linux-kernel - Re: [PATCH] x86 (64): make calibrate_APIC

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Message-ID: <488886C8.3020904@fujitsu-siemens.com>
Date:	Thu, 24 Jul 2008 15:42:32 +0200
From:	Martin Wilck <martin.wilck@...itsu-siemens.com>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Wichert, Gerhard" <Gerhard.Wichert@...itsu-siemens.com>
Subject: Re: [PATCH] x86 (64): make calibrate_APIC_clock() SMI-safe

H. Peter Anvin wrote:

> The other thing we may want to consider is doing multiple runs of the
> calibration loop.  I suspect we'd get a more accurate result running,
> say, 9 loops of the 1/9 the current duration and looking either for the
> best result or the median (NOT the mean.)
> 
>         -hpa

We considered that. However I think if you know what result to expect 
(here: TICK_COUNT + a few cycles) it is both easier and safer to compare 
against the expected value than to do statistics. SMI protection is all 
about outliers (thus, the median is a good suggestion but I'd say it 
would be over-enigineered).

Martin

PS: I am sorry for the stupid, misleading typo (SMi -> smp) in the mail 
subject.

-- 
Martin Wilck
PRIMERGY System Software Engineer
FSC IP ESP DEV 6

Fujitsu Siemens Computers GmbH
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