The Application of the Original Priority Ceiling Protocol to Mixed Criticality Systems

Mixed Criticality Systems (MCSs) have been the focus of considerable study over the last six years. This work has lead to the definition of a standard model that allows processors to be shared efficiently between tasks of different criticality levels. This model, however, often assumes that the tasks are independent of each other; which is an unrealistic restriction. In this paper tasks of the same criticality are allowed to shared resources that require mutually exclusive access. Such resources are usually protected by some form of priority ceiling protocol (PCP). But it is not clear that the more common form of this protocol (sometimes called the Immediate Priority Ceiling Protocol, or the Stack Resource Protocol) is appropriate for MCS. Here we evaluate the original form of the PCP and propose a new MCS-aware protocol called MCS OPCP. This protocol allows lower criticality tasks to transfer budgets when a task runs out of its own, or the resource, budget before it has completed its use of the resource. Without this provision the task would need to be suspended whilst holding a resource lock. We show that higher criticality tasks are not impacted by the protocol. An assessment of MCS OPCP in terms of its impact on schedulability analysis is provided.