Bi-Level Deadline Scaling for Admission Control in Mixed-Criticality Systems

In some cases, tasks may be allowed to migrate from one processor to another, e.g., Due to hardware failures or for workload balancing. If a mixed-criticality setting is considered, it is necessary to decide whether new tasks with different levels of criticality may be accepted by a processor without compromising the already running tasks. Since this decision has to be taken on-line, there is a need for fast but yet accurate schedulability tests for mixed-criticality systems. In this paper, we consider that the EDF-VD algorithm is used to schedule tasks on the different processors. EDF-VD assigns virtual deadlines to high-criticality tasks, i.e., It uniformly downscales their real deadlines, to account for a potential increase in their execution demand. A deadline scaling factor is hence computed for the whole processor. However, in the case where the increase in computation demand strongly differs from one task to another, scaling deadlines uniformly makes EDF-VD incur pessimism. Of course, a scaling factor can be computed for every single task, however, this leads to a considerably more complex algorithm which cannot be used in an on-line setting. As a result, we propose an intermediate solution by introducing a bi-level deadline scaling. This way, high-criticality tasks that experience a small increase of workload are assigned one scaling factor, whereas tasks with a large increase of workload are assigned a separate scaling factor. Our experiments show that the proposed approach dominates the original EDF-VD algorithm while it does not increase complexity allowing for constant-time admission control in mixed-criticality systems.