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

[Miroslav Lichvar <mlichvar@redhat.com>]

On Thu, Sep 10, 2015 at 11:14:25AM -0700, John Stultz wrote:
> On Thu, Sep 10, 2015 at 10:42 AM, John Stultz <john.stultz@linaro.org> wrote:
> > On Thu, Sep 10, 2015 at 5:02 AM, Miroslav Lichvar <mlichvar@redhat.com> wrote:
> >> 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?
> >
> > I think its due to the ntp_error being large enough prior (or during
> > the freq transition) that we're still applying a single unit freq
> > adjustment for that error. But I'm guessing on the acpi_pm clocksource
> > the shift is low enough that a single unit adjustment is coarse enough
> > to affect the ppm, since I see the same consistently measured ppm
> > result if I both increase the settling time measurement sleep times.
> > If I left it for a long long time, the single unit correction would
> > likely null the error out and we'd get the desired result, but I don't
> > think the test has time for that.

I ran few tests and it doesn't seem to be a problem with large
ntp_error or an extremely slow adjustment of the multiplier for the
new frequency.

I think it really is the xtime_remainder correction. It is a fixed
offset added to the ntp error on each tick to compensate for the
cycle_interval rounding error. With the acpi_pm clocksource and 1000Hz
update rate xtime_remainder is -127 ns, which effectively speeds up
the clock by 127 ppm. When NTP slows the clock down by 10%, the
correction is not decreased by 10% and we can observe the clock is
running faster by 12.7 ppm than expected.

Is there a cheap way to calculate this?
xtime_remainder * (ntp_tick >> ntp_error_shift) / NTP_INTERVAL_LENGTH

> So bumping the fail level to > 100ppm avoids false positives due to
> long-term error correction with coarse clocksources, but still is
> tight enough to catch the dampened approximation issue caused by the
> abs(s64) problem.
> 
> Any objection to moving to that? It is still a 0.01% error bound.

No objection from me as long as we understand where that error is
coming from.

-- 
Miroslav Lichvar