Re: clocksource changes in 2.6.31 - possible regression

[Stephen Hemminger <shemminger@vyatta.com>]

On Mon, 17 Aug 2009 16:17:54 -0700
john stultz <johnstul@us.ibm.com> wrote:

> On Mon, 2009-08-17 at 16:02 -0700, Stephen Hemminger wrote:
> > On Mon, 17 Aug 2009 15:23:22 -0700
> > john stultz <johnstul@us.ibm.com> wrote:
> > 
> > > On Mon, 2009-08-17 at 14:45 -0700, Stephen Hemminger wrote:
> > > > On Mon, 17 Aug 2009 14:37:57 -0700
> > > > john stultz <johnstul@us.ibm.com> wrote:
> > > > > However, I think its fair, that as your TSC is being disqualified for
> > > > > being an old AMD SMP box, and there is a *possibility* that if you don't
> > > > > run with cpufreq and the SUMA-ness of the box didn't get in the way of
> > > > > TSC synchronization, you might have an argument for overriding the
> > > > > unsynchronized_tsc() heuristics.
> > > > > 
> > > > > Luckily the option is already there. :)
> > > > > 
> > > > > So try booting with "tsc=reliable" to override those checks, and I think
> > > > > you'll be able to do what you want to do.
> > > > > 
> > > > 
> > > > Good idea, doesn't work.
> > > > 
> > > > vyatta@amd1:~$ cat /proc/cmdline 
> > > > BOOT_IMAGE=/boot/vmlinuz-2.6.31-rc6 root=/dev/sda1 ro tsc=reliable
> > > > vyatta@amd1:~$ cat /sys/devices/system/clocksource/clocksource0/available_clocksource
> > > > acpi_pm 
> > > 
> > > Bah! My apologies for half-assing this. 
> > > 
> > > How about with the following *tested* patch (includes a variant of
> > > Thomas' fix). 
> > > 
> > > thanks
> > > -john
> > > 
> > > 
> > > Signed-off-by: John Stultz <johnstul@us.ibm.com>
> > > 
> > > diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
> > > index 71f4368..648fb26 100644
> > > --- a/arch/x86/kernel/tsc.c
> > > +++ b/arch/x86/kernel/tsc.c
> > > @@ -825,6 +825,9 @@ __cpuinit int unsynchronized_tsc(void)
> > >  
> > >  	if (boot_cpu_has(X86_FEATURE_CONSTANT_TSC))
> > >  		return 0;
> > > +
> > > +	if (tsc_clocksource_reliable)
> > > +		return 0;
> > >  	/*
> > >  	 * Intel systems are normally all synchronized.
> > >  	 * Exceptions must mark TSC as unstable:
> > > @@ -832,10 +835,10 @@ __cpuinit int unsynchronized_tsc(void)
> > >  	if (boot_cpu_data.x86_vendor != X86_VENDOR_INTEL) {
> > >  		/* assume multi socket systems are not synchronized: */
> > >  		if (num_possible_cpus() > 1)
> > > -			tsc_unstable = 1;
> > > +			return 1;
> > >  	}
> > >  
> > > -	return tsc_unstable;
> > > +	return 0;
> > >  }
> > >  
> > >  static void __init init_tsc_clocksource(void)
> > > 
> > > 
> > 
> > This adds tsc, but makes it first?  it is reliable, but do I want
> > to make it most important?
> > 
> > $ cat /sys/devices/system/clocksource/clocksource0/available_clocksource
> > tsc acpi_pm
> 
> 
> Well, if you're overriding the system saying that its safe, then sure,
> its better then anything else, why wouldn't we?
> 

That's acceptable, maybe add change to Documentation/kernel-parameters.txt

	tsc=		Disable clocksource-must-verify flag for TSC.
			Format: <string>
			[x86] reliable: mark tsc clocksource as reliable and
                        makes tsc the default clocksource; this
			disables clocksource verification at runtime.
			Used to enable high-resolution timer mode on older
			hardware, and in virtualized environment.


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