Periodic and aperiodic tasks co-exist in many real-time systems. The periodic tasks typically arise from sensor data or control loops, while the aperiodic tasks generally arise from arbitrary events. Their time constraints need to be met even in the presence of faults. Considering the unpredictability of aperiodic tasks, this paper proposes a fault-tolerant reservation-based strategy (FTRB) to schedule aperiodic tasks by utilizing the processor time left unused by periodic tasks. The least upper bound of reserved processor time is derived analytically such that all available processor time may be exploited for servicing aperiodic tasks. Any newly arrived aperiodic task is scheduled on the first-fit processor by using an extended dynamic schedulability criterion. A primary/backup approach is used to schedule the primary and backup copy of each task on different processors to tolerate a processor failure. Our analysis and simulation results show that the processors can achieve high utilization and that the on-line implementation of aperiodic task scheduling is feasible.
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