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MetaIRQ Thread Priority Demonstration

Overview

This sample demonstrates the use of a thread running at a MetaIRQ priority level to implement “bottom half” style processing synchronously with the end of a hardware ISR. It implements a simulated “device” that produces messages that need to be dispatched to asynchronous queues feeding several worker threads, each running at a different priority. The dispatch is handled by a MetaIRQ thread fed via a queue from the device ISR (really just a timer interrupt).

Each message has a random (and non-trivial) amount of processing that must happen in the worker thread. This implements a “bursty load” environment where occassional spikes in load require preemption of running threads and delay scheduling of lower priority threads. Messages are accompanied by a timestamp that allows per-message latencies to be computed at several points:

  • The cycle time between message creation in the ISR and receipt by the MetaIRQ thread for dispatch.

  • The time between ISR and receipt by the worker thread.

  • The real time spent processing the message in the worker thread, for comparison with the required processing time. This provides a way to measure preemption overhead where the thread is not scheduled.

Aspects to note in the results:

  • On average, higher priority (lower numbered) threads have better latencies and lower processing delays, as expected.

  • Cooperatively scheduled threads have significantly better processing delay behavior than preemtible ones, as they can only be preempted by the MetaIRQ thread.

  • Because of queueing and the bursty load, all worker threads of any priority will experience some load-dependent delays, as the CPU occasionally has more work to do than time available.

  • But, no matter the system load or thread configuration, the MetaIRQ thread always runs immediately after the ISR. It shows reliable, constant latency under all circumstances because it can preempt all other threads, including cooperative ones that cannot normally be preempted.

Requirements

This sample should run well on any Zephyr platform that provides preemption of running threads by interrupts, a working timer driver, and working log output. For precision reasons, it produces better (and more) data on systems with a high timer tick rate (ideally 10+ kHz).

Note that because the test is fundamentally measuring thread preemption behavior, it does not run without modification on native_posix platforms. In that emulation environment, threads will not be preempted except at specific instrumentation points (e.g. in k_busy_wait()) where they will voluntarily release the CPU.