linux-kernel - Re: division and cpu usage

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Message-ID: <Pine.LNX.4.61.0708240731130.8096@chaos.analogic.com>
Date:	Fri, 24 Aug 2007 07:34:50 -0400
From:	"linux-os \(Dick Johnson\)" <linux-os@...logic.com>
To:	"Luka Napotnik" <luka.napotnik@...il.com>
Cc:	<linux-kernel@...r.kernel.org>
Subject: Re: division and cpu usage

On Thu, 23 Aug 2007, Luka Napotnik wrote:
> Hello.
>
> I'm new to kernel development and have some questions.
>
> 1. Why can't I divide with regular casting to double ((double)a /
> (double)b)? It gives me strange errors when compiling:
>
> WARNING: "__divdf3" [/root....] undefined!
> WARNING: "__addf3" [/root/...] undefined!
> WARNING: "__floatsidf" [/root/...] undefined!
>
> And if I compile with normal integers, I get zero as the result.
>
> 2. I'm trying to get the percentage of CPU used for a certain
> task_struct and figured the following formula:
>
> (task->utime + task->stime) / jiffies
>
> Before calculating I convert all the variables to jiffies. Is this correct?
>
> Please help.
>
> Greets,
> Lukla

Floating point operations are not allowed in the kernel. Often,
when you think you are simply creating a constant, the compiler
generates runtime code so you have to watch out for that as well.

You can use "long long" for high precision math if necessary.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.22.1 on an i686 machine (5588.30 BogoMips).
My book : http://www.AbominableFirebug.com/
_

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