Re: [PATCH] x86, tsc: Skip refined tsc calibration on systems withreliable TSC.

[john stultz]

On Tue, 2012-02-21 at 18:19 -0800, Alok Kataria wrote:
> [Oops forgot to copy LKML, now it is, sorry for the duplicates]
> 
> While running the latest Linux as guest under VMware in highly
> over-committed situations, we have seen cases when the refined TSC
> algorithm fails to get a valid tsc_start value in
> tsc_refine_calibration_work from multiple attempts. As a result the
> kernel keeps on scheduling the tsc_irqwork task for later. Subsequently
> after several attempts when it gets a valid start value it goes through
> the refined calibration and either bails out or uses the new results.
> Given that the kernel originally read the TSC frequency from the
> platform, which is the best it can get, I don't think there is much
> value in refining it. 
> 
> So IMO, for systems which get the TSC frequency from the platform we
> should skip the refined tsc algorithm.
>         
> We can use the TSC_RELIABLE cpu cap flag to detect this, right now it is
> set only on VMware and for Moorestown Penwell both of which have there
> own TSC calibration methods.

So this looks ok to me, only one nit below...

> 
> Index: linux-2.6/arch/x86/kernel/tsc.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/tsc.c	2012-02-21 17:31:01.000000000 -0800
> +++ linux-2.6/arch/x86/kernel/tsc.c	2012-02-21 17:39:05.000000000 -0800
> @@ -874,6 +874,13 @@ static void tsc_refine_calibration_work(
>  		goto out;
> 
>  	/*
> +	 * Trust the results of the earlier calibration on systems
> +	 * exporting a reliable TSC.
> +	 */
> +	if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
> +		goto out;
> +
> +	/*

Instead of dropping out in the function called by the work-queue, why
not just avoid scheduling the work-queue to begin with?

The FEATURE_TSC_RELIABLE isn't something that is set late, and needs the
delay, right?

Here's what I queued up, let me know if it looks ok to you and I'll push
it on to Thomas.

thanks
-john