FreeRtos Context Switch

Question

If higher priority task is waiting for an event and lower priority task is running. Before tick interrupt higher priority task gets ready then how context switch will happen before tick interrupt? Till tick interrupt higher priority task will be waiting for tick interrupt to context switch?

I need to know that in freertos context switch happen only at tick interrupt? If it is not then how higher priority task preempt the lower priority task before tick interrupt, if it get ready before tick interrupt?

Answer 1

If configUSE_PREEMPTION is set to 1 in FreeRTOSConfig.h then FreeRTOS will always run the highest priority task that is able to run even if a task becomes available to run in between tick interrupts. This happens because sending an event that unblocks a task requires you to call a FreeRTOS API function, and the context switch happens in the API function. I would recommend reading the free book to learn more about the scheduling policy.

(repost of response already provided here https://forums.freertos.org/t/freertos-context-switch/8333)

Answer 2

To clarify your question and provide a more complete anwser, let's name things first. You mentioned you have two tasks - let's call them HighPriorityTask and LowPriorityTask. In such scenario, LowPriorityTask will only ever run if HighPriorityTask is blocked, which happens when either of the following is done:

• HighPriorityTask calls vTaskDelay function or similar that explicitly stops its execution for given amount of time or forever e.g. with vTaskSuspend,
• HighPriorityTask calls xQueueReceive or similar with non-zero blocking time and there's no elements in the queue,
• HighPriorityTask calls xQueueSend function or similar with non-zero blocking time and there's no free space left in the queue.

If neither of the above or similar cases happen that cause HighPriorityTask to be blocked, then HighPriorityTask will continue to run forever. That includes task preemption (which I assume you refer to as "tick interrupt") which will not cause LowPriorityTask to run because both tasks are not the same priority and only tasks with same priority are given their time slices with preemption. This even includes calling taskYIELD from HighPriorityTask which explicitly causes FreeRTOS to perform a context switch, but in this case all it's going to do is to go back to executing HighPriorityTask code, as this is still the highest non-blocked task currently executing.

Therefore to summarize - if you have two different priority tasks in a "running" state, then the lower priority task will never run (task preemption won't assign any time to lower priority task) unless higher priority task explicitly blocks.