Schedulability analysis for a combination of preemptive strict periodic tasks and sporadic tasks

International audienceWe consider the problem of scheduling tasks with strict periods combined with sporadic tasks. Both types of task have fixed priorities and are preemptive. For a task with a strict period, %it is necessary to prove that for any job of the task, the difference between its starting time and its release time must be identical for every job. Tasks with strict periods are typically in charge of controlling the activities of a system (sensor/actuator, feedback control, ect.). The freshness of the information they use and/or the reactivity of the system are constrained. Indeed, for control tasks, it might be important to control their jitters (the difference between the worst case and the minimum response time) to ensure the stability of the control loop. In this paper, we consider for controlled tasks, the solution satisfying property that minimizes the jitter of tasks with strict periods. For any task with strict period, (i) the start time of any job of the task must be equal to its release time and (ii) the Worst Case Response Time (WCRT) of the task must be equal to its Worst Case Execution Time (WCET). In this paper, we provide a sufficient schedulability condition for the schedulability of tasks with strict periods. We show how to define their first release times such that property \ref{property-P1} is met (based on paper \cite{rtns10}). Tasks with strict periods have the same fixed priority, the highest one. Sporadic tasks all have a lower priority than any task with a strict period. We show in this paper how to define the worst case scenario for the schedulability of sporadic tasks in the presence of tasks with strict periods. Then we propose a schedulability condition for sporadic tasks based on the worst case response time computation