linux-kernel - Re: division and cpu usage

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Date:	Sun, 26 Aug 2007 01:25:55 +0200
From:	Luka Napotnik <luka.napotnik@...il.com>
To:	Jan Engelhardt <jengelh@...putergmbh.de>
CC:	"linux-os (Dick Johnson)" <linux-os@...logic.com>,
	linux-kernel@...r.kernel.org
Subject: Re: division and cpu usage

Hello again.

I have the following code:
========================================
clock_t c_sum, j, p;
cputime_t j_tmp;
...
c_sum = cputime64_to_clock_t(task->utime) +
cputime64_to_clock_t(task->stime);
cur_j = jiffies;
j_tmp =  jiffies64_to_cputime64(cur_j);
j = cputime64_to_clock_t(j_tmp);
p = (c_sum * 100) / j;
========================================

And if I check the p value of a certain process it gives wrong results.
For example for a process using 99% of the CPU it shows 20. What am I
doing wrong?

Greets,
Luka

Jan Engelhardt pravi:
> On Aug 24 2007 07:34, linux-os (Dick Johnson) wrote:
>>> 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?
> 
> * So use integer math: (task->utime + task->stime) * 100 / jiffies
>   and you get the 'common' percentage. In integer, that is.
> 
> * I am not sure about the use of jiffies when it comes to CONFIG_NO_HZ=y.
> 
>> Floating point operations are not allowed in the kernel. Often,
> 
> IIRC they are allowed since ... recently (2.6.16, .17? can't remember). When
> the kernel tries to execute an FP instruction (and traps as a result), more
> kernel code will enable that the FP stack gets properly switched when a process
> changes between userspace-kernelspace.
> 
>> You can use "long long" for high precision math if necessary.
> 
> That will give link failure for __udivdi3. Use do_div().
> 
> 
> 	Jan

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