Support for Nested Preemption in nxtOSEK

In real-time computing scenarios, we are concerned with logical as well as temporal accuracy of the results. While developing algorithms or performing schedulability analysis, preemption overheads are usually ignored even, though it’s an operation that consumes finite amount of time. Preemption allows higher priority tasks to halt the lower priority tasks, though this need not be necessarily required to meet task deadlines. Our concern is not to get the results as soon as possible but soon enough to just meet the deadlines of all the tasks. Algorithms which meet deadlines and minimize preemptions in order to decrease overheads have been developed. The nxtOSEK is a Real-Time Operating System for the AT91SAM7S256 ARM7TDMI controller, with capabilities of ANSI-C/C++ programmability. The nxtOSEK kernel cannot be used to demonstrate any such algorithm, since it does not support nested preemption. The task scheduling in the nxtOSEK kernel is governed by a 1ms ISR routine. When a higher priority task preempts a lower priority task, it has been observed that this 1ms ISR(and hence the task scheduler) is suppressed for the duration of execution of the higher priority task. This causes an equivalent shift in the release times of all the subsequent tasks and the entire timing behaviour of the nxtOSEK kernel is skewed. The nxtOSEK kernel has been purposefully designed to prevent successive instances of preemption i.e. the kernel does not support a case where after one preemption event, while the preempting task is yet to finish its execution, it is preempted in turn by an even higher priority task. In this project, we attempted to resolve this by modifying the current kernel behavior. The article has been divided into five sections, Background, that explains the current kernel behavior and highlights the problem in it, Design, that encompasses the suggested modifications in the kernel, Implementation, depicting the changes done to achieve the goal, Validation, defines the test-cases used to test the modified kernel and Results,presenting the output of the modification.