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Analyses pour l'ordonnançabilité et la flexibilité de systèmes temps-réel

Christophe Prévot 1, 2
2 SPADES [2013-2015] - Sound Programming of Adaptive Dependable Embedded Systems [2013-2015]
Inria Grenoble - Rhône-Alpes, LIG [2007-2015] - Laboratoire d'Informatique de Grenoble [2007-2015]
Abstract : Real time system are often used for applications in avionic or automotive domains. For those systems, timing constraints are as important as functional constraints. Thus, for each functionality it is necessary to compute the maximum duration between the acquisition of the inputs and the production of the corresponding outputs, duration denoted worst case latency. Given the needs that we identify at Thales, we consider uni-processor systems scheduled under fixed priority preemptive scheduling with task chains mapped on them. Each chain implements a functionality and each task has a fix and unique priority. The finishing time of a task corresponds to the activation of the following one in the chain. The processor always executes the task with the highest priority. Then, we use scheduling analysis to characterize the timing behavior of systems and to compute the worst case latency of chains. If the worst case latency of each chain is lower than or equal to its timing constraint, then the system is schedulable. To guarantee the schedulability of a system, we compute upper bounds that are higher than or equal to the worst case latency. Depending on the precision of the analysis, these bounds may be more of less over-approximated. For a given system, if the over-approximations are too large, then it is necessary to over-dimension the system in order to guarantee its schedulability, which is not desirable in a industrial context. To solve this over-dimensioning problem, we compute more precise upper bounds and on more general systems than in the state of the art. When computing these upper bounds, it is useful to measure the gap between the worst case latency and its upper bound. To achieve this, we compute a lower bound on the worst case latency to evaluate the precision of the upper bound. In the state of the art, lower bounds are computed using simulation. We propose instead to use schedulability analysis to compute lower bounds that we define as execution scenarios that are realizable considering the system model. Our lower bounds are computed with equations similar to those established to compute our upper bounds. Finally, considering the very long lifetime of systems and the quick evolution of technologies, many systems have to evolve during their lifetime. A relevant evolution in an industrial context consists in adding a new chain to an existing system. To guarantee the schedulability of a system in this context, we present a methodology to compute the worst-case latency of a new chain while providing guarantees on the schedulability of the new system. This analysis can be used to determine the sensitivity and/or the robustness of a system with respect to timing parameter changes, such as the execution times of its tasks. This consists in finding extreme values of its timing parameters such that the system remains schedulable. In this thesis, we present analyses to compute upper and lower bounds on the worst case latency, and to schedule a new chain while giving guarantee on its schedulability. These results may be extended in the future to handle more complex systems, and the computation of lower bound may be adapted to other analyses. Finally, the developed analyses are complex, so it would be interesting to certify that they are correct using a proof assistant to guarantee that they are exact.
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Christophe Prévot. Analyses pour l'ordonnançabilité et la flexibilité de systèmes temps-réel. Systèmes embarqués. Université Grenoble Alpes, 2019. Français. ⟨NNT : 2019GREAM045⟩. ⟨tel-02513594v2⟩

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