Task priority assignment in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is task priority in FreeRTOS?

Task priority is a number that tells FreeRTOS which task is more important. Higher priority tasks run before lower priority ones.

beginner

How do you assign a priority to a task when creating it in FreeRTOS?

You assign priority by passing a number as the 'uxPriority' parameter in the xTaskCreate() function.

intermediate

What happens if two tasks have the same priority in FreeRTOS?

If two tasks have the same priority, FreeRTOS switches between them in a round-robin way, sharing CPU time fairly.

intermediate

Can task priority be changed after a task is created? How?

Yes, you can change a task's priority anytime using the vTaskPrioritySet() function.

advanced

Why should you avoid giving all tasks the same high priority?

Giving all tasks the same high priority can cause poor system performance because tasks will compete and may starve lower priority tasks.

In FreeRTOS, which task will run first?

A The task with the highest priority number

B The task created first

C The task with the lowest priority number

D The task with the longest name

FreeRTOS runs the task with the highest priority number first.

How do you set the priority of a task when creating it?

A By passing the priority as a parameter to xTaskCreate()

B By calling vTaskPrioritySet() before creating the task

C By naming the task with a priority number

D By setting a global variable

The priority is set by passing it as the 'uxPriority' parameter in xTaskCreate().

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

A Runs the task with the lower priority

B Runs them one after the other in round-robin fashion

C Runs the task with the longer name

D Runs only the first created task

FreeRTOS shares CPU time between tasks of equal priority using round-robin scheduling.

Which function changes a task's priority after creation?

A xTaskCreate()

B vTaskDelete()

C vTaskPrioritySet()

D xTaskResume()

vTaskPrioritySet() is used to change a task's priority at runtime.

Why is it bad to give all tasks the same high priority?

A It saves memory

B It makes the system run faster

C It disables task switching

D It can cause tasks to compete and starve others

All tasks with high priority compete for CPU, which can cause poor performance and starvation.

Explain how task priority affects task scheduling in FreeRTOS.

Think about which tasks get CPU time first and how tasks with same priority share time.

Describe how to assign and change task priority in FreeRTOS.

Consider both when creating a task and after it is running.

Practice

1.

In FreeRTOS, what does a higher task priority number mean?

easy

A. The task runs only when idle

B. The task runs after lower priority tasks

C. The task runs before lower priority tasks

D. The task is deleted automatically

Solution

1. Step 1: Understand task priority meaning

   In FreeRTOS, tasks with higher priority numbers get CPU time before lower priority tasks.

2. Step 2: Relate priority number to execution order

   A higher number means the task is more important and runs first.

3. Final Answer:

   The task runs before lower priority tasks -> Option C

4. Quick Check:

   Higher priority = runs first [OK]

Hint: Higher number means higher priority, runs first [OK]

Common Mistakes:

• Thinking lower number means higher priority
• Confusing priority with task state
• Assuming priority controls memory usage

2.

Which of the following is the correct way to assign a priority of 3 to a task in FreeRTOS using xTaskCreate?

void vTaskFunction(void *pvParameters) { /* Task code */ }

xTaskCreate(vTaskFunction, "Task1", 1000, NULL, ?, NULL);

easy

A. taskPriority = 3

B. "3"

C. priority = 3

D. 3

Solution

1. Step 1: Check xTaskCreate parameters

   The fifth parameter is the task priority and expects an integer value.

2. Step 2: Identify correct priority assignment

   Passing 3 directly as an integer is correct; strings or assignments are invalid here.

3. Final Answer:

   3 -> Option D

4. Quick Check:

   Priority parameter = integer value [OK]

Hint: Priority is an integer, no quotes or assignments [OK]

Common Mistakes:

• Using quotes around priority number
• Writing assignment expressions inside parameters
• Passing variable names without declaration

3.

Given the following FreeRTOS code snippet, which task will run first?

void TaskA(void *pvParameters) { while(1) {} }
void TaskB(void *pvParameters) { while(1) {} }

xTaskCreate(TaskA, "A", 1000, NULL, 2, NULL);
xTaskCreate(TaskB, "B", 1000, NULL, 4, NULL);

medium

A. TaskB

B. TaskA

C. Both run simultaneously

D. Neither runs

Solution

1. Step 1: Identify task priorities

   TaskA has priority 2, TaskB has priority 4.

2. Step 2: Determine which runs first

   TaskB has higher priority, so it runs before TaskA.

3. Final Answer:

   TaskB -> Option A

4. Quick Check:

   Higher priority task runs first [OK]

Hint: Higher priority number runs first [OK]

Common Mistakes:

• Assuming tasks run in creation order
• Thinking equal priority means simultaneous run
• Ignoring priority values

4.

What is wrong with this FreeRTOS task creation code snippet?

void vTask(void *pvParameters) { /* code */ }

xTaskCreate(vTask, "Task", 1000, NULL, 5, NULL);
xTaskCreate(vTask, "Task", 1000, NULL, -1, NULL);

medium

A. Priority cannot be negative

B. Task name must be unique

C. Stack size must be zero

D. Task function must return int

Solution

1. Step 1: Check priority values

   FreeRTOS priorities must be zero or positive integers; negative values are invalid.

2. Step 2: Validate other parameters

   Task name can be reused, stack size 1000 is valid, and task function returns void as required.

3. Final Answer:

   Priority cannot be negative -> Option A

4. Quick Check:

   Priority >= 0 required [OK]

Hint: Priority must be zero or positive [OK]

Common Mistakes:

• Using negative priority values
• Thinking task names must be unique
• Expecting task function to return int

5.

You have three tasks with priorities 1, 3, and 3. How should you assign priorities to ensure the most important task runs first and the least important runs last?

Choose the best priority assignment:

• A: Task1=1, Task2=3, Task3=3
• B: Task1=3, Task2=3, Task3=1
• C: Task1=3, Task2=1, Task3=3
• D: Task1=1, Task2=1, Task3=3

hard

A. Task1=1 (lowest), Task2=3, Task3=3 (highest)

B. Task1=3 (highest), Task2=3, Task3=1 (lowest)

C. Task1=3 (highest), Task2=1, Task3=3 (highest)

D. Task1=1, Task2=1, Task3=3 (highest)

Solution

1. Step 1: Identify importance order

   The most important task should have highest priority (3), least important lowest (1).

2. Step 2: Match priorities to tasks

   Task1=3 (highest), Task2=3, Task3=1 (lowest) assigns highest priority (3) to Task1 and Task2, and lowest (1) to Task3, ensuring important tasks run first.

3. Final Answer:

   Task1=3 (highest), Task2=3, Task3=1 (lowest) -> Option B

4. Quick Check:

   Highest priority = most important task [OK]

Hint: Assign highest number to most important task [OK]

Common Mistakes:

• Assigning same priority to all tasks
• Giving lowest priority to important tasks
• Ignoring task importance order