linux-kernel - Re: Changing Kernel thread priorities

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Date:	Tue, 07 Jun 2011 11:40:02 +0200
From:	Armin Steinhoff <armin@...inhoff.de>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Johannes Bauer <hannes_bauer@....at>,
	Peter Zijlstra <peterz@...radead.org>,
	Monica Puig-Pey <puigpeym@...can.es>,
	Rolando Martins <rolando.martins@...il.com>,
	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: Changing Kernel thread priorities


Hi,

when I read all these confusing statements here ( in german it looks 
like an "Eiertanz")  ... I can only say:

- do the basic stuff in a minimal kernel driver
- use UIO (or VFIO for PCI devices)

and you get clean control about your real-time priorities.

I think changing the priorities of "interrupt threads" inside the kernel 
could lead to strange race conditions in the kernel.
That seems to be the reason for that "Eiertanz" here :)

--Armin





Thomas Gleixner wrote:
> On Tue, 7 Jun 2011, Johannes Bauer wrote:
>
> Please stop top-posting and use proper line breaks at 78
>
>>> Peter Zijlstra wrote:
>>>> "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.
>> 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.
> Feel free to do that for your embedded system and carry the patch for
> yourself if you think it's worth to avoid the extra init script.
>
> But we do _not_ add stuff like this to the mainline simply because
> there is no way to find a prio setting which is appropriate for all
> users of a particular driver.
>
> Aside of that the extra init script is definitely less annoying to
> maintain than the crap you need to hack into random drivers.
>
> Thanks,
>
> 	tglx
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