On the Safety of Mixed-Criticality Scheduling

A common assumption for scheduling mixed-criticality systems is the degradation of less critical tasks when the system is in the emergency mode; which is entered when critical tasks overrun their expected WCET. In the meanwhile, common safety standards enforce strict safety requirements on all criticality levels. However, the impact of degrading less critical tasks on the overall system safety is not well understood. By introducing probabilistic distributions of task execution times, we show in this paper a first analysis of safety of mixed-criticality systems. Inspired by standards, we provide the probability-of-failure-per-hour (PFH) of a system when no overrun is detected. We also introduce and analyze the expected mode switching time, which describes how often emergency mode is entered. Our results reveal a fundamental trade-off between these two values. Finally, we discuss other possible safety measures that form a complete overview of a system's safety