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

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Message-ID: <e598eaa0-3b3b-4c16-9d7c-ff35dd150baa@kenogo.org>
Date: Fri, 9 May 2025 21:40:31 +0200
From: Keno Goertz <contact@...ogo.org>
To: John Stultz <jstultz@...gle.com>
Cc: tglx@...utronix.de, zippel@...ux-m68k.org, mingo@...e.hu,
 linux-kernel@...r.kernel.org, Miroslav Lichvar <mlichvar@...hat.com>
Subject: Re: ntp: Adjustment of time_maxerror with 500ppm instead of 15ppm

Hey!

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.

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.

Best regards
Keno