linux-kernel - Re: [PATCH RFC 0/3] Improve stability of system clock

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Message-ID: <CALAqxLVjzOAf_UmY_5F4cYevqFD2Qa4+-uPB-2HRYNk93qvsXw@mail.gmail.com>
Date:   Wed, 17 May 2017 09:30:31 -0700
From:   John Stultz <john.stultz@...aro.org>
To:     Miroslav Lichvar <mlichvar@...hat.com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Prarit Bhargava <prarit@...hat.com>,
        Richard Cochran <richardcochran@...il.com>
Subject: Re: [PATCH RFC 0/3] Improve stability of system clock

On Wed, May 17, 2017 at 9:13 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> This is an attempt to improve stability and accuracy of the system clock
> with very accurate time sources like the new PTP KVM clock or NTP/PTP
> using hardware timestamping. It affects mainly kernels running with
> NOHZ. It requires updating of the old ia64 and powerpc vsyscalls.
>
> The main problem is that the error accumulated in the ntp_error register
> takes too long to correct and this cannot be easily fixed. There are
> four sources of the error:
> - rounding of time for old vsyscalls
> - alignment of frequency adjustments to ticks
> - iterative correction of the multiplier
> - limited resolution of the multipler
>
> Instead of trying to correct the error faster, the patches remove the
> first three sources. With the only remaining source the correction logic
> can be simplified and the frequency of the clock is much more stable and
> accurate.
>
> Simulations of a frequency step in linux-tktest (values are in ppm and
> nanoseconds):

So thanks for sending these out. I still need to look them over in
depth, but can I make another ask here?  :)

Could you submit your linux-tktest infrastructure to the kselftests dir?

It would be really nice for folks to be able to reproduce your results
be able to do similar testing for regressions.

thanks
-john