Re: [patch 13/19] GTOD: Mark TSC unusable for highres timers

[Posted November 13, 2006 by corbet]

From:		Thomas Gleixner <tglx-AT-linutronix.de>
To:		Andi Kleen <ak-AT-suse.de>
Subject:		Re: [patch 13/19] GTOD: Mark TSC unusable for highres timers
Date:		Sat, 11 Nov 2006 12:14:37 +0100
Cc:		Andrew Morton <akpm-AT-osdl.org>, Ingo Molnar <mingo-AT-elte.hu>, john stultz <johnstul-AT-us.ibm.com>, LKML <linux-kernel-AT-vger.kernel.org>, Len Brown <lenb-AT-kernel.org>, Arjan van de Ven <arjan-AT-infradead.org>, Roman Zippel <zippel-AT-linux-m68k.org>

On Fri, 2006-11-10 at 10:29 +0100, Andi Kleen wrote:
> > But that's different.
> > 
> > We're limping along in a semi-OK fashion with the TSC.  But now Thomas is
> > proposing that we effectively kill it off for all x86 because of hrtimers.
> 
> I'm totally against that.

I'm working on that. The general disable is indeed overkill. All I need
to prevent is to switch over to highres/dyntick in case that there is no
fallback (e.g. pm_timer) available. Else I end up in a circular
dependency as the emulated tick depends on the monotonic clock.

> > And afaict the reason for that is that we're using jiffies to determine if
> > the TSC has gone bad, and that test is getting false positives.
> 
> The i386 clocksource had always trouble with that. e.g.  I have a box
> where the TSC works perfectly fine on a 64bit kernel, but since the new i386
> clocksource code is in it always insists on disabling it shortly after boot.
> My guess is that some of the checks in there are just broken and need
> to be fixed.

It's the unconditional mark_unstable call in ACPI C2 state. /me looks.

	tglx