linux-kernel - Re: Linux 2.6.29-rc6

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Date:	Mon, 9 Mar 2009 21:26:48 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Jesper Krogh <jesper@...gh.cc>
cc:	john stultz <johnstul@...ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Len Brown <len.brown@...el.com>
Subject: Re: Linux 2.6.29-rc6

On Mon, 9 Mar 2009, Jesper Krogh wrote:
> 
> First boot.
> [    0.000000] ref_freq: 2311825  pit_freq: 2310386
> Second boot:
> [    0.000000] ref_freq: 2311803  pit_freq: 2310190
> Third boot:
> [    0.000000] ref_freq: 2311824  pit_freq: 2310080
> Fourth boot:
> [    0.000000] ref_freq: 2311831  pit_freq: 2310130

It's really quite impressively stable, but the fast-PIT calibration 
frequency is reliably about 3/4 of a promille low. Or, put another way, 
the TSC difference over the pit calibration is just a _tad_ too small 
compared to the value we'd expect if that loop of pit_expect_msb() would 
really run at the expected delay of a 1.193182MHz clock divided by 256.

And it's stable in that it really always seems to be off by a very similar 
amount. It's not moving around very much.

I also wonder why it seems to happen mainly just to _you_. There's 
absolutely nothing odd in your system, neither a slow CPU or anything 
else that would stand out.

Grr. Very annoyingly non-obvious.

		Linus
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