linux-kernel - Re: Missing CLOCK_BOOTTIME

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Message-ID: <87ftbxxz55.fsf@nanos.tec.linutronix.de>
Date:   Mon, 18 May 2020 13:26:14 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Pali Rohár <pali@...nel.org>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: Missing CLOCK_BOOTTIME_RAW?

Pali Rohár <pali@...nel.org> writes:
> On Saturday 09 May 2020 11:49:27 Thomas Gleixner wrote:
>> Sure, but what's the problem? The adjustemt is done to make the observed
>> time as correct as possible.
>
> Yes. But correction may take lot of time, e.g. also more then one day.
>
> So during this period when correction is in progress, measuring time
> difference via CLOCK_MONITONIC will have incorrect results.
>
> It already happened for me, system clock was shifted by one hour and
> chronyd started adjustment, it slow down system clock. 6 real hours
> passed and via system clock was measured time difference only about
> 5 hours and 20 minutes (correction was still in progress as system
> clock at that time was still shifted by about 20 minutes).

System clock shifted by one hour? You mean DST change? If chronyd
adjusts that by smoothing the frequency, that's just broken and really
not the kernel's problem.

Thanks,

        tglx