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

On Tue, Dec 22, 2015 at 9:49 AM, Russell King - ARM Linux
[off-list ref] wrote:

On Tue, Dec 22, 2015 at 09:30:30AM -0500, Patrick Doyle wrote:

quoted

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