Preemptive scheduling behavior in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the FreeRTOS scheduler.

FreeRTOS

vTaskStartScheduler[1];

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A()

B{}

C[]

D;

Attempts:

3 left

💡 Hint

Common Mistakes

Forgetting the parentheses and writing just the function name.

Adding curly braces instead of parentheses.

The function vTaskStartScheduler() starts the FreeRTOS scheduler and takes no parameters, so it requires parentheses ().

2fill in blank

medium

Complete the code to create a task with priority 2.

FreeRTOS

xTaskCreate(TaskFunction, "Task1", 1000, NULL, [1], NULL);

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A2

B1

C3

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using priority 0 which is the lowest priority.

Confusing the stack size with priority.

The priority parameter in xTaskCreate sets the task priority. Here, priority 2 is required.

3fill in blank

hard

Fix the error in the task delay function to yield control correctly.

FreeRTOS

vTaskDelay([1]);

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AportTICK_PERIOD_MS

B1000

C1000 * portTICK_PERIOD_MS

D1000 / portTICK_PERIOD_MS

Attempts:

3 left

💡 Hint

Common Mistakes

Passing milliseconds directly without conversion.

Multiplying instead of dividing by portTICK_PERIOD_MS.

The vTaskDelay() function expects a delay in ticks. To delay 1000 milliseconds, divide by portTICK_PERIOD_MS.

4fill in blank

hard

Fill both blanks to create a preemptive scheduler configuration.

FreeRTOS

configUSE_PREEMPTION = [1];
configMAX_PRIORITIES = [2];

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A1

B0

C5

D10

Attempts:

3 left

💡 Hint

Common Mistakes

Setting configUSE_PREEMPTION to 0 disables preemption.

Choosing too few or too many priorities.

Setting configUSE_PREEMPTION to 1 enables preemptive scheduling. configMAX_PRIORITIES defines the number of priority levels; 5 is a common choice.

5fill in blank

hard

Fill all three blanks to correctly implement a task function that yields control.

FreeRTOS

void TaskFunction(void *pvParameters) {
    for(;;) {
        // Task code here
        [1]();
        [2](10 / [3]);
    }
}

Drag options to blanks, or click blank then click option'

AvTaskDelay

BvTaskDelayUntil

CportTICK_PERIOD_MS

DtaskYIELD

Attempts:

3 left

💡 Hint

Common Mistakes

Using vTaskDelayUntil without proper tick count variable.

Forgetting to convert milliseconds to ticks.

taskYIELD() forces the task to yield control immediately. vTaskDelay() delays the task for a number of ticks, which is calculated by dividing milliseconds by portTICK_PERIOD_MS.

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