prefer tsc timecounter when it's good

Jung-uk Kim jkim at
Thu Apr 7 20:00:32 UTC 2011

On Thursday 07 April 2011 01:12 pm, Andriy Gapon wrote:
> Guys,
> what do you think about the following change?
> The idea is mark TSC as the best timecounter when it's invariant
> and synchronized between cores. Unfortunately I don't have code to
> auto-detect the synchronization and keep relying on the
> corresponding tunable. 

I know Intel is claiming that TSCs for all cores/packages reset to 
zero when they receive a "synchronous" INIT/RESET IPI.  I haven't 
really verified their claim but I think it may be good enough for our 
AP startup code.  However, AMD processors never had such guarantee, 

> I thought about auto-setting it for single-package configurations,
> but even that information is currently not trivial to get out of our
> mp (i386/amd64) machdep code.

It isn't easy ATM. :-/

> --- a/sys/x86/x86/tsc.c
> +++ b/sys/x86/x86/tsc.c
> @@ -169,6 +169,9 @@ init_TSC_tc(void)
>  			printf("TSC timecounter disabled: APM enabled.\n");
>  	}
> +	if (tsc_is_invariant)
> +		tsc_timecounter.tc_quality = 1200;
> +
>  #ifdef SMP
>  	/*
>  	 * We can not use the TSC in SMP mode unless the TSCs on all CPUs

Although it looks okay, please don't commit it just yet.  I am working 
in this area actively.  Also, if the Intel's claim is true, i.e., 
TSCs reset to zero when APs start, we cannot use TSC as a timecounter 
hardware until all APs are started properly.


Jung-uk Kim

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