There's an intermediate point between pre-emptive and cooperative that I've found useful. For the lack of a better name let's call it "you-better-cooperate".
Suppose a real-time system using cooperative scheduling where well-behaved tasks yield within a guaranteed time window. Suppose also a system that has the ability to launch and restart processes for example in the case of error. In such a system, a poorly-behaved process can hang the system because it doesn't yield.
Introducing pre-emption to such a system avoids the potential hangs, but (a) adds the complexity of pre-emption; (b) only gets exercised in the case that you're already in a failure state (process failed to yield); and (c) allows processes in a known failure state to continue.
Instead, when a process is scheduled, set a timer interrupt for a time period after its guaranteed yield. When the process yields, cancel that timer (or re-schedule it for the next process). If the timer fires, just kill and restart the non-yielding process.
In a limited set of cases, this is a simpler, more robust, and equally powerful system compared to both full pre-emption and cooperation.
Suppose a real-time system using cooperative scheduling where well-behaved tasks yield within a guaranteed time window. Suppose also a system that has the ability to launch and restart processes for example in the case of error. In such a system, a poorly-behaved process can hang the system because it doesn't yield.
Introducing pre-emption to such a system avoids the potential hangs, but (a) adds the complexity of pre-emption; (b) only gets exercised in the case that you're already in a failure state (process failed to yield); and (c) allows processes in a known failure state to continue.
Instead, when a process is scheduled, set a timer interrupt for a time period after its guaranteed yield. When the process yields, cancel that timer (or re-schedule it for the next process). If the timer fires, just kill and restart the non-yielding process.
In a limited set of cases, this is a simpler, more robust, and equally powerful system compared to both full pre-emption and cooperation.