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

[john stultz <johnstul@us.ibm.com>]

On Thu, 2006-11-09 at 23:38 +0000, Thomas Gleixner wrote:
> plain text document attachment
> (gtod-mark-tsc-unusable-for-highres-timers.patch)
> From: Thomas Gleixner <tglx@linutronix.de>
> 
> The TSC is too unstable and unreliable to be used with high resolution timers.
> The automatic detection of TSC unstability fails once we switched to high
> resolution mode, because the tick emulation would use the TSC as reference. 
> This results in a circular dependency.  Mark it unusable for high res upfront.
> 
> [akpm@osdl.org: updated for i386-time-avoid-pit-smp-lockups.patch]
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Ingo Molnar <mingo@elte.hu>
> 
> diff -puN arch/i386/kernel/tsc.c~gtod-mark-tsc-unusable-for-highres-timers arch/i386/kernel/tsc.c
> --- a/arch/i386/kernel/tsc.c~gtod-mark-tsc-unusable-for-highres-timers
> +++ a/arch/i386/kernel/tsc.c
> @@ -459,10 +459,23 @@ static int __init init_tsc_clocksource(v
>  		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. 

Mind adding a warning so users know why a system that might use the TSC
normally does not use the TSC w/ highres timers?

Otherwise looks ok.

Acked-by: John Stultz <johnstul@us.ibm.com>

thanks
-john