Task starvation and priority inversion in FreeRTOS - Mini Project: Build & Apply

Task Starvation and Priority Inversion in FreeRTOS

📖 Scenario: You are working on a FreeRTOS-based embedded system where multiple tasks share a resource. You want to understand how task starvation and priority inversion can happen and how to detect them.

🎯 Goal: Build a simple FreeRTOS program with three tasks of different priorities sharing a mutex. Observe how priority inversion can cause task starvation and learn how to use priority inheritance to fix it.

📋 What You'll Learn

Create three tasks: HighPriorityTask, MediumPriorityTask, and LowPriorityTask

Use a mutex called xMutex to protect a shared resource

Demonstrate priority inversion by having LowPriorityTask hold the mutex while HighPriorityTask waits

Add priority inheritance to the mutex to prevent starvation

Print messages to the console to show task execution order

💡 Why This Matters

🌍 Real World

Embedded systems often have tasks with different priorities sharing resources. Understanding priority inversion helps avoid delays in critical tasks.

💼 Career

Embedded software engineers must manage task priorities and synchronization to ensure real-time system responsiveness and reliability.

1

Create three tasks and a mutex

Create a mutex called xMutex using xSemaphoreCreateMutex(). Then create three tasks named LowPriorityTask, MediumPriorityTask, and HighPriorityTask with priorities 1, 2, and 3 respectively. Use xTaskCreate() for each task. Do not start the scheduler yet.

FreeRTOS

// Create the mutex and three tasks here
// Your code here

Hint

Use xSemaphoreCreateMutex() to create the mutex. Use xTaskCreate() to create each task with the correct priority.

2

Add mutex locking in LowPriorityTask

Modify LowPriorityTask to take the mutex xMutex using xSemaphoreTake() before printing "Low priority task running". Hold the mutex for 3000 milliseconds using vTaskDelay(), then release it with xSemaphoreGive(). Keep the other tasks unchanged.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include "semphr.h"
#include 

SemaphoreHandle_t xMutex;

void LowPriorityTask(void *pvParameters) {
    // Take the mutex, print message, hold for 3000ms, then give mutex
    // Your code here
}

void MediumPriorityTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void HighPriorityTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

int main(void) {
    xMutex = xSemaphoreCreateMutex();
    xTaskCreate(LowPriorityTask, "LowPriorityTask", 1000, NULL, 1, NULL);
    xTaskCreate(MediumPriorityTask, "MediumPriorityTask", 1000, NULL, 2, NULL);
    xTaskCreate(HighPriorityTask, "HighPriorityTask", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    for(;;);
}

Hint

Use xSemaphoreTake(xMutex, portMAX_DELAY) to wait for the mutex. Print the message, delay for 3000 ms, then release the mutex with xSemaphoreGive(xMutex).

3

Make HighPriorityTask try to take the mutex and print messages

Modify HighPriorityTask to try taking the mutex xMutex with xSemaphoreTake() using portMAX_DELAY. When it gets the mutex, print "High priority task running", then release the mutex with xSemaphoreGive(). Add a short delay of 1000 milliseconds after releasing the mutex. Keep MediumPriorityTask unchanged.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include "semphr.h"
#include 

SemaphoreHandle_t xMutex;

void LowPriorityTask(void *pvParameters) {
    for (;;) {
        if (xSemaphoreTake(xMutex, portMAX_DELAY) == pdTRUE) {
            printf("Low priority task running\n");
            vTaskDelay(pdMS_TO_TICKS(3000));
            xSemaphoreGive(xMutex);
        }
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void MediumPriorityTask(void *pvParameters) {
    for (;;) {
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void HighPriorityTask(void *pvParameters) {
    // Take the mutex, print message, give mutex, delay 1000ms
    // Your code here
}

int main(void) {
    xMutex = xSemaphoreCreateMutex();
    xTaskCreate(LowPriorityTask, "LowPriorityTask", 1000, NULL, 1, NULL);
    xTaskCreate(MediumPriorityTask, "MediumPriorityTask", 1000, NULL, 2, NULL);
    xTaskCreate(HighPriorityTask, "HighPriorityTask", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    for(;;);
}

Hint

Use xSemaphoreTake(xMutex, portMAX_DELAY) to wait for the mutex. Print the message, release the mutex, then delay 1000 ms.

4

Enable priority inheritance and observe output

Modify the mutex creation to enable priority inheritance by using xSemaphoreCreateMutex() (already done). Run the program and print the messages from all tasks to observe how priority inversion is resolved. Add print statements in MediumPriorityTask to print "Medium priority task running" every 1000 milliseconds. Finally, print the messages from all tasks to the console.

FreeRTOS

#include "FreeRTOS.h"
#include "task.h"
#include "semphr.h"
#include 

SemaphoreHandle_t xMutex;

void LowPriorityTask(void *pvParameters) {
    for (;;) {
        if (xSemaphoreTake(xMutex, portMAX_DELAY) == pdTRUE) {
            printf("Low priority task running\n");
            vTaskDelay(pdMS_TO_TICKS(3000));
            xSemaphoreGive(xMutex);
        }
        vTaskDelay(pdMS_TO_TICKS(1000));
    }
}

void MediumPriorityTask(void *pvParameters) {
    // Print "Medium priority task running" every 1000ms
    // Your code here
}

void HighPriorityTask(void *pvParameters) {
    for (;;) {
        if (xSemaphoreTake(xMutex, portMAX_DELAY) == pdTRUE) {
            printf("High priority task running\n");
            xSemaphoreGive(xMutex);
            vTaskDelay(pdMS_TO_TICKS(1000));
        }
    }
}

int main(void) {
    xMutex = xSemaphoreCreateMutex();
    xTaskCreate(LowPriorityTask, "LowPriorityTask", 1000, NULL, 1, NULL);
    xTaskCreate(MediumPriorityTask, "MediumPriorityTask", 1000, NULL, 2, NULL);
    xTaskCreate(HighPriorityTask, "HighPriorityTask", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    for(;;);
}

Hint

In MediumPriorityTask, add a print statement inside the infinite loop and delay 1000 ms each time.

Practice

1. What does task starvation mean in FreeRTOS?

easy

A. Some tasks never get CPU time because others run too much

B. All tasks get equal CPU time regardless of priority

C. Tasks run in a random order without priority

D. Tasks share CPU time only when a mutex is free

Solution

1. Step 1: Understand task scheduling in FreeRTOS

   FreeRTOS schedules tasks based on priority, so higher priority tasks run more often.

2. Step 2: Define task starvation

   Task starvation happens when low priority tasks never get CPU time because higher priority tasks keep running.

3. Final Answer:

   Some tasks never get CPU time because others run too much -> Option A

4. Quick Check:

   Task starvation = some tasks starve for CPU [OK]

Hint: Starvation means some tasks get no CPU time [OK]

Common Mistakes:

• Confusing starvation with equal CPU sharing
• Thinking tasks run randomly
• Assuming mutex controls CPU time directly

2. Which FreeRTOS API is used to create a mutex that supports priority inheritance?

easy

A. xQueueCreate()

B. xSemaphoreCreateMutex()

C. vTaskDelay()

D. xTaskCreate()

Solution

1. Step 1: Identify mutex creation functions

   FreeRTOS uses xSemaphoreCreateMutex() to create mutexes with priority inheritance.

2. Step 2: Differentiate other APIs

   xQueueCreate() creates queues, vTaskDelay() delays tasks, and xTaskCreate() creates tasks, not mutexes.

3. Final Answer:

   xSemaphoreCreateMutex() -> Option B

4. Quick Check:

   Mutex with priority inheritance = xSemaphoreCreateMutex() [OK]

Hint: Mutex with priority inheritance uses xSemaphoreCreateMutex() [OK]

Common Mistakes:

• Choosing xQueueCreate() for mutex
• Confusing task creation with mutex creation
• Using delay functions instead of mutex APIs

3. Given three tasks with priorities High (3), Medium (2), and Low (1). Low task holds a mutex needed by High task. What happens without priority inheritance?

medium

A. Medium task is blocked until High task finishes

B. Low task immediately releases the mutex

C. High task waits indefinitely while Medium task runs

D. High task preempts Low task and runs

Solution

1. Step 1: Understand priority inversion scenario

   Low priority task holds mutex needed by High priority task, so High task blocks waiting.

2. Step 2: Effect without priority inheritance

   Medium priority task runs, preventing Low task from running and releasing mutex, causing High task to wait indefinitely.

3. Final Answer:

   High task waits indefinitely while Medium task runs -> Option C

4. Quick Check:

   Priority inversion without inheritance = High task blocked [OK]

Hint: Without inheritance, medium task blocks low task, blocking high task [OK]

Common Mistakes:

• Assuming low task releases mutex immediately
• Thinking high task preempts low task despite mutex
• Believing medium task is blocked

4. Identify the problem in this FreeRTOS code snippet causing priority inversion:

void lowTask(void *pvParameters) {
  xSemaphoreTake(mutex, portMAX_DELAY);
  // long processing
  xSemaphoreGive(mutex);
  vTaskDelete(NULL);
}

void highTask(void *pvParameters) {
  xSemaphoreTake(mutex, portMAX_DELAY);
  // quick processing
  xSemaphoreGive(mutex);
  vTaskDelete(NULL);
}

medium

A. Mutex created without priority inheritance support

B. High task does not take the mutex

C. Low task never releases the mutex

D. Tasks have the same priority

Solution

1. Step 1: Analyze mutex usage in code

   Both tasks take and give the mutex correctly, so no misuse in code logic.

2. Step 2: Check for priority inversion cause

   If mutex was created without priority inheritance, low task holding mutex blocks high task causing priority inversion.

3. Final Answer:

   Mutex created without priority inheritance support -> Option A

4. Quick Check:

   Priority inversion fix needs mutex with priority inheritance [OK]

Hint: Check if mutex supports priority inheritance to fix inversion [OK]

Common Mistakes:

• Assuming high task does not take mutex
• Thinking low task never releases mutex
• Believing tasks have same priority

5. How can you prevent task starvation and priority inversion in a FreeRTOS system with multiple tasks sharing resources?

hard

A. Use busy-wait loops in low priority tasks to keep CPU busy

B. Assign all tasks the same priority to avoid conflicts

C. Avoid using any synchronization primitives like mutexes

D. Use mutexes with priority inheritance and design balanced task priorities

Solution

1. Step 1: Understand causes of starvation and inversion

   Starvation happens when low priority tasks never run; inversion when low priority blocks high priority.

2. Step 2: Apply solutions in FreeRTOS

   Using mutexes with priority inheritance prevents inversion; balanced priorities avoid starvation.

3. Final Answer:

   Use mutexes with priority inheritance and design balanced task priorities -> Option D

4. Quick Check:

   Priority inheritance + balanced priorities prevent issues [OK]

Hint: Combine priority inheritance mutexes with balanced priorities [OK]

Common Mistakes:

• Setting all tasks to same priority causing inefficiency
• Avoiding mutexes causing race conditions
• Using busy-wait wastes CPU and worsens starvation