[Xenomai-core] CPU utilization question

[makarand pradhan <makarand_u_pradhan@domain.hid>]

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Greetings all,

I am working on a real time power pc embedded system which uses xenomai. I am presently noticing a CPU utilization which I am not able to explain. Will highly appreciate your opinion on the problem described below.

I'm summarizing my understanding. Pl feel free to correct me.
I'm using /proc/xenomai/stat to get the CPU util. I'm assuming that the ROOT thread captures the %CPU spent in Linux/secnodary and each xenomai thread listed, captures the %CPU in primary domain. If a thread transitions to the secondary, then I'm assuming that the %util in secondary domain will get accounted against ROOT and the actual % usage in primary will get accounted against the thread.

Problem:
I'm starting a xenomai thread using rt_task_spawn. 
        rt_task_spawn(&task2, "Task 2 ", STACK_SIZE, 11, 0, task2_func, NULL);

The thread invoking rt_task_spawn, sleeps for a long long time without doing anything, or I can pause it.
sleep(5000000);

The xenomai task does rt_printfs periodically.

With this setup, I was hoping to see 0% CPU util for the linux task in top.

Contrary to this expectation, I notice CPU util as high as 3.1% when I top the linux process:

  PID USER      PR  NI  VIRT  RES  SHR S %CPU %MEM    TIME+  COMMAND                                                         
 7988 root      20   0  4940 4376 3688 S  3.1  0.8   0:01.65 a.out      

The CPU % varies based on the activity I do in the primary domain. i.e. more rt_printfs more %CPU in both primary and secondary.


Question:

If the linux process is sleeping, why do I see a CPU util against the process? 
Why is the CPU % util vary based on the activity I do in the primary domain?

Thanks for taking the time to look at the question.

Would highly appreciate your inputs.

Warm Rgds,
Makarand.

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