Don't these threaded interrupt handlers have even more overhead than workqueues? Workqueues are at least handled sequentially in the same thread; using threaded interrupt handlers means there are even more context switches.
I'm afraid that you have missed a key point about real-time systems. Design of a hard real-time system starts with deciding which threads in the system will run at real-time priority (SCHED_FIFO or SCHED_RR) and also at which priority (1..99). This step involves all threads in the system, including threaded interrupt handlers. By assigning a higher priority to certain application threads than the threaded interrupt handlers it becomes possible to obtain a system with low and bounded event response time. Handling some of the interrupt work via workqueues instead of threaded interrupt handlers would result in a larger event response time.
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