linux-kernel - Re: TSC to Mono-raw Drift

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Date:   Fri, 2 Nov 2018 12:27:16 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Miroslav Lichvar <mlichvar@...hat.com>
cc:     John Stultz <john.stultz@...aro.org>,
        Christopher Hall <christopher.s.hall@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        linux-rt-users <linux-rt-users@...r.kernel.org>,
        jesus.sanchez-palencia@...el.com,
        Gavin Hindman <gavin.hindman@...el.com>,
        liam.r.girdwood@...el.com, Peter Zijlstra <peterz@...radead.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: TSC to Mono-raw Drift

Miroslav,

On Fri, 2 Nov 2018, Miroslav Lichvar wrote:
> On Thu, Nov 01, 2018 at 06:41:00PM +0100, Thomas Gleixner wrote:
> > On Wed, 24 Oct 2018, Miroslav Lichvar wrote:
> > > The error is too large to be corrected by stepping on clock updates.
> > > For a typical TSC frequency we have multiplier in the range of few
> > > millions, so that's a frequency error of up to few hundred ppb. In the
> > > old days when the clock was updated 1000 times per second that would
> > > be hidden in the resolution of the clock, but now with tickless
> > > kernels those steps would be very noticeable.
> 
> > That only happens when the system was completely idle for a second and in
> > that case it's a non issue because the clock is updated before it's
> > used. So nothing will be able to observe the time jumping forward by a few
> > or even a few hundreds of nanoseconds.
> 
> That's great news (to me). I think we should do the same with the
> mono/real clock. A periodic 4ns step would be better than a slew
> correcting tens or hundreds of nanoseconds. This would be a
> significant improvement in accuracy on idle systems, in theory
> identical to running with nohz=off.
> 
> Maybe I am missing some important detail, but I think we can just drop
> the +1 mult adjustment and step on each update by the (truncated)
> amount that has accumulated in the NTP error register. With the
> changes that have been made earlier this year the clock should never
> be ahead, so the step would always be forward.

That sounds reasonable.

> > For the regular case, where CPUs are
> > busy and the update happens 100/250/1000 times per second the jump forward
> > will not be noticable at all.
> 
> I think a 4ns jump at 100 Hz might be noticeable with a good reference
> clock and large number of measurements, but so would be the current +1
> mult adjustment.

Indeed.

Thanks,

	tglx