linux-kernel - Re: [PATCH] x86: Export tsc related information in sysfs

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Message-ID: <4BFAFE17.8060105@zytor.com>
Date:	Mon, 24 May 2010 15:30:47 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	john stultz <johnstul@...ibm.com>,
	Brian Bloniarz <bmb@...enacr.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Andi Kleen <andi@...stfloor.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Venkatesh Pallipadi <venki@...gle.com>, chris.mason@...cle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] x86: Export tsc related information in sysfs

On 05/24/2010 03:04 PM, Dan Magenheimer wrote:
>>> Is that still the case?  I thought newer versions of NTP could deal
>> with
>>> large values.  Inaccuracies of way more than 500 ppm are everyday.
>>
>> That's scary.
>>
>> Yea, in the kernel the ntp freq correction tops out at 500ppm. Almost
>> all the systems I see tend to fall in the +/- 200ppm range (if there's
>> not something terribly wrong with the hardware).
>>
>> So maybe things aren't so bad out there? Or is that wishful thinking?
> 
> Since Brian's concern is at boot-time at which point there is no
> network or ntp, and assuming that it would be unwise to vary tsc_khz
> dynamically on a clocksource==tsc machine (is it?), would optionally
> lengthening the TSC<->PIT calibration beyond 25ms result in a more
> consistent tsc_khz between boots?  Or is the relative instability
> an unavoidable result of skew between the PIT and the fixed constant
> PIT_TICK_RATE combined with algorithmic/arithmetic error?  Or is
> the jitter of the (spread-spectrum) TSC too extreme?  Or ???
> 
> If better more consistent calibration is possible, offering
> that as an optional kernel parameter seems better than specifying
> a fixed tsc_khz (stamped or user-specified) which may or may
> not be ignored due to "too different from measured tsc_khz".
> Even an (*optional*) extra second or two of boot time might
> be perfectly OK if it resulted in an additional five or six
> bits of tsc_khz precision.
> 
> Thoughts, Brian?

Making the calibration time longer should give a more precise result,
but of course at the expense of longer boot time.

A longer sample would make sense if the goal is to freeze it into a
kernel command line variable, but the real question is how many people
would actually do that (and how many people would then suffer problems
because they upgraded their CPU/mobo and got massive failures on post-boot.)

	-hpa
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