Linux is an operating system that is widely used in embedded systems such as consumer electronics, networking equipment, machine control, industrial, automation and so on and so forth. However, all systems do not have the same requirements in term of determinism and sometimes determinism, the ability to schedule the highest priority tasks in a consistent and predictable way, really matters. This is the case for Financial Services, Networking (QoS), Robots, Air Traffic Control Systems...
Compared to other real-time operating systems, Linux has the advantage to be open source with great hardware support. Yet it was not designed to be real-time. It was originally designed to be a time-sharing system where the goal is to give the best throughput from the hardware using all the resources at the maximum. This is the opposite requirement of the real-time constraints that needs determinism even at a low global throughput.
Throughout the years, different approaches have emerged to overcome this problem. The first approach is to modify the Linux Kernel itself in order to get the required latencies or the real-time APIs. This approach is covered by the project PREEMPT_RT led by the Linux kernel developers Ingo Molnar, Thomas Gleixner and Steven Rostedt. The second approach is to add a layer between the Hardware and the Linux kernel to handle the real-time requirements so that the Linux kernel behaviour can stay as it is. This approach has been taken into account by different project like RTLinux, RTAI and Xenomai. Since only the last one is maintained actively on ARM, we will only talk about it.