Priority-based scheduling in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is priority-based scheduling in FreeRTOS?

Priority-based scheduling is a method where tasks with higher priority get CPU time before lower priority tasks. FreeRTOS uses this to decide which task runs next.

beginner

How does FreeRTOS handle tasks with the same priority?

FreeRTOS uses time slicing (round-robin) for tasks with the same priority, giving each task a turn to run for a fixed time slice if configUSE_TIME_SLICING is set to 1.

intermediate

What happens if a higher priority task becomes ready while a lower priority task is running?

The scheduler immediately switches to the higher priority task, pausing the lower priority one until the higher priority task blocks or finishes.

intermediate

Explain the role of configMAX_PRIORITIES in FreeRTOS.

configMAX_PRIORITIES sets the maximum number of priority levels available for tasks. It defines how many different priority levels you can assign.

advanced

Why is priority inversion a concern in priority-based scheduling?

Priority inversion happens when a low priority task holds a resource needed by a high priority task, causing the high priority task to wait and reducing system responsiveness.

In FreeRTOS, which task runs first?

A The task with the highest priority

B The task that was created first

C The task with the lowest priority

D The task with the longest runtime

FreeRTOS always runs the task with the highest priority that is ready to run.

What does FreeRTOS do when two tasks have the same priority?

A Runs only the first created task

B Runs them in a round-robin fashion

C Runs the lower priority task first

D Runs tasks randomly

FreeRTOS uses time slicing to share CPU time fairly among tasks with the same priority if configUSE_TIME_SLICING is enabled.

What is the effect of a higher priority task becoming ready while a lower priority task runs?

A The lower priority task continues until it finishes

B The higher priority task waits

C Both tasks run simultaneously

D The scheduler switches immediately to the higher priority task

FreeRTOS preempts the lower priority task to run the higher priority one immediately.

What does configMAX_PRIORITIES control in FreeRTOS?

A Maximum stack size

B Maximum number of tasks

C Maximum number of task priorities

D Maximum CPU frequency

configMAX_PRIORITIES sets how many priority levels tasks can have.

What problem does priority inversion cause?

A High priority task waits for low priority task

B Low priority task runs too fast

C Scheduler crashes

D Tasks run out of memory

Priority inversion delays high priority tasks because they wait on resources held by lower priority tasks.

Describe how FreeRTOS decides which task to run next using priority-based scheduling.

Think about how the scheduler picks tasks based on priority and what happens when priorities are equal.

Explain what priority inversion is and why it can be a problem in FreeRTOS scheduling.

Consider how a low priority task holding a resource can delay a high priority task.

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