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

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Date:	Fri, 10 Nov 2006 00:50:20 -0800
From:	Andrew Morton <akpm@...l.org>
To:	tglx@...utronix.de
Cc:	Andi Kleen <ak@...e.de>, john stultz <johnstul@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...e.hu>,
	Len Brown <lenb@...nel.org>,
	Arjan van de Ven <arjan@...radead.org>,
	Roman Zippel <zippel@...ux-m68k.org>
Subject: Re: [patch 13/19] GTOD: Mark TSC unusable for highres timers

On Fri, 10 Nov 2006 09:10:06 +0100
Thomas Gleixner <tglx@...utronix.de> wrote:

> On Fri, 2006-11-10 at 06:10 +0100, Andi Kleen wrote:
> > > >  		verify_tsc_freq_timer.function = verify_tsc_freq;
> > > >  		verify_tsc_freq_timer.expires =
> > > 
> > > 
> > > Hmmm. I wish this patch was unnecessary, but I don't see an easy
> > > solution. 
> > 
> > Very sad. This will make a lot of people unhappy, even to the point
> > where they might prefer disabling noidlehz over super slow gettimeofday. 
> > I assume you at least have a suitable command line option for that, right?
> 
> Yes it is sad. And the sadest part is that AMD and Intel have been asked
> to fix that more than 5 years ago. They did not get their brain straight
> and now we are the dimwits.
> 
> > Can we get a summary on which systems the TSC is considered unstable?
> > Normally we assume if it's stable enough for gettimeofday it should
> > be stable enough for longer delays too.
> 
> TSC is simply a nightmare:
> 
> - Frequency changes with CPU clock
> - Unsynced across CPUs
> - Stops in C3, which makes it completely unusable
> 
> Once you take away periodic interrupts it is simply broken. AMD and
> Intel can run in circels, it does not get better.
> 

What is the actual problem?  verify_tsc_freq()?

If so, could that function use the PIT/pmtimer/etc for working out if
the TSC is bust, rather than directly using jiffies?
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