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

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Message-ID: <20080603182014.79a38d03@infradead.org>
Date:	Tue, 3 Jun 2008 18:20:14 -0700
From:	Arjan van de Ven <arjan@...radead.org>
To:	akataria@...are.com
Cc:	Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>,
	Dan Hecht <dhecht@...are.com>, Tim Mann <mann@...are.com>,
	Zachary Amsden <zach@...are.com>,
	Sahil Rihan <srihan@...are.com>
Subject: Re: [RFC PATCH] x86:Use cpu_khz for loops_per_jiffy calculation

On Tue, 03 Jun 2008 17:41:09 -0700
Alok Kataria <akataria@...are.com> wrote:

> On X86 platform we can use the value of cpu_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.
\

can you guarantee that the rate tsc ticks at is the same as the current
CPU frequency? Answer: You can't....

sadly we do need to calibrate this...

In addition, clearly you can have different cpus in a system run at a
different rate (both in terms of cpu_khz and, independently, in terms
of tsc rate)


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