I am using FreeRTOS on an STM32F4-Nucleo board and `vTaskGetRunTimeStats()` to measure task CPU usage with the following code:

void configureTimerForRunTimeStats(void)
{
    g_osRuntimeCounter = 0;
}

unsigned long getRunTimeCounterValue(void)
{
    return g_osRuntimeCounter;
}

The `g_osRuntimeCounter` is incremented in a timer callback, and I can measure CPU usage for tasks and IDLE time. However, I need to track CPU usage for specific ISRs (CAN reception and USB-OTG), which are interrupt-based.

Context:
– The interrupts for CAN and USB-OTG are triggered frequently , which are 10ms and 5ms respectively . Understanding the impact of ISRs on task performance is important in this setup.
– The hardware timer used for `g_osRuntimeCounter` runs at a frequency of 1MHz which determines how accurately I can track time.

Challenges:
– I am unsure how to extend this setup to measure CPU usage inside ISRs, especially with interrupt nesting complicating things.
– Timer increments within ISRs may affect the accuracy of the measurement.

How can I accurately measure CPU usage for these ISRs while addressing nested interrupts? Should ISR CPU usage be tracked separately from task CPU usage?