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

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Date:	Mon, 24 May 2010 16:30:36 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	"H. Peter Anvin" <hpa@...or.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 Mon, 2010-05-24 at 16:16 -0700, Dan Magenheimer wrote:
> > From: john stultz [mailto:johnstul@...ibm.com]
> > Also, you don't really need extra accuracy, you just need it to be the
> > same from boot to boot. NTP keeps the correction factor persistent from
> > boot to boot via the drift file. The boot argument is just trying to
> > save the time (possibly hours depending on ntp config) after a reboot
> > for NTP to correct for the new error introduced by calibration.
> 
> I was assuming that extra accuracy would decrease the ntp
> convergence time by about the same factor (5-6 bits of extra
> accuracy would decrease ntp convergence time by 32-64x).
> Is that an incorrect assumption?

Sorry, this is sort of mixing points. I was saying you don't need more
accuracy (as opposed to what H. Peter mentioned below) when setting the
tsc_khz= option I proposed. Since it will be constant from boot to boot,
and thus will reduce the ntp convergence time.

However, without such a boot option, more accuracy from an increased
calibration time would help. However, the tradeoff of a longer boot time
is one not many will probably want.


> > From: H. Peter Anvin [mailto:hpa@...or.com]
> > 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.)
> 
> Not sure why upgraded mobo's would fail due to a longer sample?

Again, this is mixing the discussion. The concern was users of a
tsc_khz= boot option might have problems when they upgrade, as the
actual TSC freq might not match what was specified at boot.

> As more and more systems become dependent on clocksource==tsc
> and more and more people assume nanosecond-class measurements
> are relatively accurate, I'd expect the accuracy of tsc_khz
> to become more important.  While desktop users might bristle
> at an extra second of boot delay, I'll bet many server
> farm administrators would gladly pay that upfront cost
> if they know an option exists.

Maybe something like a tsc_long_calibration=1 option would allow for
this?  

However, I really do like the idea of pulling the stamped value from the
MSR and if its close to what we quickly calibrated, use that.

thanks
-john

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