linux-kernel - Re: [PATCH] x86, tsc: Skip refined tsc calibration on systems with reliable TSC.

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Date:	Tue, 28 Feb 2012 11:09:16 -0800
From:	Alok Kataria <akataria@...are.com>
To:	John Stultz <johnstul@...ibm.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	the arch/x86 maintainers <x86@...nel.org>,
	dirk.brandewie@...il.com, alan@...ux.intel.com, stable@...nel.org,
	Dan Hecht <dhecht@...are.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86, tsc: Skip refined tsc calibration on systems with
	reliable TSC.

Ping...any comments on this one.

Thanks,
Alok

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.
>         
> Thanks,
> Alok
> 
> --
> 
> From: Alok N Kataria <akataria@...are.com>
> 
> For systems which get the TSC frequency directly from the platform
> and don't go through the native TSC calibration algorithm, we should
> trust those values and not try to refine those.
> 
> This patch is applicable for kernel from v2.6.38 to current mainline.
> 
> Signed-off-by: Alok N Kataria <akataria@...are.com>
> Cc: John Stultz <johnstul@...ibm.com>
> Cc: Dirk Brandewie <dirk.brandewie@...il.com>
> Cc: Alan Cox <alan@...ux.intel.com>
> Cc: stable@...nel.org
> 
> 
> 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;
> +
> +	/*
>  	 * Since the work is started early in boot, we may be
>  	 * delayed the first time we expire. So set the workqueue
>  	 * again once we know timers are working.
> 

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