Task priority assignment in FreeRTOS - Step-by-Step Execution

Concept Flow - Task priority assignment

Create Task

↓

Assign Priority

↓

Add to Ready List

↓

Scheduler Chooses Highest Priority Task

↓

Task Runs

↓

Task Completes or Yields

↓

Scheduler Picks Next Highest Priority Task

When a task is created, it is given a priority. The scheduler always runs the highest priority task that is ready.

Execution Sample

FreeRTOS

xTaskCreate(task1, "Task1", 1000, NULL, 2, NULL);
xTaskCreate(task2, "Task2", 1000, NULL, 3, NULL);
// Scheduler runs task2 first because priority 3 > 2

Creates two tasks with different priorities; the scheduler runs the higher priority task first.

Execution Table

Step	Action	Task1 Priority	Task2 Priority	Scheduler Decision	Running Task
1	Create Task1	2	N/A	No tasks ready yet	None
2	Create Task2	2	3	Task2 has higher priority	Task2
3	Task2 runs	2	3	Task2 running	Task2
4	Task2 yields/completes	2	3	Task1 is next highest	Task1
5	Task1 runs	2	3	Task1 running	Task1
6	Task1 completes	2	3	No tasks ready	None

💡 All tasks completed or no ready tasks left

Variable Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
Task1 Priority	N/A	2	2	2	2
Task2 Priority	N/A	N/A	3	3	3
Running Task	None	None	Task2	Task1	None

Key Moments - 2 Insights

Why does Task2 run before Task1 even though Task1 was created first?

Because Task2 has a higher priority (3) than Task1 (2), the scheduler always runs the highest priority ready task first as shown in step 2 of the execution_table.

What happens when the running task yields or completes?

The scheduler picks the next highest priority ready task to run, as seen in step 4 where Task2 yields and Task1 starts running.

Visual Quiz - 3 Questions

Look at the execution_table, which task is running at step 3?

A Task1

B Task2

C None

D Both

💡 Hint

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

At which step does the scheduler switch from Task2 to Task1?

A Step 2

B Step 3

C Step 4

D Step 5

💡 Hint

Look at the 'Scheduler Decision' and 'Running Task' columns around steps 3 to 5.

If Task1 had priority 4 instead of 2, which task would run first after creation?

A Task1

B Task2

C None

D Both at the same time

💡 Hint

Refer to the 'Task1 Priority' and 'Task2 Priority' columns in variable_tracker and execution_table.

Concept Snapshot

Task priority assignment in FreeRTOS:
- Assign priority when creating a task.
- Scheduler runs highest priority ready task.
- Higher number means higher priority.
- Tasks with same priority share CPU time.
- Yielding or completing switches to next highest priority task.

Full Transcript

In FreeRTOS, when you create a task, you assign it a priority number. The scheduler always runs the task with the highest priority that is ready to run. For example, if Task1 has priority 2 and Task2 has priority 3, Task2 will run first because 3 is higher than 2. When Task2 finishes or yields, the scheduler switches to Task1. This process continues until all tasks complete or no tasks are ready. The priority number controls which task runs first, not the order tasks are created.