I heard that the best way to use a watch dog timer in a preemptive kernel is to assign it to the lowest task/idle task and refresh it there, I fail to understand why though,what if high priority tasks keeps running and idle task doest run before timeout.
any clarifications?
Thanks.
fail to understand why though,what if high priority tasks keeps running and idle task doest run before timeout.
Well, that is kind of the point. If the lowest priority thread (or better, the idle thread) is starved, then your system will be missing deadlines, and is either poorly designed or some unexpected condition has occurred.
If it were reset at a high priority or interrupt, then all lower priority threads could be in a failed state, either running busy or never running at all, and the watchdog would be uselessly maintained while not providing any protection whatsoever.
It is nonetheless only a partial solution to system integrity monitoring. It addresses the problem of an errant task hogging the CPU, but it does not deal with the issue of a task blocking and never being scheduled as intended. There are any number of ways of dealing with that, but a simple approach is to have "software watchdogs", counters that get reset by each task, and decremented in a high- priority timer thread or handler. If any thread counter reaches zero, then the respective thread blocked for longer than intended, and action may be taken. This requires that each thread runs at an interval shorter than its watchdog counter reset value. For threads that otherwise block indefinitely waiting for infrequent aperiodic events, you might use a blocking timeout just to update the software watchdog.
There is no absolute rule about the priority of a watchdog task. It depends on your design and goals.
Generally speaking, if the watchdog task is the lowest priority task then it will fail to run (and the watchdog will reset) if any higher priority task becomes stuck or consumes too much of the CPU time. Consider that if the high priority task(s) is running 100% of the time then that's probably too much because lower priority tasks are getting starved. And maybe you want the watchdog to reset if lower priority tasks are getting starved.
But that general idea isn't a complete design. See this answer, and especially the "Multitasking" section of this article (https://www.embedded.com/watchdog-timers/) for a more complete watchdog task design. The article suggests making the watchdog task the highest priority task but discusses the trade-offs of the alternative.
