Since 2004 a project has been going on trying to make the Linux kernel into a true hard Real-Time operating system. This project has become know as PREEMPT_RT (formally the "real-time patch"). Over the past decade, there was a running joke that this year PREEMPT_RT would be merged into the mainline kernel, but that has never happened. In actuality, it has been merged in pieces. Examples of what came from PREEMPT_RT include: mutexes, high resolution timers, lockdep, ftrace, RT scheduling, SCHED_DEADLINE, RCU_PREEMPT, generic interrupts, priority inheritance futexes, threaded interrupt handlers and more. The only thing left is turning spin_locks into mutexes, and that is now mature enough to make its way into mainline Linux. This year could possibly be the year PREEMPT_RT is merged!
Getting PREEMPT_RT into the kernel was a battle, but it is not the end of the war. Once PREEMPT_RT is in mainline, there's still much more work to be done. The RT developers have been so focused on getting RT into mainline, that little has been thought about what to do when it is finally merged. There is a lot to discuss about what to do after RT is in mainline. The game is not over yet.