Proper handling of interrupts in cyber-physical systems

Interrupt handling plays a fundamental role in Cyber-Physical Systems (CPS), particularly for those whose complexity cannot go with simplistic sensing-control-actuation loops designed around ordinary polling operations. Such systems usually rely on some sort of Real-Time Operating System (RTOS) to support the concurrent execution (or parallel execution, for multicore platforms) of periodic threads that interact with hardware devices mainly through interrupts. Since hardware interrupts are asynchronous with respect to the execution flow of threads, inappropriately handling them has the potential to disrupt the system´s real-time specification, causing undesirable jitter and potential deadline losses. In this paper, we investigate interrupt handling strategies for RTOS considering CPSs designed around specific Models of Computation (MoC). We compare traditional Interrupt Service Routines (ISR), which perform both interrupt reception and the servicing all together, with a time-predictable mechanism that decouples interrupt reception from servicing, making the first deterministic in terms of time and scheduling the second along with other real-time threads. We compare them in terms of latency and jitter considering two different scenarios: that of systems designed around the Discrete Events (DE) MoC and that of those designed around the Time-Triggered (TT) MoC. Our results show that the time-predictable mechanism is essential for TT, while the traditional interrupt handling mechanism is more suitable for DE, thus demonstrating that RTOS need to be configurable in this respect in order to support forthcoming Cyber-Physical Systems.