Real-Time Scheduling: Non-Preemption, Critical Sections and Round Robin

The main property of a (hard) real-time system is feasibility. It is the guarantee that tasks do always meet their deadlines, when the system is running under a given scheduling policy. An optimal policy produces a feasible schedule whenever a feasible schedule exists. It is therefore the most appropriate choice if feasibility is the only matter of concern. However, if a set of tasks is feasible under several policies, it is possible to impose additional constraints, which improve in a certain way the quality of a system. A refined choice is only possible if feasibility tests or, more broadly, timing analysis is available for other policies than the optimal ones. In this document we derive timing analysis for policies obtained by combining known policies in hierarchical layers. These layered priorities are motivated by the Posix 1003.1c standard, which allows such a combination of Fixed Preemptive Priorities and the Round Robin scheduling policy. In this context we extend the trajectory based model developed in [8] for systems scheduled under real-time constraints to account for non-preemptive resources and the associated priority ceiling protocol. Furthermore, timing analysis of the Round Robin policy is derived.