Priority-based scheduling in FreeRTOS - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to create a task with priority 2.

FreeRTOS

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

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

A1

B2

C3

D0

Attempts:

3 left

💡 Hint

Common Mistakes

Using 0 as priority which is the lowest priority.

Confusing stack size with priority.

The priority parameter sets the task priority. Here, 2 is the correct priority.

2fill in blank

medium

Complete the code to start the FreeRTOS scheduler.

FreeRTOS

int main(void) {
    // Setup code
    [1]();
    while(1) {}
}

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

AxTaskCreate

BvTaskSuspend

CvTaskDelay

DvTaskStartScheduler

Attempts:

3 left

💡 Hint

Common Mistakes

Calling xTaskCreate instead of starting the scheduler.

Forgetting to start the scheduler.

vTaskStartScheduler() starts the FreeRTOS scheduler to run tasks based on priority.

3fill in blank

hard

Fix the error in the task priority assignment.

FreeRTOS

xTaskCreate(taskFunction, "Task2", 1000, NULL, [1], NULL);

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

A-1

B5

CconfigMAX_PRIORITIES - 1

DconfigMAX_PRIORITIES

Attempts:

3 left

💡 Hint

Common Mistakes

Using a negative priority.

Using a priority equal to configMAX_PRIORITIES.

Task priority must be less than configMAX_PRIORITIES. The highest valid priority is configMAX_PRIORITIES - 1.

4fill in blank

hard

Fill both blanks to create a task and set its priority to 3.

FreeRTOS

xTaskCreate([1], "Task3", 1000, NULL, [2], NULL);

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

AtaskFunction3

BtaskFunction2

C1

D3

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up function names.

Using wrong priority values.

The task function is taskFunction3 and the priority is 3 as requested.

5fill in blank

hard

Fill all three blanks to create a task with priority 4 and start the scheduler.

FreeRTOS

xTaskCreate([1], "Task4", 1000, NULL, [2], NULL);
[3]();

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

AtaskFunction4

B4

CvTaskStartScheduler

DtaskFunction1

Attempts:

3 left

💡 Hint

Common Mistakes

Not starting the scheduler after creating the task.

Using incorrect function names or priorities.

The task function is taskFunction4, priority is 4, and the scheduler is started with vTaskStartScheduler().

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