Preemptive scheduling behavior in FreeRTOS - Step-by-Step Execution

Concept Flow - Preemptive scheduling behavior

Task A running

↓

Higher priority task ready?

No→Continue Task A

Yes↓

Preempt Task A

↓

Switch to Higher Priority Task

↓

Higher Priority Task runs

↓

Higher Priority Task completes or blocks

↓

Resume Task A or next highest priority task

The scheduler checks if a higher priority task is ready. If yes, it stops the current task and switches to the higher priority one immediately.

Execution Sample

FreeRTOS

void vTaskA(void *pvParameters) {
  while(1) {
    // Task A work
  }
}

void vTaskB(void *pvParameters) {
  while(1) {
    // Higher priority task work
  }
}

Two tasks run: Task A (low priority) and Task B (high priority). When Task B becomes ready, it preempts Task A immediately.

Execution Table

Step	Current Task	Event	Scheduler Action	Running Task
1	Task A	Start system	Run Task A	Task A
2	Task A	Task B becomes ready	Preempt Task A	Task B
3	Task B	Task B runs	Continue Task B	Task B
4	Task B	Task B blocks or finishes	Resume Task A	Task A
5	Task A	No higher priority ready	Continue Task A	Task A

💡 No higher priority tasks ready, Task A continues running

Variable Tracker

Variable	Start	After Step 2	After Step 4	Final
Running Task	Task A	Task B	Task A	Task A
Task B Ready	No	Yes	No	No

Key Moments - 3 Insights

Why does Task A stop immediately when Task B becomes ready?

Because Task B has higher priority, the scheduler preempts Task A right away as shown in step 2 of the execution_table.

What happens if Task B blocks or finishes?

The scheduler resumes Task A or the next highest priority ready task, as shown in step 4.

Does Task A get to finish its current work before Task B runs?

No, preemptive scheduling interrupts Task A immediately when a higher priority task is ready, as seen in step 2.

Visual Quiz - 3 Questions

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

A Task A

B Task B

C No task running

D Both tasks running

💡 Hint

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

At which step does the scheduler preempt Task A?

A Step 4

B Step 1

C Step 2

D Step 5

💡 Hint

Look for 'Preempt Task A' in the 'Scheduler Action' column

If Task B never becomes ready, what would the running task be after step 5?

A Task A

B No task

C Task B

D Both tasks

💡 Hint

Refer to the 'Running Task' variable in variable_tracker after final step

Concept Snapshot

Preemptive scheduling in FreeRTOS:
- Scheduler always runs highest priority ready task.
- If a higher priority task becomes ready, it preempts the current task immediately.
- The preempted task is paused and resumed later.
- Ensures critical tasks run without delay.
- Key for real-time responsiveness.

Full Transcript

In FreeRTOS preemptive scheduling, the system runs the highest priority task that is ready. When a higher priority task becomes ready while a lower priority task is running, the scheduler immediately stops the lower priority task and switches to the higher priority one. This is called preemption. The higher priority task runs until it blocks or finishes, then the scheduler resumes the lower priority task. This behavior ensures that important tasks get CPU time as soon as they need it. The execution table shows this step-by-step: Task A starts running, then Task B becomes ready and preempts Task A, runs, then Task A resumes after Task B blocks. Variables track which task is running and when Task B is ready. Common confusions include why Task A stops immediately and what happens when Task B finishes. The quiz questions help check understanding of these steps.