you will often find that making the RTOS handle your hardware takes the same effort as handling it without the RTOS.
I am a great fan of employing a RTOS in a microprocessor syatem, I do not see their feasibility in a microcontroller system.
The '51 is designed as a microcontroller and attempting to use it where a microprocessor belong will always be a mistake.

Erik