[lttng-dev] LTTng - Xenomai : different results between timestamp-lttng and rt_time_read()

[MONTET Julien via lttng-dev <lttng-dev@lists.lttng.org>]

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Hi the developers !

CONTEXT
I am currently working on a Raspberry pi 3B with Xenomai and LTTng tools.
Raspbian 10.9 Buster - kernel 4.19.85
uname -a : Linux raspberrypi 4.19.85-v7+ #5 SMP PREEMPT Wed May 12 10:13:37
Both tools are working, but I wonder about the accuracy of LTTng libraries.


METHOD
The code used is quite simple, it is written with the alchemy skin.
A rt_task_spawn calls a function that has rt_task_set_periodic(NULL, TM_NOW, period) and rt_task_wait_period(NULL).
->The rt_task_set_periodic is based on 1ms.
->The  rt_task_wait_period(NULL) is of course inside a while loop (see below at the very end).

My goal is to get accurate traces from Xenomai.
I took two methods to do so :
-> lttng
-> basic calculation based on  rt_timer_read()

What a surprise when I found both method have two different results.
-> LTTng shows me traces [0.870;1.13] ms (or even less precise)
-> rt_time_read shows me traces [0.980;1.020] ms

Thing to note :
-> The use of LTTng has no influence on rt_time_read(), you can use both methods at the same time.

Then, I saved the output of rt_time_read inside a tracepoint.
It appeared the LTTng is always called at the right time because the value got by rt_time_read () is really good.


QUESTIONS
These are now my questions :
- What is the method I should trust ?
- I have searched on the forum and I found LTTng uses a MONOTONIC clock for the timestamp. Can/Should I modify it ?


CODE
-----------------------------------------------------------------------
A small part of my function called by rt_task_spawn :
[...]
    RTIME period = 1000*1000; // in ns
    RTIME now;
    RTIME previous = 0;
    RTIME duration;
[...]
 while(1)
    {
        overruns = 0;
        err = rt_task_wait_period(&overruns);
        now = rt_timer_read();
        tracepoint(tp_provider, tracepoint_tick_ms, now, "tick");

        if (previous != 0)
        {
            duration=now-previous;
            rt_printf("%llu\n \n", duration/1000);
        }
       previous=now;
   [...]
}
------------------------------------------------------------------------

Regards,

Julien

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