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

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Message-ID: <87bjtmxtuc.ffs@tglx>
Date: Thu, 27 Mar 2025 18:32:27 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Miroslav Lichvar <mlichvar@...hat.com>
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 16:42, Miroslav Lichvar wrote:
> On Thu, Mar 27, 2025 at 10:22:31AM +0100, Thomas Gleixner wrote:
>> 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.

Can you point me to that code?

It would be probably useful to create a test mechanism which allows to
exercise all of this in a simulated way so we actually don't have to
wonder every time we change a bit what the consequences are.

Thanks,

        tglx