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The increasing functional and nonfunctional requirements of real-time applications, the advent of mixed criticality computing, and the necessity of reducing costs are leading to an increase in the interest for employing COTS hardware in real-time domains. In this scenario, the Linux kernel is emerging as a valuable solution on the software side, thanks to the rich support for hardware devices and peripherals, along with a well-established programming environment. However, Linux has been developed as a general-purpose operating system, followed by several approaches to introduce actual real-time capabilities in the kernel. Among these, the PREEMPT_RT patch, developed by the kernel maintainers, has the goal to increase the predictability and reduce the latencies of the kernel directly modifying the existent kernel code. This article aims at providing a survey of the state-of-the-art approaches for building real-time Linux-based systems, with a focus on PREEMPT_RT, its evolution, and the challenges that should be addressed in order to move PREEMPT_RT one step ahead. Finally, we present some applications and use cases that have already benefited from the introduction of this patch.

The increasing interest in the integration of Mixed-Criticality Systems (MCS) in Commercial-Off-The-Shelf (COTS) platforms leads to an increasing number of challenges. The possibility of sharing computing resources among applications with different time criticalities is a key goal for COTS systems, but still hard to achieve. Classical approaches in real-time systems are not feasible when platform and operating system may introduce unpredictability in the task execution. Moreover, if the system must also meet non-functional requirements (e.g., thermal and power management), dynamic approaches of computing resources allocation are more effective than static ones. Unfortunately, this contributes to increasing the complexity of the scenario. In MCS, the overheads and the unpredictability caused by sharing resources like cache memories have been well studied. However, in some cases we could also consider the operating system itself as a potential source of unexpected and unpredictable latencies, if several running tasks perform system calls. This work aims at proposing a model for the intra-core and inter-core interferences and the analysis of the OS-induced latencies in a Linux real-time system, both essential for the creation of smart and effective run-time resource management policies.

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