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SubjectRe: 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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