linux-kernel - Re: [RFC][PATCH] timekeeping: Cap adjustments so they don't exceede the maxadj value

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Message-ID: <CALAqxLUdB+G+Qp0=V65av1DBg7_f1rFDYF9gFTzwwkeCcocFJQ@mail.gmail.com>
Date:	Mon, 7 Dec 2015 17:14:38 -0800
From:	John Stultz <john.stultz@...aro.org>
To:	Miroslav Lichvar <mlichvar@...hat.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Richard Cochran <richardcochran@...il.com>,
	Prarit Bhargava <prarit@...hat.com>,
	Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC][PATCH] timekeeping: Cap adjustments so they don't exceede
 the maxadj value

On Mon, Dec 7, 2015 at 6:43 AM, Miroslav Lichvar <mlichvar@...hat.com> wrote:
> On Fri, Dec 04, 2015 at 01:57:54PM -0800, John Stultz wrote:
>> Thus its been occasionally noted that users have seen
>> confusing warnings like:
>>
>>     Adjusting tsc more than 11% (5941981 vs 7759439)
>
> Thanks for looking into this, John.
>
>> Thus this patch adds some extra logic to enforce the max adjustment
>> cap in the internal steering.
>>
>> Note: This has the potential to slow corrections when the ADJ_TICK
>> value is furthest away from the default value. So it would be good to
>> get some testing from folks using Chrony, to make sure we don't
>> cause any troubles there.
>
> I ran few tests with chronyd, forcing it to make the largest possible
> adjustment of the tick and it seems to work fine.
>
> However, I'm still able to trigger the warning by feeding a large
> offset to the NTP PLL using a short time constant. When the multiplier
> is already at the maximum and it wants to move it even further, there
> should be no timekeeping_apply_adjustment() call as it always adjusts
> the multiplier at least by one.

Ack. Thanks for noticing this. I could reproduce it and have a fix.
Will send an update out later.

thanks
-john
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