Preemptive scheduling behavior in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Preemptive Scheduling: Task Priority Impact

Consider two FreeRTOS tasks: TaskA with priority 2 and TaskB with priority 3. Both tasks are ready to run at the same time. TaskA starts running first. What happens next?

FreeRTOS

void TaskA(void *pvParameters) {
    for (;;) {
        // TaskA code
    }
}

void TaskB(void *pvParameters) {
    for (;;) {
        // TaskB code
    }
}

int main() {
    xTaskCreate(TaskA, "TaskA", 1000, NULL, 2, NULL);
    xTaskCreate(TaskB, "TaskB", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    return 0;
}

A TaskB immediately preempts TaskA and starts running because it has higher priority.

B TaskA and TaskB run in a round-robin fashion regardless of priority.

C TaskA continues running until it blocks or yields, then TaskB runs.

D Both tasks run simultaneously on separate cores.

💡 Hint

Remember, in preemptive scheduling, the highest priority ready task runs immediately.

FreeRTOS uses preemptive scheduling. When a higher priority task becomes ready, it preempts the currently running lower priority task immediately.

❓ Predict Output

intermediate

Effect of vTaskDelay on Preemptive Scheduling

In FreeRTOS, if a running task calls vTaskDelay(), what happens to the CPU scheduling?

FreeRTOS

void TaskExample(void *pvParameters) {
    for (;;) {
        // Do some work
        vTaskDelay(pdMS_TO_TICKS(100));
    }
}

A The scheduler stops all tasks until the delay expires.

B The task yields but remains ready to run immediately after delay.

C The task continues running without interruption.

D The task blocks and the scheduler runs the highest priority ready task.

💡 Hint

Think about what happens when a task delays itself in FreeRTOS.

Calling vTaskDelay() blocks the task for the specified time, allowing the scheduler to run other ready tasks.

❓ Predict Output

advanced

Preemptive Scheduling with Equal Priority Tasks

Two tasks, Task1 and Task2, have the same priority in FreeRTOS. Both are ready to run. What is the expected behavior?

FreeRTOS

void Task1(void *pvParameters) {
    for (;;) {
        // Task1 code
    }
}

void Task2(void *pvParameters) {
    for (;;) {
        // Task2 code
    }
}

int main() {
    xTaskCreate(Task1, "Task1", 1000, NULL, 2, NULL);
    xTaskCreate(Task2, "Task2", 1000, NULL, 2, NULL);
    vTaskStartScheduler();
    return 0;
}

A Only Task1 runs because it was created first.

B The scheduler randomly picks one task to run forever.

C Tasks run in strict alternation using time slicing if enabled.

D Both tasks run simultaneously on different cores.

💡 Hint

Consider how FreeRTOS handles tasks with the same priority.

FreeRTOS uses time slicing (if configUSE_TIME_SLICING is enabled) to share CPU time fairly among tasks of equal priority.

❓ Predict Output

advanced

Preemption Behavior with Interrupts and Task Priorities

In FreeRTOS, an interrupt service routine (ISR) makes a higher priority task ready to run. What happens immediately after the ISR completes?

FreeRTOS

void vAnISR(void) {
    BaseType_t xHigherPriorityTaskWoken = pdFALSE;
    xTaskNotifyFromISR(xHighPriorityTaskHandle, 0, eNoAction, &xHigherPriorityTaskWoken);
    portYIELD_FROM_ISR(xHigherPriorityTaskWoken);
}

A The higher priority task preempts the currently running task immediately after ISR.

B The scheduler ignores the higher priority task until the next tick.

C The ISR blocks the higher priority task from running until manually resumed.

D The higher priority task runs only if the current task yields voluntarily.

💡 Hint

Think about how FreeRTOS handles context switches triggered by ISRs.

Using portYIELD_FROM_ISR() causes an immediate context switch to the higher priority task after the ISR ends.

🧠 Conceptual

expert

Preemptive Scheduling and Priority Inversion Scenario

In a FreeRTOS system, TaskLow (priority 1) holds a mutex needed by TaskHigh (priority 3). TaskMedium (priority 2) is ready to run. What problem can occur and how does FreeRTOS help prevent it?

A No problem occurs; TaskHigh preempts all tasks immediately.

B Priority inversion occurs; FreeRTOS uses priority inheritance to temporarily raise TaskLow's priority.

C TaskMedium blocks TaskHigh indefinitely; FreeRTOS disables preemption to fix this.

D TaskLow is forced to release the mutex immediately; FreeRTOS kills TaskMedium.

💡 Hint

Think about what happens when a low priority task blocks a high priority task and a medium priority task runs.

Priority inversion happens when a low priority task holds a resource needed by a high priority task. FreeRTOS uses priority inheritance to raise the low priority task's priority temporarily to prevent medium priority tasks from blocking progress.

Practice

1. What happens in FreeRTOS when a higher priority task becomes ready while a lower priority task is running under preemptive scheduling?

easy

A. The scheduler ignores the higher priority task until the lower priority task yields.

B. The lower priority task continues running until it finishes.

C. Both tasks run at the same time.

D. The higher priority task immediately preempts the lower priority task and starts running.

Solution

1. Step 1: Understand preemptive scheduling behavior

   In preemptive scheduling, the system always runs the highest priority task that is ready.

2. Step 2: Apply to the scenario

   When a higher priority task becomes ready, it interrupts the lower priority task immediately.

3. Final Answer:

   The higher priority task immediately preempts the lower priority task and starts running. -> Option D

4. Quick Check:

   Preemptive scheduling = immediate switch to higher priority task [OK]

Hint: Higher priority tasks always interrupt lower ones immediately [OK]

Common Mistakes:

• Thinking tasks run simultaneously on a single core
• Believing lower priority tasks finish before switching
• Confusing preemptive with cooperative scheduling

2. Which of the following is the correct way to enable preemptive scheduling in FreeRTOS configuration?

easy

A. #define configUSE_PREEMPTION 1

B. #define configENABLE_PREEMPTION true

C. #define configPREEMPTIVE_SCHEDULING 1

D. #define configUSE_PREEMPTION 0

Solution

1. Step 1: Recall FreeRTOS config option for preemption

   The correct macro to enable preemption is configUSE_PREEMPTION.

2. Step 2: Identify correct value to enable preemption

   Setting configUSE_PREEMPTION to 1 enables preemptive scheduling.

3. Final Answer:

   #define configUSE_PREEMPTION 1 -> Option A

4. Quick Check:

   configUSE_PREEMPTION = 1 enables preemption [OK]

Hint: Enable preemption by setting configUSE_PREEMPTION to 1 [OK]

Common Mistakes:

• Using 0 instead of 1 to enable preemption
• Using wrong macro names
• Using boolean true instead of 1

3. Consider two tasks in FreeRTOS: TaskA with priority 2 and TaskB with priority 3. TaskA is running when TaskB becomes ready. What will be the output if both tasks print their names when running?

medium

A. TaskA prints first, then TaskB prints after TaskA finishes.

B. TaskB prints immediately, preempting TaskA.

C. Both tasks print simultaneously.

D. No output because tasks block each other.

Solution

1. Step 1: Identify task priorities and running state

   TaskA has priority 2 and is running; TaskB has higher priority 3 and becomes ready.

2. Step 2: Apply preemptive scheduling rules

   TaskB will preempt TaskA immediately and run first, printing its name.

3. Final Answer:

   TaskB prints immediately, preempting TaskA. -> Option B

4. Quick Check:

   Higher priority task runs first = TaskB prints first [OK]

Hint: Higher priority task prints first by preempting lower priority [OK]

Common Mistakes:

• Assuming lower priority task finishes before switching
• Thinking tasks run at the same time
• Ignoring task priorities

4. Given this FreeRTOS snippet, what is the error that prevents preemptive scheduling from working?

#define configUSE_PREEMPTION 0

void Task1(void *pvParameters) {
  while(1) {
    // Task code
  }
}

void Task2(void *pvParameters) {
  while(1) {
    // Task code
  }
}

medium

A. configUSE_PREEMPTION is set to 0, disabling preemption.

B. Tasks are missing priority assignments.

C. Task functions must return int, not void.

D. Tasks need to call vTaskDelay to allow switching.

Solution

1. Step 1: Check configUSE_PREEMPTION value

   The macro is set to 0, which disables preemptive scheduling.

2. Step 2: Understand effect on scheduling

   With preemption disabled, tasks will not be interrupted automatically by higher priority tasks.

3. Final Answer:

   configUSE_PREEMPTION is set to 0, disabling preemption. -> Option A

4. Quick Check:

   configUSE_PREEMPTION = 0 disables preemption [OK]

Hint: Preemption disabled if configUSE_PREEMPTION = 0 [OK]

Common Mistakes:

• Thinking tasks must return int
• Assuming missing priorities cause preemption failure
• Believing vTaskDelay is required for preemption

5. You have three tasks: TaskX (priority 2), TaskY (priority 3), and TaskZ (priority 3). TaskX is running, then both TaskY and TaskZ become ready at the same time. Which task will FreeRTOS run first under preemptive scheduling, and why?

hard

A. TaskY runs first because it was created before TaskZ.

B. TaskZ runs first because it has the same priority as TaskY but higher than TaskX.

C. Either TaskY or TaskZ runs first; FreeRTOS uses time slicing between equal priorities.

D. TaskX continues running until it yields.

Solution

1. Step 1: Identify task priorities and ready state

   TaskX priority 2 is running; TaskY and TaskZ both have priority 3 and become ready simultaneously.

2. Step 2: Understand FreeRTOS behavior with equal priority tasks

   FreeRTOS uses time slicing (round-robin) between tasks of equal highest priority.

3. Step 3: Determine which task runs first

   Either TaskY or TaskZ can run first; the scheduler switches between them fairly.

4. Final Answer:

   Either TaskY or TaskZ runs first; FreeRTOS uses time slicing between equal priorities. -> Option C

5. Quick Check:

   Equal priority tasks share CPU time via time slicing [OK]

Hint: Equal priority tasks share CPU time by time slicing [OK]

Common Mistakes:

• Assuming creation order decides first run
• Thinking lower priority task continues running
• Believing one equal priority task always runs first