linux-kernel - Re: [Patch] Fix the possibility of insane return value of hpet

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Date:	Tue, 17 Mar 2009 09:12:55 -0400
From:	Paul Gortmaker <paul.gortmaker@...driver.com>
To:	Yasunori Goto <y-goto@...fujitsu.com>
CC:	Clemens Ladisch <clemens@...isch.de>,
	Linux Kernel ML <linux-kernel@...r.kernel.org>,
	robert.picco@...com, venkatesh.pallipadi@...el.com,
	vojtech@...e.cz, mingo@...hat.com
Subject: Re: [Patch] Fix the possibility of insane return value of hpet_calibrate()
 against SMI.

Yasunori Goto wrote:
> Sorry for late response.
>
>   
>> Yasunori Goto wrote:
>>     
>>>> I've no sense of feel for how long each calibration run would take.
>>>> Would doing it 5 times show up as a significant increase in the boot
>>>> time for those that care about boot time being as quick as possible?
>>>>         
>>> Hmm. The loop times is trade off against reliable value....
>>> Though SMI is rare interruption, I don't know how frequent
>>> hypervisor's switch is.
>>>       
>> Could you, just for testing, run the calibration five thousand times or
>> so instead of five times, and count how often you get insane values?
>> (And how much delay does such an SMI add?)
>>     
>
> I tried 50000 times. But insane value was nothing.
> I think SMI is very rare.
>
>   
>>> Each calibration of this has 1 milli second.
>>> Do you think 5 msec is too long?
>>>       
>> This shouldn't matter when booting.  Anyway, I think it's possible to
>> increase TICK_CALIBRATE without losing too much accuracy.
>>     
>
> Hmm. If the person who is trying to reduce boot time for fastboot dislikes this impact,
> then I'll try Vojtech-san's way.
>   

That probably makes sense -- if it is extremely rare, and if you get two
values that are the same (within some small delta) then you probably
are 99.99% confident that you have the right data.

Paul.

>
> Thanks.
>   

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