Safety- and Task-Drop-Aware Mixed-Criticality Task Scheduling

In Mixed-Criticality (MC) systems, a fault may occur due to different reasons, e.g., hardware defects, software errors, or the arrival of unexpected events. In order to tolerate faults in MC systems, the re-execution technique is typically employed, which may lead to overrun of High-Criticality (HC) tasks, which necessitates the drop of Low-Criticality (LC) tasks or degrading their quality. However, frequent drops or relatively long execution times of LC tasks (especially mission-critical tasks) are not always desirable, and it may impose a negative impact on the performance, or the functionality of MC systems. In this regard, this chapter proposes a parameter for MC tasks and develops a design-time task-drop-aware schedulability analysis based on the EDF-VD scheduling algorithm. According to this analysis and the proposed scheduling policy based on the new MC task parameter, in the HIgh-criticality mode (HI mode), when an HC task overruns and the system switches to the HI mode, the number of drops per LC tasks is prohibited from passing a predefined threshold. In addition, to guarantee the real-time constraints and safety requirements of MC tasks in the presence of faults (assuming transient faults in this chapter), a corresponding scheduling mechanism has been developed. According to evaluation through realistic avionic applications and synthetic task sets, the proposed method can improve the acceptance ratio significantly, compared to state of the art.