linux-kernel - Re: ntp: Adjustment of time_maxerror with 500ppm instead of 15ppm

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Message-ID: <CANDhNCqZRZPE=s5q+K2-jKJnBycAxXBJH_mUmC1d+jWEvOPGZA@mail.gmail.com>
Date: Fri, 9 May 2025 12:49:33 -0700
From: John Stultz <jstultz@...gle.com>
To: Keno Goertz <contact@...ogo.org>
Cc: tglx@...utronix.de, zippel@...ux-m68k.org, linux-kernel@...r.kernel.org, 
	Miroslav Lichvar <mlichvar@...hat.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: ntp: Adjustment of time_maxerror with 500ppm instead of 15ppm

On Fri, May 9, 2025 at 12:40 PM Keno Goertz <contact@...ogo.org> wrote:
> On 5/8/25 21:45, John Stultz wrote:
> > Have you tried a patch introducing PHI (likely setting it to 15000 as
> > MAXFREQ is represented as nsec/sec) and using it instead of MAXFREQ in
> > the calculation? Do you see any behavioral change in fixing it, or is
> > this just a reporting  correctness issue?
>
> I haven't tried a patch, but I couldn't find any place in the kernel
> itself that uses time_maxerror.  So I wouldn't expect behavioral changes
> within the kernel.  Of course, user space applications may depend on the
> values returned by the adjtimex system call.

Agreed. Established userspace expectations (of the current behavior)
are what I'm worried about.

> In fact, I only noticed this behavior in the first place because I am
> writing a distributed time-stamping program and the maxerror reported by
> libc's ntp_gettime (which calls adjtimex on Linux) just felt way too large.
>
> I'm curious to hear what Miroslav might know about other user space
> applications that take an interest in this value.

It might be good to share a kernel patch so that folks can test such a
change as well.

thanks
-john