linux-kernel - Re: [PATCH v2 1/2] time/timekeeping: Fix possible inconsistencies in

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Message-ID: <Z-Vx8kV4M3khPknC@localhost>
Date: Thu, 27 Mar 2025 16:42:42 +0100
From: Miroslav Lichvar <mlichvar@...hat.com>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: John Stultz <jstultz@...gle.com>, LKML <linux-kernel@...r.kernel.org>,
	Stephen Boyd <sboyd@...nel.org>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Shuah Khan <shuah@...nel.org>, linux-kselftest@...r.kernel.org,
	kernel-team@...roid.com, Lei Chen <lei.chen@...rtx.com>
Subject: Re: [PATCH v2 1/2] time/timekeeping: Fix possible inconsistencies in
 _COARSE clockids

On Thu, Mar 27, 2025 at 10:22:31AM +0100, Thomas Gleixner wrote:
> On Tue, Mar 25 2025 at 12:32, Miroslav Lichvar wrote:
> > Resetting the NTP error causes a small time step. An NTP/PTP client
> > can be setting the frequency very frequently, e.g. up to 128 times per
> > second and the interval between updates can be random. If the timing
> 
> I never observed that behaviour, but I'm not a NTP/PTP wizard/power user.

On a machine that has a /dev/ptp device available, a simple test to
observe such a high update rate is to run:

1) phc_ctl /dev/ptp0 set
2) phc2sys -m -q -O 0 -s /dev/ptp0 -R 128
or alternatively
2) chronyd -d 'refclock PHC /dev/ptp0 poll -7'

> The original implementation respected this base period, but John's
> approach of forwarding, which cures the coarse time getter issue,
> violates it. As a consequence the previous error accumulation is not
> longer based on the base period because the period has been reset to the
> random point in time when adjtimex() was invoked, which makes the error
> accumulation a random number.

I see, so that value of the NTP error is already wrong at that point
where it's reset to 0.

To clearly see the difference with the new code, I made an attempt
to update the old linux-tktest simulation that was used back when the
multiplier adjustment was reworked, but there are too many missing
things now and I gave up.

Maybe I could simply patch the kernel to force a small clock
multiplier to increase the rate at which the error accumulates.

-- 
Miroslav Lichvar