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

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Message-Id: <200611100610.13957.ak@suse.de>
Date:	Fri, 10 Nov 2006 06:10:13 +0100
From:	Andi Kleen <ak@...e.de>
To:	john stultz <johnstul@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...l.org>,
	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

	current_tsc_khz = tsc_khz;
> >  		clocksource_tsc.mult = clocksource_khz2mult(current_tsc_khz,
> >  							clocksource_tsc.shift);
> > +#ifndef CONFIG_HIGH_RES_TIMERS
> >  		/* lower the rating if we already know its unstable: */
> >  		if (check_tsc_unstable())
> >  			clocksource_tsc.rating = 0;
> > -
> > +#else
> > +		/*
> > +		 * Mark TSC unsuitable for high resolution timers. TSC has so
> > +		 * many pitfalls: frequency changes, stop in idle ...  When we
> > +		 * switch to high resolution mode we can not longer detect a
> > +		 * firmware caused frequency change, as the emulated tick uses
> > +		 * TSC as reference. This results in a circular dependency.
> > +		 * Switch only to high resolution mode, if pm_timer or such
> > +		 * is available.
> > +		 */
> > +		clocksource_tsc.rating = 50;
> > +		clocksource_tsc.is_continuous = 0;
> > +#endif
> >  		init_timer(&verify_tsc_freq_timer);
> >  		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?

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.

-Andi

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