linux-kernel - Re: Regression in 2.6.27 caused by commit bfc0f59

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Message-ID: <alpine.LFD.1.10.0809022239220.3243@apollo.tec.linutronix.de>
Date:	Tue, 2 Sep 2008 23:13:22 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Larry Finger <Larry.Finger@...inger.net>,
	LKML <linux-kernel@...r.kernel.org>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Alok Kataria <akataria@...are.com>,
	Michael Buesch <mb@...sch.de>
Subject: Re: Regression in 2.6.27 caused by commit bfc0f59

On Tue, 2 Sep 2008, Linus Torvalds wrote:
> 
> You're definitely right that this could easily be the _real_ problem. 
> Especially as your TSC min value of 2160 is (a) pretty close to the 
> expected time of a microsecond and (b) so stable that I actually do not 
> believe that the PIT itself is at all emulated or the problem.

On that box, the PIT is probably real hardware or a damned good
emulation. When you look at the 10 loop values you see that it does
50% perfectly fine calibration loops. The others are just SMI
interruptions caused by random unknown crap in the BIOS.
 
> Btw - as to caring about the average value: that's pointless. If you only 
> look at the average time the PIT read takes place, then it is going to 
> approximate that "pit_count" thing in the end that I already did. 
> 
> Why? Because the average value should essentially end up being "(end_tsc - 
> start_tsc) / pit_count". And if you just compare that to "min_tsc", then 
> that should always be about a microsecond (on normal machines where the 
> PIT is essentially on the old emulated internal "ISA" bus on the 
> southbridge). So you end up with what I already posted, and you already 
> dismissed.
> 
> So average TSC is not any more interesting than "pit_count". 

Yeah, you're right. Math is hard :)

Thanks,

	tglx
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