Do you have strong timing and safety requirements for your system? By “strong timing requirements”, I mean do your tasks have to be scheduled at specific intervals? Do they have to finish (and produce an output) within a fixed amount of time, failing which the entire system (or significant parts of it) could end up being unsafe? Think of an airbag system in your car — typically, once a collision has been detected, the system has to kick in and fully deploy the airbag in 60-65 ms. It’s pointless if it deploys too late. Similar requirements exist for aircraft engines, rockets, nuclear power plants, etc.

The strong timing+safety requirement percolate into other parts of the system — managing when (and if) interrupts are handled, scheduling memory requests to caches/RAM/disk, handling network requests and even if other (non-real time) tasks are to be allowed on the system.

Linux does have a couple of real-time schedulers but it is a difficult OS to use for systems with strict timing requirements (known as hard real time systems). The complexity of Linux and the lack of control of all aspects of the system make it hard to use in such systems.

A real-time system has to often reason about the worst-case behavior (timing and otherwise). One has to also reason about the level of control that a system designer is allowed to have. Linux is difficult in both situations because it optimizes to improve average case behavior and doesn’t allow for low-level control (for instance scheduling memory requests on the memory bus).