[RTOS Support] Does the highest priority task run at all times?

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Does the highest priority task run at all times?

Posted by bupthebroker on May 9, 2016

Hi, i read the scheduler pages but am still confused about if it is always the highest priority task in FreeRTOS that is executing? Or does the scheduler yield the highest priority task to give processing time to others as well?

Regards

Does the highest priority task run at all times?

Posted by rtel on May 9, 2016

By default, configUSETIMESLICING is set to 1, and configUSE_PREEMPTION is set to 1. When this is the case, the scheduler will always run the highest priority task that is able to run. A task that is able to run is a task that is not in the Blocked or Suspended state.

Therefore, if a low priority task is running when a high priority task becomes able to run, then a switch to the higher priority task will occur immediately. Also, while a higher priority task is running (is not in the Blocked or Suspended state) all lower priority tasks will be starved of processing time - which is why tasks must enter the Blocked state.

If there is more than one task at any given priority, and that priority is the highest priority that has any tasks that are able to run, then the scheduler will time slice the tasks - each time the tick interrupt occurs it will switch to another task of equal priority.