vTaskGetRunTimeStats() for CPU usage in FreeRTOS - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of vTaskGetRunTimeStats() after two tasks run?

Consider two FreeRTOS tasks running with different CPU times. The vTaskGetRunTimeStats() function fills a buffer with CPU usage stats. What will the output string contain?

FreeRTOS

char buffer[100];
vTaskGetRunTimeStats(buffer);
printf("%s", buffer);

A Task1 100%\r\nTask2 0%\r\n

B Task1 30%\r\nTask2 70%\r\n

C Task1 70%\r\nTask2 30%\r\n

D Task1 50%\r\nTask2 50%\r\n

💡 Hint

Remember CPU usage is proportional to the run time of each task relative to total run time.

The output string shows each task's CPU usage percentage. If Task1 ran 70% of the time and Task2 30%, the buffer will reflect that.

🧠 Conceptual

intermediate

What does vTaskGetRunTimeStats() measure?

Which of the following best describes what vTaskGetRunTimeStats() reports?

A The total time each task has spent running since system start, as a percentage of total CPU time.

B The current stack usage of each task in bytes.

C The number of times each task has been switched in and out by the scheduler.

D The priority level of each task.

💡 Hint

Think about what CPU usage means in a multitasking system.

vTaskGetRunTimeStats() reports how much CPU time each task has used relative to total CPU time since the stats timer started.

🔧 Debug

advanced

Why does vTaskGetRunTimeStats() show zero CPU usage for all tasks?

You call vTaskGetRunTimeStats() but the output shows zero CPU usage for every task. What is the most likely cause?

A The buffer passed to <code>vTaskGetRunTimeStats()</code> is too small.

B The configGENERATE_RUN_TIME_STATS macro is not enabled or the timer for run time stats is not configured.

C The tasks have not been created yet.

D The scheduler is not running.

💡 Hint

Check your FreeRTOS configuration related to run time stats.

If configGENERATE_RUN_TIME_STATS is not enabled or the timer used to measure run time is not set up, the stats will always be zero.

📝 Syntax

advanced

Which code snippet correctly uses vTaskGetRunTimeStats()?

Choose the code snippet that correctly calls vTaskGetRunTimeStats() and prints the CPU usage stats.

A char stats[200];\nvTaskGetRunTimeStats(&stats);\nprintf("CPU Usage:\n%s", stats);

B char stats[200];\nvTaskGetRunTimeStats();\nprintf("CPU Usage:\n%s", stats);

C char *stats;\nvTaskGetRunTimeStats(stats);\nprintf("CPU Usage:\n%s", stats);

D char stats[200];\nvTaskGetRunTimeStats(stats);\nprintf("CPU Usage:\n%s", stats);

💡 Hint

Check the function parameter type and how the buffer is passed.

vTaskGetRunTimeStats() requires a pointer to a char array buffer to fill with stats. Passing the buffer name (array) is correct.

🚀 Application

expert

How to use vTaskGetRunTimeStats() to monitor CPU usage in a running system?

You want to monitor CPU usage of tasks periodically in a FreeRTOS system. Which approach correctly uses vTaskGetRunTimeStats() to achieve this?

A Create a high priority monitoring task that calls <code>vTaskGetRunTimeStats()</code> every second, stores the output, and prints it via UART.

B Call <code>vTaskGetRunTimeStats()</code> once at system start and store the output for later use.

C Modify each task to call <code>vTaskGetRunTimeStats()</code> inside their loop to get their own CPU usage.

D Use <code>vTaskGetRunTimeStats()</code> inside an interrupt service routine to get real-time CPU usage.

💡 Hint

Think about how to safely and periodically collect stats without blocking or causing errors.

A dedicated monitoring task calling vTaskGetRunTimeStats() periodically is the correct way. Calling it once at start won't update usage. Calling inside tasks or ISRs is unsafe or incorrect.