Efficient schedulability tests for real-time embedded systems with urgent routines

Task scheduling is one of the key mechanisms to ensure timeliness in embedded real-time systems. Such systems have often the need to execute not only application tasks but also some urgent routines (e.g. error-detection actions, consistency checkers, interrupt handlers) with minimum latency. Although fixed-priority schedulers such as Rate-Monotonic (RM) are in line with this need, they usually make a low processor utilization available to the system. Moreover, this availability usually decreases with the number of considered tasks. If dynamic-priority schedulers such as Earliest Deadline First (EDF) are applied instead, high system utilization can be guaranteed but the minimum latency for executing urgent routines may not be ensured. In this paper we describe a scheduling model according to which urgent routines are executed at the highest priority level and all other system tasks are scheduled by EDF. We show that the guaranteed processor utilization for the assumed scheduling model is at least as high as the one provided by RM for two tasks, namely 2(2√−1). Seven polynomial time tests for checking the system timeliness are derived and proved correct. The proposed tests are compared against each other and to an exact but exponential running time test

Complex scheduling models and analyses for property-based real-time embedded systems

Modern multi core architectures and parallel applications pose a significant challenge to the worst-case centric real-time system verification and design efforts. The involved model and parameter uncertainty contest the fidelity of formal real-time analyses, which are mostly based on exact model assumptions. In this dissertation, various approaches that can accept parameter and model uncertainty are presented. In an attempt to improve predictability in worst-case centric analyses, the exploration of timing predictable protocols are examined for parallel task scheduling on multiprocessors and network-on-chip arbitration. A novel scheduling algorithm, called stationary rigid gang scheduling, for gang tasks on multiprocessors is proposed. In regard to fixed-priority wormhole-switched network-on-chips, a more restrictive family of transmission protocols called simultaneous progression switching protocols is proposed with predictability enhancing properties. Moreover, hierarchical scheduling for parallel DAG tasks under parameter uncertainty is studied to achieve temporal- and spatial isolation. Fault-tolerance as a supplementary reliability aspect of real-time systems is examined, in spite of dynamic external causes of fault. Using various job variants, which trade off increased execution time demand with increased error protection, a state-based policy selection strategy is proposed, which provably assures an acceptable quality-of-service (QoS). Lastly, the temporal misalignment of sensor data in sensor fusion applications in cyber-physical systems is examined. A modular analysis based on minimal properties to obtain an upper-bound for the maximal sensor data time-stamp difference is proposed

Blocking analysis of spin locks under partitioned fixed-priority scheduling

Partitioned fixed-priority scheduling is widely used in embedded multicore real-time systems. In multicore systems, spin locks are one well-known technique used to synchronize conflicting accesses from different processor cores to shared resources (e.g., data structures). The use of spin locks can cause blocking. Accounting for blocking is a crucial part of static analysis techniques to establish correct temporal behavior. In this thesis, we consider two aspects inherent to the partitioned fixed-priority scheduling of tasks sharing resources protected by spin locks: (1) the assignment of tasks to processor cores to ensure correct timing, and (2) the blocking analysis required to derive bounds on the blocking. Heuristics commonly used for task assignment fail to produce assignments that ensure correct timing when shared resources protected by spin locks are used. We present an optimal approach that is guaranteed to find such an assignment if it exists (under the original MSRP analysis). Further, we present a well-performing and inexpensive heuristic. For most spin lock types, no blocking analysis is available in prior work, which renders them unusable in real-time systems. We present a blocking analysis approach that supports eight different types and is less pessimistic than prior analyses, where available. Further, we show that allowing nested requests for FIFO- and priority-ordered locks renders the blocking analysis problem NP-hard.Partitioned Fixed-Priority Scheduling ist in eingebetteten Multicore-Echtzeitsystemen weit verbreitet. In Multicore-Systemen sind Spinlocks ein bekannter Mechanismus um konkurrierende Zugriffe von unterschiedlichen Prozessorkernen auf geteilte Resourcen (z.B. Datenstrukturen) zu koordinieren. Bei der Nutzung von Spinlocks können Blockierungen auftreten, die in statischen Analysetechniken zum Nachweis des korrekten zeitlichen Verhaltens eines Systems zu berücksichtigen sind. Wir betrachten zwei Aspekte von Partitioned Fixed-Priority Scheduling in Verbindung mit Spinlocks zum Schutz geteilter Resourcen: (1) die Zuweisung von Tasks zu Prozessorkernen unter Einhaltung zeitlicher Vorgaben und (2) die Analyse zur Entwicklung oberer Schranken für die Blockierungsdauer. Übliche Heuristiken finden bei der Nutzung von Spinlocks oft keine Taskzuweisung, bei der die Einhaltung zeitlicher Vorgaben garantiert ist. Wir stellen einen optimalen Ansatz vor, der dies (mit der ursprünglichen MSRP Analyse) garantiert, falls eine solche Zuweisung existiert. Zudem präsentieren wir eine leistungsfähige Heuristik. Die meisten Arten von Spinlocks können mangels Analyse der Blockierungsdauer nicht für Echtzeitsysteme verwendet werden. Wir stellen einen Analyseansatz vor, der acht Spinlockarten unterstützt und weniger pessimistische Schranken liefert als vorherige Analysen, soweit vorhanden. Weiterhin zeigen wir, dass die Analyse bei verschachtelten Zugriffen mit FIFO- und prioritäts-geordneten Locks ein NP-hartes Problem ist

A Review of Priority Assignment in Real-Time Systems

It is over 40 years since the first seminal work on priority assignment for real-time systems using fixed priority scheduling. Since then, huge progress has been made in the field of real-time scheduling with more complex models and schedulability analysis techniques developed to better represent and analyse real systems. This tutorial style review provides an in-depth assessment of priority assignment techniques for hard real-time systems scheduled using fixed priorities. It examines the role and importance of priority in fixed priority scheduling in all of its guises, including: preemptive and non-pre-emptive scheduling; covering single- and multi-processor systems, and networks. A categorisation of optimal priority assignment techniques is given, along with the conditions on their applicability. We examine the extension of these techniques via sensitivity analysis to form robust priority assignment policies that can be used even when there is only partial information available about the system. The review covers priority assignment in a wide variety of settings including: mixed-criticality systems, systems with deferred pre-emption, and probabilistic real-time systems with worstcase execution times described by random variables. It concludes with a discussion of open problems in the area of priority assignment