[Ovmsdev] Time spent in ticker event handlers / new use for housekeeping task

[Michael Balzer]

A question to the FreeRTOS gurus:

How much time may we spend in a ticker event handler, or generally a software timer callback, without risking system stability?

The FreeRTOS manual explicitly warns about using any kind of blocking calls within the timer service task, as all software timer callbacks are executed in that
context:

https://freertos.org/RTOS-software-timer.html

    /"//Timer callback functions execute in the context of the timer service task. It is therefore// /*/essential/*/ //that timer callback functions never
    attempt to block. For example, a timer callback function must not call vTaskDelay(), vTaskDelayUntil(), or specify a non zero block time when accessing a
    queue or a semaphore.//"/

The timer service queue currently has 10 slots. That's tunable, but I'm also worried about other system components possibly relying on regular timer periods of
higher frequency.

My specific issue: I'm producing the notifications from my  ticker1 handler. I now found out a simple standard info notification needs already around 10 ms, and
my battery status update (15 data messages) needs 60-70 ms. I thought these would run faster, not sure where the time is spent. I'm using the command
notifications, nice to use but with quite some overhead of course.

So if for example the wifi or bluetooth stack needs a 50 ms timer, that may already be a problem. But do system components use software timers for time critical
tasks?

The solution would be moving the notification generator to a separate task. A new task would need RAM, but there's also the housekeeping task, which could
generally be made available for such needs. It's basically idle after the init process and already has a large stack. I think about adding a callback execution
queue to it, so my ticker handler would simply queue the notification generator call there.

Thanks,
Michael

-- 
Michael Balzer * Helkenberger Weg 9 * D-58256 Ennepetal
Fon 02333 / 833 5735 * Handy 0176 / 206 989 26

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