A Concurrency Framework for Priority-Aware Intercomponent Requests in CAmkES on seL4

Component-based design can encapsulate and isolate state and the operations on it, but timing semantics crosscut these boundaries when a real-time task’s control flow spans multiple components. Under priority-based scheduling, inter-component control flow should be coupled with priority information, so that task execution can be prioritized appropriately end-to-end. However, the CAmkES component architecture for the seL4 microkernel does not adequately support priority propagation across intercomponent requests: component interfaces are bound to threads that execute at fixed priorities provided at compile-time in the component specification. In this paper, we present a new library for CAmkES with a thread model that supports (1) multiple concurrent requests to the same component endpoint; (2) propagation and enforcement of priority metadata, such that those requests are appropriately prioritized; and (3) implementations of Non-Preemptive Critical Sections, the Immediate Priority Ceiling Protocol and the Priority Inheritance Protocol for components encapsulating critical sections of exclusive access to a shared resource. We measure overheads and blocking times for these new features and use existing theory to perform schedulability analysis. Evaluations on both Intel x86 and ARM platforms show that our new library allows CAmkES to provide suitable end-to-end timing for real-time systems.