Re: [Xenomai-help] 0 CPU time problem

[Philippe Gerum <rpm@xenomai.org>]

On 11/04/2011 10:36 PM, Gilles Chanteperdrix wrote:
> On 11/04/2011 10:03 PM, Tom Z wrote:
>>> We do not know what skin you use, we will assume that you use
>>> Xenomai posix skin, so, compile with the right flags.
>>
>> Here is how I
>> build my program, say, fooTest: gcc -I/usr/xenomai/include
>> -D_GNU_SOURCE -D_REENTRANT -Wall -pipe -D__XENO__ -lnative
>> -L/usr/xenomai/lib -lxenomai -lpthread -lnative -lrtdk fooTest.c -o
>> fooTest
>
> You are using the native skin then.
>
>>> CLOCK_PROCESS_CPUTIME_ID is not implemented for xenomai posix skin,
>>> so, clock_gettime probably returns an error, and the struct timespec
>>> is unchanged. If somehow the glibc clock_gettime is used, and this
>>> call does not cause a system call, then all bets are off, and even a
>>> lockup could happen.
>>
>> Thanks for explaining this. So can I trust the
>> CPU time I obtained with my method when the function is running in
>> the primary mode? It seems you explanation implies that the results I
>> obtained are not reliable, if so, are there other reliable ways to
>> obtain the CPU time?
>
> No, CLOCK_PROCESS_CPUTIME is a clock implemented by the Linux kernel,
> so, it does not see the time spent in primary mode. If the foo function
> does not suspend, you can use rt_timer_tsc(), but the answer is no,
> there is currently no way to get the cpu time used when in primary mode.
>

There is one, but deeply buried in the API basement, via rt_task_inquire():

RT_TASK_INFO info;
RTIME start;

ret = rt_task_inquire(NULL, &info); /* NULL means current task */
if (ret)
	goto Houston_handles_the_problem;
start = info.exectime;
... do work ....
ret = rt_task_inquire(NULL, &info);
delta = info.exectime - start; /* elapsed CPU time in ns */

rt_task_inquire() will beget a syscall each time invoked though, so it 
does have some overhead. You need CONFIG_XENO_OPT_STATS enabled in your 
Kconfig for this feature (defaults to on).

-- 
Philippe.