Re: [PATCH 2/2 (v2)] kselftest: timers: Add adjtick test to validate adjtimex() tick adjustments

[Miroslav Lichvar <mlichvar@redhat.com>]

On Wed, Sep 09, 2015 at 04:07:31PM -0700, John Stultz wrote:
> Recently an issue was reported that was difficult to detect except
> by tweaking the adjtimex tick value, and noticing how quickly the
> adjustment took to be made:
> 	https://lkml.org/lkml/2015/9/1/488
> 
> Thus this patch introduces a new test which manipulates the adjtimex
> tick value and validates the results are what we expect.

> +	if (llabs(eppm - ppm) > 10) {
> +		printf("	[FAILED]\n");
> +		return -1;
> +	}
> +	printf("	[OK]\n");
> +	return  0;

This seems to work nicely with the tsc and hpet clocksources, but for
some reason 10 ppm is not enough with the acpi_pm clocksource on both
machines I tried this on. They both show -99988 ppm for the first
test. When I modify the program to go through errors I get:

Estimating tick (act: 9000 usec, -100000 ppm): 9001 usec, -99988 ppm    [FAILED]
Estimating tick (act: 9250 usec, -75000 ppm): 9251 usec, -74991 ppm     [OK]
Estimating tick (act: 9500 usec, -50000 ppm): 9501 usec, -49994 ppm     [OK]
Estimating tick (act: 9750 usec, -25000 ppm): 9751 usec, -24997 ppm     [OK]
Estimating tick (act: 10000 usec, 0 ppm): 10000 usec, 0 ppm     [OK]
Estimating tick (act: 10250 usec, 25000 ppm): 10249 usec, 24996 ppm     [OK]
Estimating tick (act: 10500 usec, 50000 ppm): 10499 usec, 49993 ppm     [OK]
Estimating tick (act: 10750 usec, 75000 ppm): 10749 usec, 74990 ppm     [OK]

The precision of the clock is better than microsecond, so that
wouldn't explain a 12 ppm error over the 15 second interval. I guess
it's due to a larger xtime_remainder, which basically is a hidden
frequency offset added (and not multiplied) to the NTP frequency
offset. Would that explain it?

-- 
Miroslav Lichvar