Priority-based scheduling in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of task execution order with different priorities

Consider three FreeRTOS tasks with priorities 2, 3, and 1 respectively. Each task prints its name once when it runs. What is the order of printed task names?

FreeRTOS

void Task1(void *pvParameters) {
    printf("Task1\n");
    vTaskDelete(NULL);
}
void Task2(void *pvParameters) {
    printf("Task2\n");
    vTaskDelete(NULL);
}
void Task3(void *pvParameters) {
    printf("Task3\n");
    vTaskDelete(NULL);
}

int main() {
    xTaskCreate(Task1, "Task1", 1000, NULL, 2, NULL);
    xTaskCreate(Task2, "Task2", 1000, NULL, 3, NULL);
    xTaskCreate(Task3, "Task3", 1000, NULL, 1, NULL);
    vTaskStartScheduler();
    return 0;
}

A Task1\nTask3\nTask2\n

B Task2\nTask1\nTask3\n

C Task3\nTask1\nTask2\n

D Task1\nTask2\nTask3\n

💡 Hint

Remember, FreeRTOS runs the highest priority task that is ready.

Task2 has the highest priority (3), so it runs first. Then Task1 (priority 2), then Task3 (priority 1).

❓ Predict Output

intermediate

Output when two tasks have the same priority

Two FreeRTOS tasks have the same priority and both are ready to run. Each prints its name once. What is the possible output order?

FreeRTOS

void TaskA(void *pvParameters) {
    printf("TaskA\n");
    vTaskDelete(NULL);
}
void TaskB(void *pvParameters) {
    printf("TaskB\n");
    vTaskDelete(NULL);
}

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

A No output, deadlock

B TaskA\nTaskB\n

C TaskB\nTaskA\n

D Either TaskA\nTaskB\n or TaskB\nTaskA\n

💡 Hint

Tasks with the same priority are scheduled round-robin.

When tasks have the same priority, FreeRTOS time-slices between them, so either task can run first.

🔧 Debug

advanced

Identify the cause of priority inversion

Given three tasks with priorities 3 (high), 2 (medium), and 1 (low). The low priority task holds a mutex needed by the high priority task. The medium priority task runs continuously. What problem occurs and why?

FreeRTOS

/*
TaskLow (priority 1) locks mutex and starts long processing
TaskHigh (priority 3) tries to lock mutex and blocks
TaskMedium (priority 2) runs continuously
*/

A No problem, TaskHigh preempts others immediately

B Deadlock because TaskHigh and TaskLow wait on each other

C Priority inversion because TaskMedium prevents TaskLow from running and releasing mutex

D Starvation of TaskMedium because TaskHigh blocks it

💡 Hint

Think about how medium priority tasks affect low priority tasks holding resources needed by high priority tasks.

Priority inversion happens because the medium priority task keeps running, preventing the low priority task from releasing the mutex needed by the high priority task.

📝 Syntax

advanced

Identify the syntax error in task creation

Which option contains the correct syntax to create a FreeRTOS task with priority 4?

A xTaskCreate(TaskFunc, "Task", 1000, NULL, 4, NULL);

B xTaskCreate(TaskFunc, Task, 1000, NULL, 4, NULL);

C xTaskCreate(TaskFunc, "Task", 1000, NULL, "4", NULL);

D xTaskCreate(TaskFunc, "Task", 1000, NULL, 4);

💡 Hint

Check the parameter types and count for xTaskCreate.

Option A uses correct string for task name and integer for priority with all required parameters.

🚀 Application

expert

Determine the number of context switches in priority scheduling

In a FreeRTOS system, three tasks with priorities 3, 2, and 1 are ready. The highest priority task runs for 5 ticks, then blocks. The next highest runs for 3 ticks, then blocks. The lowest priority task runs for 4 ticks. How many context switches occur in total?

A 4

B 3

C 5

D 6

💡 Hint

Count switches when tasks block and scheduler switches to next ready task.

Initially, highest priority runs (no switch). When it blocks, switch to medium priority (1). When medium blocks, switch to low priority (2). When low finishes, no more tasks, so total 2 switches. But starting scheduler counts as first switch (0 to highest), total 3. Also, when tasks delete themselves, scheduler switches back to idle task (3). Total 4 context switches.

Practice

1.

What does priority-based scheduling in FreeRTOS ensure?

easy

A. Tasks run randomly without any priority

B. Tasks run in the order they were created

C. The highest priority task runs first

D. All tasks run equally regardless of priority

Solution

1. Step 1: Understand priority-based scheduling

   Priority-based scheduling means tasks with higher priority get CPU time before lower priority tasks.

2. Step 2: Apply this to FreeRTOS task execution

   FreeRTOS runs the highest priority task that is ready to run first, preempting lower priority tasks.

3. Final Answer:

   The highest priority task runs first -> Option C

4. Quick Check:

   Priority-based scheduling = highest priority runs first [OK]

Hint: Highest priority task always runs first in FreeRTOS [OK]

Common Mistakes:

• Thinking tasks run in creation order
• Assuming equal CPU time for all tasks
• Believing tasks run randomly

2.

Which of the following is the correct way to set a task priority when creating a task in FreeRTOS?

xTaskCreate(TaskFunction, "TaskName", 1000, NULL, ___, &TaskHandle);

easy

A. task name

B. stack size

C. task handle

D. priority

Solution

1. Step 1: Identify parameters of xTaskCreate

   The fifth parameter in xTaskCreate is the task priority.

2. Step 2: Match parameter to correct argument

   Passing 'priority' sets the task's priority level correctly.

3. Final Answer:

   priority -> Option D

4. Quick Check:

   Fifth parameter in xTaskCreate = priority [OK]

Hint: Priority is the fifth argument in xTaskCreate [OK]

Common Mistakes:

• Confusing stack size with priority
• Using task handle as priority
• Passing task name instead of priority

3.

Given the following FreeRTOS tasks with priorities, which task will run first?

Task A priority = 2
Task B priority = 3
Task C priority = 1

medium

A. Task A

B. Task B

C. Task C

D. All run simultaneously

Solution

1. Step 1: Compare task priorities

   Task B has priority 3, which is higher than Task A (2) and Task C (1).

2. Step 2: Determine which task runs first

   FreeRTOS runs the highest priority task first, so Task B runs before others.

3. Final Answer:

   Task B -> Option B

4. Quick Check:

   Highest priority (3) = Task B [OK]

Hint: Highest number priority runs first in FreeRTOS [OK]

Common Mistakes:

• Choosing task with lowest priority
• Assuming tasks run simultaneously
• Ignoring priority values

4.

Identify the error in this FreeRTOS task creation code snippet:

BaseType_t result = xTaskCreate(TaskFunc, "Task1", 500, NULL, 0, NULL);
if(result != pdPASS) {
    // Handle error
}

What is wrong with the priority value?

medium

A. Priority 0 means lowest priority but is valid

B. Priority 0 is invalid and causes error

C. Priority must be greater than 5

D. Priority should be a negative number

Solution

1. Step 1: Understand priority 0 in FreeRTOS

   Priority 0 is the lowest priority and is valid in FreeRTOS.

2. Step 2: Check if priority 0 causes error

   Priority 0 does not cause an error; task creation can succeed with it.

3. Final Answer:

   Priority 0 means lowest priority but is valid -> Option A

4. Quick Check:

   Priority 0 is valid lowest priority [OK]

Hint: Priority 0 is valid and lowest priority in FreeRTOS [OK]

Common Mistakes:

• Thinking priority 0 is invalid
• Assuming priority must be > 5
• Using negative priority values

5.

You have three tasks with priorities 2, 4, and 3. Task with priority 4 is blocked waiting for an event. Which task will FreeRTOS schedule next?

hard

A. Task with priority 3

B. Task with priority 2

C. Task with priority 4

D. No task runs until priority 4 task is ready

Solution

1. Step 1: Understand task states and scheduling

   FreeRTOS schedules the highest priority task that is ready (not blocked).

2. Step 2: Identify ready tasks

   Task with priority 4 is blocked, so it cannot run now. Tasks with priority 3 and 2 are ready.

3. Step 3: Choose highest priority ready task

   Between priority 3 and 2, priority 3 is higher, so it runs next.

4. Final Answer:

   Task with priority 3 -> Option A

5. Quick Check:

   Highest ready priority = 3 [OK]

Hint: Blocked tasks don't run; next highest ready priority runs [OK]

Common Mistakes:

• Trying to run blocked task
• Ignoring task states
• Assuming no task runs if highest priority blocked