Re: [chrony-users] Fwd: Chrony malfunctioning at beaglebones

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On Sun, Nov 15, 2015 at 05:31:50PM +0000, Nuno Gonçalves wrote:
> On Tue, Aug 25, 2015 at 9:27 AM, Miroslav Lichvar <mlichvar@xxxxxxxxxx> wrote:
> > It seems disabling nohz doesn't fix this. With the maxslewrate setting
> > and a good drift file chronyd stabilized, but when running without
> > driftfile and maxslewrate the skew and frequency are insane again.
> > Running adjtimex -t 9000 and observing the offset confirms the clock
> > is not slowing down by 0.1s per second as it should.

> Considering this previous test we did with the nohz cmdline option,
> considering the bug outcome (that was related to nohz), and also
> considering this other thread:
> Do you believe there is some problem with the nohz cmdline option?

The problem with abs64() and the one in the thread were both actually
not related directly to the nohz code. The effect of disabling nohz in
3.17+ kernels should be pretty small anyway and even if the nohz
option was broken it would probably not explain the difference in the
PPS jitter you see when the CPU is forced to wakeup.

> I'm asking this because It also doesn't seem to have any effect to me
> with the beaglebone running linux 4.1.
> Consider the two graphs in attach. I can see clearly a effect on PPS
> performance when I run some program that wakes every 1ms, but setting
> nohz=off doesn't have any effect.

Is the CPU on BBB entering some deep sleep states like C2 and C3?
Maybe the idle=halt kernel option could reduce the observed jitter.

Miroslav Lichvar

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