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

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Message-ID: <CALAqxLWHdZn9RCcYbYicKXeu3QiCuVna27mWBQPbFvaj9gm-fw@mail.gmail.com>
Date:   Fri, 19 Oct 2018 11:48:53 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     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@...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

On Fri, Oct 19, 2018 at 11:37 AM, Thomas Gleixner <tglx@...utronix.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