Scheduling Self-Suspending Real-Time Tasks with Rate-Monotonic Priorities

Recent results have shown that the feasibility problem of scheduling periodic tasks with self-suspensions is NP-hard in the strong sense. We observe that a variation of the problem statement that includes sporadic tasks instead of periodic tasks results in a simple characterization of the critical scheduling instant. This in turn leads to an exact characterization of the critical instant for self-suspending tasks with respect to the interference (preemption) from higher-priority sporadic tasks. Using this characterization, we provide pseudo-polynomial response-time tests for analyzing the schedulability of such self-suspending tasks. Self-suspending tasks can also result in more worst-case interference to lower-priority tasks than their equivalent non-suspending counterparts with zero suspension intervals. Hence, we develop a dynamic slack enforcement scheme, which guarantees that the worst-case interference caused by suspending sporadic tasks is no more than the worst-case interference arising from equivalent non-suspending sporadic tasks without suspension intervals. The worst-case response time of self-suspending sporadic tasks themselves is also shown to be unaffected by dynamic slack enforcement, thereby making it optimal. In order to reduce the runtime complexity of slack enforcement, a static slack enforcement scheme is also developed. Empirical analysis of these schemes and the previously studied period enforcement algorithm shows that static slack enforcement achieves within 3% of the breakdown utilization of dynamic slack enforcement, while period enforcement achieves within 14% of dynamic slack enforcement. System designers can take advantage of these different execution control policies depending on their taskset utilizations and implementation constraints.