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SubjectRe: [PATCH] make tsc=reliable override boot time stability checks
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Hi John,

On Mon, 2009-08-17 at 16:40 -0700, john stultz wrote:
> On Mon, 2009-08-17 at 16:27 -0700, Stephen Hemminger wrote:
> > 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:
> > > > 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.
> >
>
> Sounds good. Thanks so much for the bug report and testing!
>
>
> This patch makes the tsc=reliable option disable the boot time stability
> checks. Currently the option only disables the runtime watchdog checks.
> This change allows folks who want to override the boot time TSC
> stability checks and use the TSC when the system would otherwise
> disqualify it.
>
> There still are some situations that the TSC will be disqualified, such
> as cpufreq scaling. But these are situations where the box will hang if
> allowed.
>

I had purposefully kept the tsc=reliable separate from the TSC
synchronous checks.
With this patch TSC is marked as usable though the hardware below
doesn't export a CONSTANT_TSC, it might not be a problem generally, but
since TSC has the highest rating, don't you think that timekeeping might
be wayward on such systems ?
Having said that, I don't think I have a particular problem with the
patch as far as we are explicitly mentioning the fact that TSC=reliable
means TSC is blindly trusted on this system, and time might be little
off on some systems.


Alok


> Patch also includes a fix for an issue found by Thomas Gleixner, where
> the TSC disqualification message wouldn't be printed after a call to
> unsynchronized_tsc().


>
> I'd recommend queuing this for 2.6.32, since it probably should get more
> testing then we have time for in 2.6.31.
>
> thanks
> -john
>
>
> Signed-off-by: John Stultz <johnstul@us.ibm.com>
>
> diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
> index 7936b80..4c6b415 100644
> --- a/Documentation/kernel-parameters.txt
> +++ b/Documentation/kernel-parameters.txt
> @@ -2484,12 +2484,13 @@ and is between 256 and 4096 characters. It is defined in the file
> Format:
> <io>,<irq>,<dma>,<dma2>,<sb_io>,<sb_irq>,<sb_dma>,<mpu_io>,<mpu_irq>
>
> - tsc= Disable clocksource-must-verify flag for TSC.
> + tsc= Disable clocksource stability checks for TSC.
> Format: <string>
> [x86] reliable: mark tsc clocksource as reliable, this
> - disables clocksource verification at runtime.
> - Used to enable high-resolution timer mode on older
> - hardware, and in virtualized environment.
> + disables clocksource verification at runtime, as well
> + as the stability checks done at bootup. Used to enable
> + high-resolution timer mode on older hardware, and in
> + virtualized environment.
>
> turbografx.map[2|3]= [HW,JOY]
> TurboGraFX parallel port interface
> 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)
>
>
>



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