linux-kernel - Re: [PATCH v2] x86: Fix sched_clock_cpu for systems with unsynchronized TSC

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Date:	Mon, 01 Mar 2010 09:58:00 -0800
From:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
To:	Dimitri Sivanich <sivanich@....com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH v2] x86: Fix sched_clock_cpu for systems with
 unsynchronized TSC


Acked-by: Venkatesh Pallipadi <venkatesh.pallipadi@...el.com>

On Mon, 2010-03-01 at 09:48 -0800, Dimitri Sivanich wrote:
> On UV systems, the TSC is not synchronized across blades.  The
> sched_clock_cpu() function is returning values that can go backwards 
> (I've seen as much as 8 seconds) when switching between cpus.
> 
> As each cpu comes up, early_init_intel() will currently set the
> sched_clock_stable flag true.  When mark_tsc_unstable() runs, it clears
> the flag, but this only occurs once (the first time a cpu comes up whose
> TSC is not synchronized with cpu 0).  After this, early_init_intel() will
> set the flag again as the next cpu comes up.
> 
> Only set sched_clock_stable if tsc has not been marked unstable.
> 
> Signed-off-by: Dimitri Sivanich <sivanich@....com>
> 
> ---
> 
> Only affects x86 arch.
> 
>  arch/x86/kernel/cpu/intel.c |    3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> Index: linux/arch/x86/kernel/cpu/intel.c
> ===================================================================
> --- linux.orig/arch/x86/kernel/cpu/intel.c
> +++ linux/arch/x86/kernel/cpu/intel.c
> @@ -70,7 +70,8 @@ static void __cpuinit early_init_intel(s
>  	if (c->x86_power & (1 << 8)) {
>  		set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
>  		set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
> -		sched_clock_stable = 1;
> +		if (!check_tsc_unstable())
> +			sched_clock_stable = 1;
>  	}
>  
>  	/*


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