linux-kernel - Re: [RFC PATCH] x86:Use cpu_khz for loops_per

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Message-ID: <20080624175753.GG5642@ucw.cz>
Date:	Tue, 24 Jun 2008 19:57:53 +0200
From:	Pavel Machek <pavel@...e.cz>
To:	Alok Kataria <akataria@...are.com>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Arjan van de Ven <arjan@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Daniel Hecht <dhecht@...are.com>, Tim Mann <mann@...are.com>,
	Zach Amsden <zach@...are.com>, Sahil Rihan <srihan@...are.com>
Subject: Re: [RFC PATCH] x86:Use cpu_khz for loops_per_jiffy calculation

Hi!

> On X86 platform we can use the value of tsc_khz computed during tsc calibration
> to calculate the loops_per_jiffy value. Its very important to keep the error in
> lpj values to minimum as any error in that may result in kernel panic in
> check_timer. 
> In virtualization environment, On a highly overloaded host the guest delay
> calibration may sometimes result in errors beyond the ~50% that timer_irq_works
> can handle, resulting in the guest panicking.

How did you adress 'khz has nothing to do with loops per jiffie'
comment?

Some cpus can do loop in cycle , some need two cycles, some need half.

								Pavel

> @@ -421,6 +422,10 @@
>  		return;
>  	}
>  
> +	lpj = ((u64)tsc_khz * 1000);
> +	do_div(lpj, HZ);
> +	lpj_tsc = lpj;
> +
>  	printk("Detected %lu.%03lu MHz processor.\n",
>  				(unsigned long)cpu_khz / 1000,
>  				(unsigned long)cpu_khz % 1000);
> 


-- 
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