Towards adaptive hierarchical scheduling of overloaded real-time systems

In a hierarchical scheduling framework, a resource can be shared among modules with different criticality levels. In our recently introduced adaptive hierarchical scheduling framework, modules receive a dynamic portion of the CPU during run-time. While providing temporal isolation is one of the main advantages of hierarchical scheduling, in an adaptive framework, for example when the CPU is overloaded, the higher priority modules can violate timing guarantees of the lower priority modules. However, the priorities of modules are assigned based on parameters other than the module criticality levels. For example the priority is often assigned according to periods and deadlines of tasks to increase the CPU utilization assuming static systems, i.e. modules parameters do not change during runtime. In an overload situation the high criticality modules should be superior to the low criticality modules in receiving resources. In this paper, extending our adaptive framework, we propose two techniques for controlling the CPU distribution among modules in an overload situation. We are taking another step towards having a complete adaptive hierarchical scheduling framework by incorporating an overload controller into our framework.