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From: John Stultz <john.stultz@linaro.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Christopher Hall <christopher.s.hall@intel.com>,
	"H. Peter Anvin" <hpa@zytor.com>,
	linux-rt-users <linux-rt-users@vger.kernel.org>,
	jesus.sanchez-palencia@intel.com, gavin.hindman@intel.com,
	liam.r.girdwood@intel.com, Peter Zijlstra <peterz@infradead.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: TSC to Mono-raw Drift
Date: Fri, 19 Oct 2018 11:48:53 -0700	[thread overview]
Message-ID: <CALAqxLWHdZn9RCcYbYicKXeu3QiCuVna27mWBQPbFvaj9gm-fw@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1810191955250.1651@nanos.tec.linutronix.de>

On Fri, Oct 19, 2018 at 11:37 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
> On Fri, 19 Oct 2018, Thomas Gleixner wrote:
>> On Mon, 15 Oct 2018, Christopher Hall wrote:
>> > TSC kHz used to calculate mult/shift value: 3312000
>>
>> Now the most interesting information here would be the resulting mult/shift
>> value which the kernel uses. Can you please provide that?
>
> But aside of the actual values it's pretty obvious why this happens. It's a
> simple rounding error. Minimal, but still. To avoid rounding errors you'd
> have to find mult and shift values which exactly result in:
>
>      (freq * mult) >> shift = 1e9
>
> which is impossible for freq = 3312 * 1e6.

Right.

> A possible fix for this is, because the error is in the low PPB range, to
> adjust the error once per second or in some other sensible regular
> interval.

Ehh.. I mean, this is basically what all the complicated ntp_error
logic does for the long-term accuracy compensation.  Part of the
allure of the raw clock is that there is no changes made over time.
Its a very simple constant calculation.

We could try to do something like pre-seed the ntpinterval value so
the default ntp_tick value corrects for this, but that's only going to
effect the monotonic/realtime clock until ntpd or anyone else sets a
different interval rate.

So  I'm not particularly eager in trying to do the type of small
frequency oscillation we do for monotonic/realtime for long-term
accuracy to the raw clock as that feels a bit antithetical to the
point of the raw clock.

I guess I'd want to understand more of the use here and the need to
tie the raw clock back to the hardware counter it abstracts.

thanks
-john

  reply	other threads:[~2018-10-19 18:49 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <20181015160945.5993-1-christopher.s.hall@intel.com>
2018-10-19 15:25 ` Thomas Gleixner
2018-10-19 18:34   ` John Stultz
2018-10-19 18:39     ` John Stultz
2018-10-19 18:37   ` Thomas Gleixner
2018-10-19 18:48     ` John Stultz [this message]
2018-10-19 18:57       ` Thomas Gleixner
2018-10-19 19:21         ` John Stultz
2018-10-19 20:50           ` Thomas Gleixner
2018-10-19 22:36             ` John Stultz
2018-10-23 18:31               ` John Stultz
2018-10-24 14:51                 ` Miroslav Lichvar
2018-10-24 17:32                   ` Christopher Hall
2018-10-25 11:49                     ` Miroslav Lichvar
2018-11-01 17:41                   ` Thomas Gleixner
2018-11-02 10:26                     ` Miroslav Lichvar
2018-11-02 11:27                       ` Thomas Gleixner
2018-11-01 17:44                 ` Thomas Gleixner
2018-11-01 17:56                   ` John Stultz
2018-11-01 18:03                     ` Thomas Gleixner
2018-11-02 11:20                       ` Miroslav Lichvar
2018-11-02 11:25                         ` Thomas Gleixner
2018-11-02 12:31                           ` Miroslav Lichvar

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