linux-kernel - Re: Changing Kernel thread priorities

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Message-ID: <1307439469.2322.235.camel@twins>
Date:	Tue, 07 Jun 2011 11:37:49 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Armin Steinhoff <armin@...inhoff.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Johannes Bauer <hannes_bauer@....at>,
	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

On Tue, 2011-06-07 at 11:40 +0200, Armin Steinhoff wrote:
> 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)

I see no requirement for any of those horrid things to be used. You can
write a full on proper kernel driver, it just cannot set kernel thread
priorities to a sane value (let them all default to 50 or so).

Then have a user space script or whatever set the kthread priorities.

> 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.

No, changing the priority in the kernel is a perfectly sound operation,
it just doesn't make any sense to do so since its impossible to
determine a proper priority.

Therefore setting a priority is a pure user policy and should not be
done by the driver itself -- it simply cannot do it right so why bother
doing it.

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