About effective resolution of cpu execution clocks

[Miguel Telleria de Esteban <miguel@mtelleria.com>]

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Dear all,

Please allow me to post this newbie question that must have been
answered many times before.

I have my doubts about the actual resolution of CPU TIME USAGE
MEASUREMENT for individual tasks, that is using the 

            clock_gettime()

            with the thread clock id.

From one side, it looks as the counter of cpu-time use in each thread
is done with a jiffy resolution:

-  utime and stime fields in task_struct seem to be unsigned long,
   therefore in 32bit archs they cannot have a very high resolution.

   http://lxr.linux.no/#linux+v3.6.3/kernel/posix-cpu-timers.c
   http://lxr.linux.no/linux+*/include/linux/sched.h#L1362
   http://lxr.linux.no/linux+*/include/asm-generic/cputime.h#L7

-  utime and stime fields in /proc/<tid>/stat come out in jiffies
   according to proc man page.

since I guess that these utime and stime fields are the basis of the
cpuclock implementation I infer that the effective resolution of
thread or process usage is limited by the jiffy resolution (4msg with
CONFIG_HZ=250) of the utime and stime fields in task_struct.

On the other side, clock_getres() returns 1 nanosec and the well known
hrtimers merge in 2.6.21 announced highly and broadly that the jiffy
barrier has been lifted with real time clocks and timers.

So my questions are:

*  What is the effective resolution of two invocations of
   clock_gettime() on the same running thread for a long period
   involving several CPU preemptions?

*  Are there other fields apart from stime and utime with the
   sufficient precision to maintain a CPU usage count?

*  Does the PREEMPT_RT branch improve this resolution somehow?

Thanks in advance for your time.
Cheers,

       Miguel Telleria


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