vTaskGetRunTimeStats() for CPU usage in FreeRTOS - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does vTaskGetRunTimeStats() function provide in FreeRTOS?

It provides a snapshot of CPU usage by each task, showing how much processing time each task has consumed since the system started or since the last reset of the stats.

beginner

What type of data does vTaskGetRunTimeStats() output?

It outputs a formatted string that lists each task's name, the amount of run time in ticks or percentage, and the percentage of total CPU time used by that task.

intermediate

What must be enabled in FreeRTOS to use vTaskGetRunTimeStats()?

The configGENERATE_RUN_TIME_STATS macro must be set to 1 in FreeRTOSConfig.h, and a timer or counter must be configured to measure run time accurately.

beginner

How do you prepare a buffer to receive the output of vTaskGetRunTimeStats()?

You create a character array (string buffer) large enough to hold the stats output, then pass its pointer to the function to fill it with the formatted CPU usage data.

intermediate

Why is vTaskGetRunTimeStats() useful for debugging FreeRTOS applications?

It helps identify which tasks are using the most CPU time, allowing developers to optimize task priorities and improve system responsiveness.

What must be enabled to use vTaskGetRunTimeStats() in FreeRTOS?

A configGENERATE_RUN_TIME_STATS set to 1

B configUSE_PREEMPTION set to 0

C configUSE_IDLE_HOOK set to 1

D configUSE_TIMERS set to 0

The configGENERATE_RUN_TIME_STATS macro must be enabled to collect runtime statistics.

What type of information does vTaskGetRunTimeStats() return?

A Task CPU usage statistics as a formatted string

B Memory usage of each task

C Task stack overflow status

D Number of context switches

It returns CPU usage stats for each task in a readable string format.

Which of these is required to measure run time for vTaskGetRunTimeStats()?

A The scheduler must be stopped

B A software delay loop

C The idle task must be disabled

D A hardware timer or counter configured for run time stats

A timer or counter is needed to track how long tasks run.

How do you use vTaskGetRunTimeStats() in code?

A Call it without parameters to print stats

B Pass a char array buffer to receive the stats string

C It returns an integer with CPU usage

D It requires a task handle as input

You provide a buffer where the function writes the stats string.

What is a practical use of vTaskGetRunTimeStats()?

A To allocate memory dynamically

B To reset task priorities automatically

C To find which tasks use the most CPU time

D To disable unused tasks

It helps identify CPU-heavy tasks for optimization.

Explain how vTaskGetRunTimeStats() helps monitor CPU usage in FreeRTOS.

Think about how you can see which tasks run the longest.

Describe the steps to use vTaskGetRunTimeStats() in a FreeRTOS application.

Consider configuration, setup, and usage.

Practice

1. What is the primary purpose of the vTaskGetRunTimeStats() function in FreeRTOS?

easy

A. To suspend all running tasks temporarily

B. To create a new task with specific priority

C. To delete a task from the scheduler

D. To display the CPU usage time of each task

Solution

1. Step 1: Understand the function's role

   vTaskGetRunTimeStats() is designed to collect and display CPU usage statistics for each task in FreeRTOS.

2. Step 2: Compare options with function purpose

   Options A, B, and C relate to task management but not CPU usage reporting, which is the key role of vTaskGetRunTimeStats().

3. Final Answer:

   To display the CPU usage time of each task -> Option D

4. Quick Check:

   Function purpose = CPU usage display [OK]

Hint: Remember: vTaskGetRunTimeStats shows CPU time per task [OK]

Common Mistakes:

• Confusing it with task creation or deletion functions
• Thinking it suspends tasks instead of reporting stats
• Assuming it modifies task priorities

2. Which of the following is the correct way to call vTaskGetRunTimeStats() to store the stats in a buffer named buffer?

easy

A. vTaskGetRunTimeStats(buffer);

B. vTaskGetRunTimeStats(&buffer);

C. vTaskGetRunTimeStats(buffer[]);

D. vTaskGetRunTimeStats(*buffer);

Solution

1. Step 1: Check function parameter type

   vTaskGetRunTimeStats() expects a pointer to a char array (string buffer), so passing the buffer name directly is correct.

2. Step 2: Analyze each option's syntax

   vTaskGetRunTimeStats(buffer); passes the buffer correctly. vTaskGetRunTimeStats(&buffer); passes address of buffer pointer (wrong). vTaskGetRunTimeStats(buffer[]); uses invalid syntax. vTaskGetRunTimeStats(*buffer); dereferences buffer incorrectly.

3. Final Answer:

   vTaskGetRunTimeStats(buffer); -> Option A

4. Quick Check:

   Pass buffer name directly [OK]

Hint: Pass buffer name directly, no & or * needed [OK]

Common Mistakes:

• Using &buffer which is wrong for arrays
• Adding [] inside function call
• Dereferencing buffer pointer incorrectly

3. Given the following code snippet, what will be the output stored in statsBuffer?

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

Assuming two tasks "TaskA" and "TaskB" run and TaskA used 70% CPU, TaskB used 30%.

medium

A. TaskA 7
TaskB 3

B. TaskA 70%
TaskB 30%

C. TaskA 7000
TaskB 3000

D. TaskA 700
TaskB 300

Solution

1. Step 1: Understand output format of vTaskGetRunTimeStats()

   The function outputs CPU usage as run time counter ticks, not percentages. If TaskA used 70% CPU, it might show 7000 ticks and TaskB 3000 ticks (assuming 10000 total ticks).

2. Step 2: Match options with expected output

   TaskA 7000 TaskB 3000 shows usage in ticks (7000 and 3000), matching expected output. Options B, C, and D show percentages or smaller numbers, which are not the direct output format.

3. Final Answer:

   TaskA 7000 TaskB 3000 -> Option C

4. Quick Check:

   Output shows ticks, not percentages [OK]

Hint: Output shows ticks, not percent values [OK]

Common Mistakes:

• Assuming output shows percentages directly
• Confusing ticks with smaller numbers
• Expecting formatted percentages in output

4. You called vTaskGetRunTimeStats(statsBuffer); but the output is empty. What is the most likely cause?

medium

A. The function requires a NULL pointer as argument

B. Run time stats collection is not enabled in FreeRTOSConfig.h

C. The buffer size is too large

D. The tasks have zero priority

Solution

1. Step 1: Identify prerequisites for run time stats

   Run time stats must be enabled in FreeRTOSConfig.h (configGENERATE_RUN_TIME_STATS set to 1) for vTaskGetRunTimeStats() to work.

2. Step 2: Analyze other options

   Buffer size being too large does not cause empty output. Passing NULL is invalid. Task priority does not affect stats collection directly.

3. Final Answer:

   Run time stats collection is not enabled in FreeRTOSConfig.h -> Option B

4. Quick Check:

   Enable run time stats in config [OK]

Hint: Check configGENERATE_RUN_TIME_STATS is enabled [OK]

Common Mistakes:

• Assuming buffer size too large causes empty output
• Passing NULL pointer mistakenly
• Thinking task priority disables stats

5. You want to monitor CPU usage of tasks over time using vTaskGetRunTimeStats(). Which approach correctly ensures accurate and meaningful stats?

hard

A. Enable run time stats, provide a large buffer, call periodically, and reset counters between calls

B. Call vTaskGetRunTimeStats() once at startup with a small buffer

C. Disable run time stats and manually count task switches

D. Call vTaskGetRunTimeStats() inside each task to get its own CPU usage

Solution

1. Step 1: Understand requirements for accurate CPU stats

   Run time stats must be enabled, buffer must be large enough to hold all task info, and stats should be collected periodically to track changes over time.

2. Step 2: Evaluate each option

   Enable run time stats, provide a large buffer, call periodically, and reset counters between calls describes the correct approach including resetting counters to avoid cumulative overflow. Call vTaskGetRunTimeStats() once at startup with a small buffer is insufficient (only once, small buffer). Disable run time stats and manually count task switches disables stats, losing automation. Call vTaskGetRunTimeStats() inside each task to get its own CPU usage is incorrect because stats are global, not per task.

3. Final Answer:

   Enable run time stats, provide a large buffer, call periodically, and reset counters between calls -> Option A

4. Quick Check:

   Enable + buffer + periodic calls + reset = accurate stats [OK]

Hint: Enable stats, use big buffer, call regularly, reset counters [OK]

Common Mistakes:

• Calling once only at startup
• Disabling run time stats
• Calling inside each task expecting per-task stats