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SubjectRe: 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.

Dick Johnson
Penguin : Linux version on an i686 machine (5588.30 BogoMips).
My book :

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