linux-kernel - Re: Changing Kernel thread priorities

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Message-ID: <4DEDE232.4090007@unican.es>
Date:	Tue, 7 Jun 2011 10:32:50 +0200
From:	Monica Puig-Pey <puigpeym@...can.es>
To:	<hannes_bauer@....at>
CC:	Peter Zijlstra <peterz@...radead.org>,
	Rolando Martins <rolando.martins@...il.com>,
	<linux-rt-users@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
	tglx <tglx@...utronix.de>
Subject: Re: Changing Kernel thread priorities

El 07/06/11 10:27, Johannes Bauer escribió:
> Absolutly correct!
>
> However, if you are running the system on an embedded platform, where the _WHOLE_ system (including priorities) is preconfigured and never touched, starting a irq thread with the right prio from start is a more straightforward method than having to invoke a script that changes it using userspace chrt tool.
>
> Regards JB
> ----- Ursprüngliche Nachricht -----
> Von: "Peter Zijlstra"<peterz@...radead.org>
> Erhalten: 07.06.2011 00:36
> An: hannes_bauer@....at
>
> "Monica Puig-Pey" wrote:
>>
>> I need to change the priority from inside the driver, when creating the
>> kernel thread.
>
> No you don't. How does you driver know about what priority is correct
> wrt all the other running RT tasks on the system?
>
> Determining the right priority in a fixed priority scheduling system is
> a system wide problem, nobody but the administrator can possibly even
> begin to solve it.
>
> There's a reason all RT irq threads are started at 50, its plain
> impossible to do better.
>

That's it!
If I work with embedded system where I know all my tasks running and 
there is not a user how could I do it?

What I tried is create the kernel thread in my init_module using:

	#include <linux/kthread.h>

	struct task_struct *kthread_create(int (*threadfn)(void *data),
				   void *data,
				   const char namefmt[], ...)
and then running it with:

	#include <linux/sched.h>
	
	extern int wake_up_process(struct task_struct *tsk);

These functions stars a Kthread which has a NON RT priority. I can see 
this using the ps command from user space.
Because it's not a real time thread is why I want, better need, to 
change its priority, to have only real time threads running in my 
driver. I want to use the Kthread as a bottom half for the interrupts.

How could I create real time kernel threads then? is kthread_create 
wrong for real time enviroment?


-- 
__________________________________________________________________________________

Mónica Puig-Pey González       E-mail: puigpeym@...can.es

Grupo de Computadores y Tiempo Real, Departamento de Electrónica y 
Computadores.
Facultad de Ciencias - Universidad de Cantabria
Av. de los Castros s/n. 39005 - Santander, España
__________________________________________________________________________________
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