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

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Date:	Mon, 24 May 2010 15:04:11 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	john stultz <johnstul@...ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>, Brian Bloniarz <bmb@...enacr.com>
Cc:	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

> > 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?
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