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Question about Preemptive Scheduler

Posted by leesp on May 23, 2017

Can a task be pre-empted at any point? For example the task below: Can it be pre-empted any time, or pre-emption can only happen while it is waiting for the semaphore? ~~~ static void taskconsole(void *p) { (void)p; for (;;) { ... ... // Can it be pre-empted here? ... if (xSemaphoreTake(dispmutex, ~0)) { // Or, pre-emption can only happen here? strwrite(str); xSemaphoreGive(dispmutex); } } } ~~~

Question about Preemptive Scheduler

Posted by rtel on May 24, 2017

In the code you show the task can be preempted at any time other than a few short critical sections that are inside inside the xSempareTake() and xSemaphoreGive() functions.

Question about Preemptive Scheduler

Posted by leesp on May 24, 2017

I read the Example 1 found in FreeRTOS book: two tasks are created with SAME priority, and each task will print a different string on the console. It was mentioned that: > "...both tasks are rapidly entering and exiting the Running state...To be able to select the next task to run, the scheduler itself must execute at the end of each time slice. A periodic interrupt, called the tick interrupt, is used for this purpose"

If configTICKRATEHZ is 1000 (ie 1ms period), but each task's for(;;) loop takes 20ms for example, will the scheduler pre-empt a task every 1ms? Which means a task will be switched in and out 10 times before it complete one loop? Or, the scheduler will wait for a task to complete 1 loop, then pre-empt it?

Question about Preemptive Scheduler

Posted by richard_damon on May 24, 2017

If you have enabled both Preempton and Time Slicing, then every tick interrupt you will switch between two equal priority execution bound tasks. The tick interrupt (which is what initiates the switch, know nothing of the loop in the task.

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