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When debugging something as involved as kernel scheduler timings, you would typically use one of the software-based debugging mechanisms available. However, in cases when software is close to bare metal, you don’t always need to do that. Instead, you can output a signal to a GPIO, and then use a logic analyzer or a scope to measure signal change timing – which is what [Albert David] did when evaluating Linux kernel’s PREEMPT_RT realtime operation patches.
When you reach for a realtime kernel, latency is what you care about – realtime means that for everything you do, you need to get a response within a certain (hopefully very short) interval. [Albert] wrote a program that reads a changing GPIO input and immediately writes the new state back, and scoped both of the signals to figure out the latency of of the real-time patched kernel as it processes the writes. Overlaying all the incoming and outgoing signals on the same scope screen, you can quickly determine just how suitable a scheduler is when it comes to getting an acceptable response times, and [Albert] also provides a ready-to-go BeagleBone image you can use for your own experiments, or say, in an educational environment.
What could you use this for? A lot of hobbyists use realtime kernels on Linux when building CNC machine controllers and robots, where things like motor control put tight constraints on how quickly a decision in your software is translated into real-world consequences, and if this sounds up your valley, check out this Linux real-time task tutorial from [Andreas]. If things get way too intense for a multi-tasking system like Linux, you might want to use a RTOS to begin with, and we have a guide on that for you, too.
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