Time Short Functions with cpu time using RTEMS operating system

Question

I am looking to profile some code in a real time operating system, RTEMS. Essentially, rtems has a bunch of functions to read the time, the most useful of which is rtems_clock_get_ticks_since_boot.

The problem here is that for whatever reason the clock ticks reported are synchronized with our state machine loop rate, 5kHz whereas the processor is running at around 200MHz (embedded system). I know this because i recorded the clock time, waited 1 sec and only 5000 ticks had gone by.

So the question is:

How can I get the actual CPU ticks from RTEMS?

PS. clock() from GNU C (has the same problem)

There is a guide that i have been looking into here, but I get impossible constraint in asm which indicates that i would need to use some different assembler keywords. Maybe someone can point me to something similar?

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Context

I want to profile some code, so essentially:

start = cpu_clock_ticks()
    //Some code
time = cpu_clock_ticks() - start;

The code runs in less than 0.125ms so the 8khz counter that clock() and other rtems functions get, wont cut it.

Answer 1

Accurate performance measurements can be made using an oscilloscope, provided that there is a GPIO, test point or pin that the software can write to (and the oscilloscope probe can attach to).

The method here is to send a pulse to the pin. The o'scope can be set up to trigger on the pulse. Some smarter o'scopes can perform statistics on the pulse width, such as mean time and maximum time.

On our embedded system, the H/W team was nice enough to bring out 8 test points for us to use. We initialize the pin to zero. At the start of the code to profile, we write a 1 to the pin. At the end of the profiling code, we write a 0 to the pin. This produces a pulse or square wave.

The o'scope is set up to trigger on the rising edge. The probe is connected to the pin and the program is run. Adjust the o'scope so the the entire pulse is visible on the screen. Re-run the program. When the o'scope triggers, measure the width of the pulse. This will be the actual time of the execution.

Answer 2

So a solution to this is to use the following function:

inline unsigned long timer_now()  {
    unsigned int time; 

    // The internal timer is accessed as special purpose register #268
    // (@24.576 MHz => 1tick=4.069010416E-8 sec,~.04µs 
    asm volatile ("mfspr %0,268; sync" : "=r" (time));
    return time;
}

timer_now will return tics that are still not at the processor speed, but much faster than 8kHz, the time taken can then be calculated as tics * 0.04µs.

NOTE This may only work for the powerPC MPC5200 BSP for rtems, since it uses an assembler routine.

Answer 3

In RTEMS 4.11 or newer you can use rtems_counter_read to obtain high-precision counters that abstracts the CPU-specific assembly code. Please see: https://docs.rtems.org/doxygen/cpukit/html/group__ClassicCounter.html

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