linux-kernel - Re: clocksource changes in 2.6.31

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Date:	Mon, 17 Aug 2009 16:17:54 -0700
From:	john stultz <johnstul@...ibm.com>
To:	Stephen Hemminger <shemminger@...tta.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: clocksource changes in 2.6.31 - possible regression

On Mon, 2009-08-17 at 16:02 -0700, Stephen Hemminger wrote:
> On Mon, 17 Aug 2009 15:23:22 -0700
> john stultz <johnstul@...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@...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@...1:~$ cat /proc/cmdline 
> > > BOOT_IMAGE=/boot/vmlinuz-2.6.31-rc6 root=/dev/sda1 ro tsc=reliable
> > > vyatta@...1:~$ 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@...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?

thanks
-john


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