How to get better precision out of getrusage on the ARM?

[Patrick Doyle]

On Wed, Dec 30, 2015 at 10:00 AM, Patrick Doyle <wpdster at gmail.com> wrote:
> Continuing on...
> I now have a CLOCKSOURCE_OF_DECLARED()'ed 10 MHz clock source running
> on my ARM processor (Atmel SAMA5D2 Xplained board).  It registers
> itself through sched_clock_register() to provide a high resolution
> sched clock.  Once I turned on "Full dynticks CPU time accounting"
> (CONFIG_VIRT_CPU_ACCOUNTING_GEN), I was able to get better than jiffy
> resolution from my calls to getrusage(RUSAGE_THREAD,..).  But things
> still aren't quite right.  I am using getrusage() to provide some
> runtime profile information to an existing application (that was
> ported to run on Linux instead of a custom RTOS).  I have code that
> looks like:
>
> tick()
> // commented out code that used to do something
> tock()
>
> where tick() & tock() are my profile "start" and "stop" points that
> call getrusage() to record and and accumulate time spent between calls
> to tick() & tock().  Most of the time, I get a delta of 0 between the
> two calls, which I expect.  But occasionally, I get a delta ranging
> between 800us and 1000us, which I don't understand at all.  It seems
> like my thread is being "charged" for time spent doing something else.
> Perhaps an interrupt occurred and its time got charged to my thread;
> perhaps a higher priority thread ran for 1ms, I don't know (yet).
>
> Does anybody have any suggestions as to where I might look, or as to
> what kernel CONFIG options might make the most sense for an
> application such as this?
>
> --wpd

A couple of more (confusing) data points...
- Changing the tick rate to 100Hz results in deltas as extreme as 9400us.
- Using clock_gettime(CLOCK_THREAD_CPUTIME_ID,...) instead of
getrusage(RUSAGE_THREAD,...) gives much more believable numbers in the
15-25us range, but still with a few bizarre excursions to 41, 69, and
172us (for one random test case).

--wpd