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

[Patrick Doyle]

On Tue, Dec 22, 2015 at 9:49 AM, Russell King - ARM Linux
<linux at arm.linux.org.uk> wrote:
> On Tue, Dec 22, 2015 at 09:30:30AM -0500, Patrick Doyle wrote:
>> Short version:
>> My application running on a Cortex-A5 processor (the SAMA5D2 from
>> Atmel) calls getrusage(RUSAGE_THREAD, ...), which returns cpu time
>> quantized to the kernel tick frequency (100Hz or 1Khz, depending on
>> CONFIG_HZ_100 vs CONFIG_HZ_1000).
>>
>> How can I get better precision for sched_clock on the (Cortex-A5) ARM?
>>  The x86 uses the TSC.
>
> You need to provide a sched_clock() implementation using
> sched_clock_register().
>
Thank you, I tried that by modifying the (Atmel specific) tcb_clcksrc,
and got a sched_clock that never incremented.  I have a question out
at the Atmel forum asking about this, and will continue to investigate
on my own, but I figured it was about time to ask the experts :-)

How does the kernel generate its tick clock?  Does it use a
clocksource (or the clkevt abstraction), or does it rely upon the
(PIT) timer registered much earlier in the boot process?

I guess I'm confused about the relationships between clocksources,
clock events, sched_clock, and the HZ tick interrupt.  Can anybody
point me at a good place to demystify these things for me?

--wpd