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Message-ID: <20090817162729.451613b4@nehalam>
Date: Mon, 17 Aug 2009 16:27:29 -0700
From: Stephen Hemminger <shemminger@...tta.com>
To: john stultz <johnstul@...ibm.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, 17 Aug 2009 16:17:54 -0700
john stultz <johnstul@...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@...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?
>
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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