Priority numbering in FreeRTOS - Step-by-Step Execution

Concept Flow - Priority numbering in FreeRTOS

Define task priorities

↓

Assign priority numbers

↓

Scheduler compares priorities

↓

Highest priority task runs

↓

Lower priority tasks wait

↓

If higher priority task blocks

↓

Next highest priority task runs

Tasks are given priority numbers; the scheduler runs the highest priority task ready to run, lower priority tasks wait unless higher priority tasks block.

Execution Sample

FreeRTOS

xTaskCreate(task1, "Task1", 1000, NULL, 3, NULL);
xTaskCreate(task2, "Task2", 1000, NULL, 1, NULL);
xTaskCreate(task3, "Task3", 1000, NULL, 5, NULL);

Creates three tasks with priorities 3, 1, and 5; the scheduler runs the highest priority task first.

Execution Table

Step	Task Created	Priority Assigned	Scheduler Decision	Running Task
1	Task1	3	Task1 is highest priority so far	Task1
2	Task2	1	Task1 (3) > Task2 (1), Task1 runs	Task1
3	Task3	5	Task3 (5) > Task1 (3), Task3 runs	Task3
4	Task3 blocks or delays	N/A	Next highest priority ready task is Task1 (3)	Task1
5	Task1 blocks or delays	N/A	Next highest priority ready task is Task2 (1)	Task2
6	All tasks blocked	N/A	Idle task runs	Idle Task

💡 Execution stops when all tasks are blocked or system is idle

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5	Final
Task1 Priority	N/A	3	3	3	3	3	3
Task2 Priority	N/A	N/A	1	1	1	1	1
Task3 Priority	N/A	N/A	N/A	5	5	5	5
Running Task	Idle Task	Task1	Task1	Task3	Task1	Task2	Idle Task

Key Moments - 3 Insights

Why does Task3 run instead of Task1 after it is created?

Because Task3 has a higher priority number (5) than Task1 (3), the scheduler runs the highest priority ready task as shown in step 3 of the execution_table.

What happens if the highest priority task blocks or delays?

The scheduler runs the next highest priority task that is ready, as shown in steps 4 and 5 where Task3 or Task1 blocks and the scheduler switches to the next ready task.

Is a higher priority number better or worse in FreeRTOS?

A higher priority number means higher priority and the task will run before lower priority tasks, as demonstrated by Task3 with priority 5 running before Task1 with priority 3.

Visual Quiz - 3 Questions

Look at the execution_table at step 3, which task runs?

A Task2

B Task1

C Task3

D Idle Task

💡 Hint

Check the 'Running Task' column at step 3 in the execution_table

At which step does the scheduler run Task2?

A Step 4

B Step 5

C Step 2

D Step 6

💡 Hint

Look at the 'Running Task' column in the execution_table for when Task2 runs

If Task3 had priority 2 instead of 5, which task would run at step 3?

A Task1

B Task3

C Task2

D Idle Task

💡 Hint

Compare priorities in variable_tracker and see which is highest at step 3

Concept Snapshot

FreeRTOS uses numeric priorities for tasks.
Higher number means higher priority.
Scheduler runs highest priority ready task.
If highest priority blocks, next highest runs.
Idle task runs if no tasks ready.

Full Transcript

In FreeRTOS, each task is assigned a numeric priority. The scheduler always runs the task with the highest priority number that is ready to run. When a new task is created with a higher priority than the currently running task, the scheduler switches to the new higher priority task immediately. If the highest priority task blocks or delays, the scheduler runs the next highest priority task that is ready. If no tasks are ready, the idle task runs. This priority numbering system ensures that important tasks get CPU time before less important ones.

Practice

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

easy

A. The task runs before lower priority tasks

B. The task runs after lower priority tasks

C. The task runs at the same time as lower priority tasks

D. The task is ignored by the scheduler

Solution

1. Step 1: Understand priority numbering

   In FreeRTOS, priority numbers start at 0 for the lowest priority and increase for higher priorities.

2. Step 2: Relate priority to task execution

   Higher priority tasks are scheduled to run before lower priority tasks to ensure responsiveness.

3. Final Answer:

   The task runs before lower priority tasks -> Option A

4. Quick Check:

   Higher priority number = runs first [OK]

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

Common Mistakes:

• Thinking higher number means lower priority
• Assuming tasks run simultaneously regardless of priority
• Confusing priority with task creation order

2. Which of the following is the correct way to define the highest priority in FreeRTOS if configMAX_PRIORITIES is 5?

easy

A. 5

B. 6

C. 4

D. 0

Solution

1. Step 1: Understand priority range

   Priority numbers start at 0 and go up to configMAX_PRIORITIES - 1.

2. Step 2: Calculate highest priority

   With configMAX_PRIORITIES = 5, highest priority is 5 - 1 = 4.

3. Final Answer:

   4 -> Option C

4. Quick Check:

   Highest priority = configMAX_PRIORITIES - 1 = 4 [OK]

Hint: Highest priority = configMAX_PRIORITIES minus one [OK]

Common Mistakes:

• Using configMAX_PRIORITIES as highest priority directly
• Using 0 as highest priority
• Choosing a number outside the allowed range

3. Given three tasks with priorities 2, 4, and 1, which task will FreeRTOS schedule to run first?

medium

A. Task with priority 4

B. Task with priority 2

C. Task with priority 1

D. All run simultaneously

Solution

1. Step 1: Identify task priorities

   Tasks have priorities 2, 4, and 1 respectively.

2. Step 2: Determine highest priority

   Priority 4 is the highest number, so that task runs first.

3. Final Answer:

   Task with priority 4 -> Option A

4. Quick Check:

   Highest priority runs first = 4 [OK]

Hint: Task with highest priority number runs first [OK]

Common Mistakes:

• Choosing the lowest priority task
• Assuming tasks run in creation order
• Thinking tasks run simultaneously

4. What is wrong with this priority assignment if configMAX_PRIORITIES is 3?
vTaskPrioritySet(taskHandle, 3);

medium

A. Priority 3 is valid and highest

B. Priority 3 is out of range; max is 2

C. Priority must be zero

D. Priority must be negative

Solution

1. Step 1: Check allowed priority range

   Priorities go from 0 to configMAX_PRIORITIES - 1, so 0 to 2 here.

2. Step 2: Validate assigned priority

   Priority 3 is outside this range and invalid.

3. Final Answer:

   Priority 3 is out of range; max is 2 -> Option B

4. Quick Check:

   Max priority = 2, 3 is invalid [OK]

Hint: Priority must be less than configMAX_PRIORITIES [OK]

Common Mistakes:

• Using configMAX_PRIORITIES as a valid priority
• Assigning negative priorities
• Assuming priority zero is highest

5. You have tasks with priorities 0, 1, and 2. You want the system to always run the task with priority 1 before the others, but currently priority 2 runs first. How can you fix this using FreeRTOS priority numbering?

hard

A. Set all tasks to priority 0

B. Set the task currently at 1 to priority 0

C. Set the task currently at 2 to priority 0

D. Set the priority of the task currently at 1 to 3 and others lower

Solution

1. Step 1: Understand current priorities

   Task with priority 2 runs first because it has highest priority.

2. Step 2: Adjust priorities to reorder execution

   Increase the priority of the task currently at 1 to a number higher than 2 (e.g., 3) so it runs first.

3. Step 3: Lower other tasks' priorities

   Set other tasks to lower priorities to ensure the desired task runs first.

4. Final Answer:

   Set the priority of the task currently at 1 to 3 and others lower -> Option D

5. Quick Check:

   Higher priority number runs first, so raise desired task's priority [OK]

Hint: Raise desired task priority above others to run first [OK]

Common Mistakes:

• Lowering all priorities to zero
• Not increasing the desired task's priority
• Assuming priority 0 is highest