Priority-based scheduling in FreeRTOS

Introduction

Priority-based scheduling helps decide which task runs first based on how important it is. This way, important tasks get done quickly.

When you have multiple tasks and some must finish before others.

When a critical task needs immediate attention over less important ones.

When you want to make sure your system responds fast to urgent events.

Syntax

FreeRTOS

BaseType_t xTaskCreate(TaskFunction_t pxTaskCode, const char * const pcName, configSTACK_DEPTH_TYPE usStackDepth, void *pvParameters, UBaseType_t uxPriority, TaskHandle_t *pxCreatedTask);

uxPriority is the task priority number; higher means more important.

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

Examples

This creates Task1 with priority 2.

FreeRTOS

xTaskCreate(Task1, "Task1", 1000, NULL, 2, NULL);

This creates Task2 with priority 5, which is higher than Task1.

FreeRTOS

xTaskCreate(Task2, "Task2", 1000, NULL, 5, NULL);

Sample Program

This program creates two tasks. The high priority task runs more often because it has a higher priority (3) than the low priority task (1). You will see the high priority task message more frequently.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include <stdio.h>

void TaskLowPriority(void *pvParameters) {
    for (;;) {
        printf("Low priority task running\n");
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void TaskHighPriority(void *pvParameters) {
    for (;;) {
        printf("High priority task running\n");
        vTaskDelay(pdMS_TO_TICKS(500));
    }
}

int main(void) {
    xTaskCreate(TaskLowPriority, "LowPriority", 1000, NULL, 1, NULL);
    xTaskCreate(TaskHighPriority, "HighPriority", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    for(;;);
    return 0;
}

Important Notes

Higher priority tasks can interrupt lower priority tasks.

If two tasks have the same priority, FreeRTOS switches between them to share CPU time.

Use priorities carefully to avoid starving lower priority tasks.

Summary

Priority-based scheduling runs the most important task first.

Set task priority when creating tasks with xTaskCreate.

Higher priority tasks run more often and can interrupt lower priority ones.

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