Section: New Results

Uniprocessor Mixed-Criticality Real-Time Scheduling

Participants : Slim Ben-Amor, Liliana Cucu, Robert Davis, Mehdi Mezouak, Yves Sorel.

In the framework of the FUI CEOS project 9.1.1.1 we mainly investigated the PX4 autopilot free software program that was chosen by the partners to be implemented on the Pixhawk electronic board. This board will be installed in the multirotor drone that the project is intended to built. The board is based on a microcontroller which contains an ARM Cortex M4 microprocessor, timers, several sensors, accelerometer, gyroscope, magnetometer, barometer, and actuators, mainly four to eight electric motors depending on the level of redundancy.

We studied the existing source code of PX4 which consists of two main layers: the flight stack, which is an estimation and flight control system, and the middleware, which is a general robotics layer providing internal/external communications and hardware integration. This study allowed us to understand the general architecture of PX4. The flight stack is split into a set of threads communicating asynchronously through a micro object request broker messaging. In the CEOS project our team is in charge to guarantee that the drone will satisfy multiple real-time criticality levels. In order to be able to perform a real-time schedulability analysis on the PX4 autopilot, first we transformed this set of communicating threads into a task dependency graph. Second, we sought the period of each task starting from input tasks which read from sensors, to output tasks which write into actuators. The partners of the project chose to run PX4 on the NuttX OS which is open source, light-weight, efficient and very stable. It provides POSIX API and some form of real-time scheduling. Thus, we had to deeply understand the scheduler and the management of interruptions and time of NuttX. We plan to modify NuttX in order to support mixed-criticality applications using to start, online real-time scheduling, and then offline real-time scheduling.

Finally, always to perform the real-time schedulability analysis of PX4, we must estimate the worst execution time (WCET) of each task. This problem is very complex due to the multiple possible paths in a task as well as the different data it consumes. Moreover, the processor and/or the microcontroller itself may have some features like memory contentions, bus accesses, caches, pipelines, speculative branchings that increase the difficulty to determine WCETs. All these variabilities lead us to introduce probabilistic reasoning in characterizing the timing behavior (WCET, schedulability analyses) of mixed-criticality real-time applications [4].