linux-kernel - Re: tsc reliability for Intel Core 2 Duo "Conroe"

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Message-ID: <AANLkTim1_8Rf0fPfycE92z5CAXpLXYga93A5qTxgXjqz@mail.gmail.com>
Date:	Wed, 26 May 2010 15:21:10 -0700
From:	Venkatesh Pallipadi <venki@...gle.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, linux-kernel@...r.kernel.org,
	Len Brown <lenb@...nel.org>
Subject: Re: tsc reliability for Intel Core 2 Duo "Conroe"

On Wed, May 26, 2010 at 10:41 AM, Dan Magenheimer
<dan.magenheimer@...cle.com> wrote:
> Looking through code following up on the separate TSC-related
> thread, I noticed that my Intel Core 2 Duo "Conroe" box
> is determined to have an unstable TSC, so falls back
> to clocksource==hpet.
>
> While a Conroe has X86_FEATURE_CONSTANT_TSC and not
> X86_FEATURE_NONSTOP_TSC, a Conroe is only able to enter
> C0 and C1 (unlike its sister Intel Core 2 Duo processor
> "Merom" which can enter C0-C3).
>
> I was under the impression (possibly from an earlier kernel
> version?) that tsc_constant PLUS inability to enter deep-C
> states would result in an acceptably stable TSC to use
> as a clocksource (assuming it passes a TSC warp test).
>
> So is this a bug?  Or is my impression incorrect?
>
> See tsc_check_state() in drivers/acpi/processor_idle.c.
> I can submit a patch, but wanted to check first.

Adding Len.

tsc_check_state(0 should only mark TSC unstable when state > C1 and
!NONSTOP_TSC. Are you seeing TSC marked even with only C1?

Or may be you are hitting the bug in acpi_pad.c that marks TSC
unstable more aggressively. I recently sent out a patch for that here
-
https://patchwork.kernel.org/patch/100633/

Thanks,
Venki
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