linux-kernel - Re: [URGENT rfc patch 0/3] tsc clocksource bug fix

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Date:	Fri, 5 Jul 2013 17:24:09 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Alex Shi <alex.shi@...el.com>, hpa@...ux.intel.com,
	tim.c.chen@...ux.intel.com, linux-kernel@...r.kernel.org,
	andi.kleen@...el.com, mingo@...e.hu
Subject: Re: [URGENT rfc patch 0/3] tsc clocksource bug fix

On Fri, 5 Jul 2013, Peter Zijlstra wrote:

> On Fri, Jul 05, 2013 at 04:23:33PM +0200, Frederic Weisbecker wrote:
> > Nope, I haven't touched that. I prefer not to fiddle with unstable
> > clocksource for now :)
> > 
> > As for unstable TSCs, if sched_clock_tick() needs to be fed, we simply
> > don't stop the tick.
> 
> Not entirely the same thing; I thought the clocksource watchdog was ran
> even when we have a 'stable' TSC, just to make sure it stays stable.
> There's known cases where the BIOS f*cks us over and wrecks TSC sync.

See arch/x86/kernel/tsc.c

We disable the watchdog for the TSC when tsc_clocksource_reliable is
set.

tsc_clocksource_reliable is set when:

 - you add tsc=reliable to the kernel command line

 - boot_cpu_has(X86_FEATURE_TSC_RELIABLE)
 
   X86_FEATURE_TSC_RELIABLE is a software flag, set by vmware and
   moorsetown. So all other machines keep the watchdog enabled.

 - On Geode LX (OLPC)

Thanks,

	tglx
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