Time-slicing for equal priority tasks in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of time-slicing with two equal priority tasks

Consider two FreeRTOS tasks with the same priority. Each task toggles an LED and then calls vTaskDelay(1). What is the expected behavior of the LED toggling?

FreeRTOS

void Task1(void *pvParameters) {
    for (;;) {
        ToggleLED1();
        vTaskDelay(1);
    }
}

void Task2(void *pvParameters) {
    for (;;) {
        ToggleLED2();
        vTaskDelay(1);
    }
}

int main(void) {
    xTaskCreate(Task1, "Task1", 1000, NULL, 2, NULL);
    xTaskCreate(Task2, "Task2", 1000, NULL, 2, NULL);
    vTaskStartScheduler();
    for(;;);
}

A Both LEDs toggle simultaneously at the same time.

B Only LED1 toggles because Task1 always runs first and blocks Task2.

C Both LEDs toggle alternately with roughly equal frequency due to time-slicing.

D Neither LED toggles because tasks block each other causing deadlock.

💡 Hint

Think about how FreeRTOS scheduler switches between tasks of equal priority when they call vTaskDelay().

When two tasks have the same priority and both call vTaskDelay(1), the scheduler time-slices between them. This causes the LEDs to toggle alternately with roughly equal frequency.

🧠 Conceptual

intermediate

Effect of preemption on equal priority tasks

In FreeRTOS, if two tasks have the same priority and preemption is enabled, what happens when one task becomes ready while the other is running?

A The currently running task is immediately preempted and the new task runs.

B The currently running task continues until it blocks or yields, then the new task runs.

C Both tasks run simultaneously on separate cores.

D The scheduler ignores the new task until the running task finishes.

💡 Hint

Consider how FreeRTOS handles tasks of equal priority with preemption enabled.

With preemption enabled, FreeRTOS does not preempt a running task for another task of the same priority immediately. The running task continues until it blocks, yields, or its time slice expires.

🔧 Debug

advanced

Identify the cause of starvation in equal priority tasks

Two tasks with equal priority are created. Task1 runs an infinite loop without any blocking calls. Task2 never runs. What is the cause?

FreeRTOS

void Task1(void *pvParameters) {
    for (;;) {
        // No blocking or yielding
    }
}

void Task2(void *pvParameters) {
    for (;;) {
        ToggleLED();
        vTaskDelay(10);
    }
}

int main(void) {
    xTaskCreate(Task1, "Task1", 1000, NULL, 3, NULL);
    xTaskCreate(Task2, "Task2", 1000, NULL, 3, NULL);
    vTaskStartScheduler();
    for(;;);
}

A Task1 starves Task2 because it never blocks or yields, preventing time-slicing.

B Task2 starves Task1 because it has a delay causing Task1 to run more.

C Both tasks run equally because they have the same priority.

D The scheduler crashes due to infinite loops.

💡 Hint

Think about how FreeRTOS schedules tasks that never block or yield.

Task1 never blocks or yields, so it keeps running continuously. This prevents Task2 from running despite equal priority, causing starvation.

📝 Syntax

advanced

Identify the syntax error in time-slicing task creation

Which option contains a syntax error in creating two equal priority tasks for time-slicing?

A

TaskCreate(Task1, "Task1", 1000, NULL, 2, NULL);
xTaskCreate(Task2, "Task2", 1000, NULL, 2, NULL);

B

xTaskCreate(Task1, "Task1", 1000, NULL, 2, NULL);
xTaskCreate(Task2, "Task2", 1000, NULL, 2, NULL);

C

;)LLUN ,2 ,LLUN ,0001 ,"2ksaT" ,2ksaT(etaerCksaTx
;)LLUN ,2 ,LLUN ,0001 ,"1ksaT" ,1ksaT(etaerCksaTx

D

xTaskCreate(Task1, "Task1", 1000, NULL, 2);
xTaskCreate(Task2, "Task2", 1000, NULL, 2, NULL);

💡 Hint

Check the number of parameters required by xTaskCreate.

xTaskCreate requires 6 parameters. Option D calls it with only 5 parameters in the first call, causing a syntax error.

🚀 Application

expert

Calculate CPU time slice for equal priority tasks

In FreeRTOS, two tasks have equal priority and the tick rate is set to 1000 Hz. If each task runs for one tick before switching, how much CPU time (in milliseconds) does each task get per time slice?

A 1 millisecond per time slice

B 100 milliseconds per time slice

C 10 milliseconds per time slice

D 0.5 milliseconds per time slice

💡 Hint

Consider the tick rate and how it relates to time slice duration.

With a tick rate of 1000 Hz, each tick is 1 millisecond. If tasks switch every tick, each gets 1 millisecond per time slice.