Subject: Re: Xterm Hangs - Possible scheduler defect?
From: Lee Revell <rlrevell@joe-job.com>
To: Paulo Marques <pmarques@grupopie.com>
Cc: Ingo Oeser <ioe-lkml@axxeo.de>, Chris Friesen <cfriesen@nortel.com>,
       "Chad N. Tindel" <chad@tindel.net>, Mike Galbraith <EFAULT@gmx.de>,
       akpm@osdl.org, linux-kernel@vger.kernel.org
In-Reply-To: <421F49E0.9090806@grupopie.com>
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	 <200502250151.41793.ioe-lkml@axxeo.de> <421F4042.3020302@nortel.com>
	 <200502251639.50238.ioe-lkml@axxeo.de>  <421F49E0.9090806@grupopie.com>
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Date: Fri, 25 Feb 2005 11:24:27 -0500
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On Fri, 2005-02-25 at 15:53 +0000, Paulo Marques wrote:
> Ingo Oeser wrote:
> > Chris Friesen wrote:
> > 
> >>Ingo Oeser wrote:
> >>[...]
> > You would need to change the priority of task 1 until it releases the
> > mutex. Ideally the owner gets the maximum priority of
> > his and all the waiters on it, until it releases his mutex, where he regains
> > its old priority after release of mutex. But this priority elevation happens
> > only, if he is runnable. If not, he gets his old priority back, until he is 
> > runnable.
> 
> This is called a "priority inversion" problem, and there was some work 
> done by Ingo Molnar to make the scheduler aware of such cases and handle 
> them appropriatelly.
> 
> You can follow this thread for more info:
> 
> http://marc.theaimsgroup.com/?l=linux-kernel&m=110106915415886&w=2
> 

The solution to your problem (which is as old as the hills) involves
priority inheriting mutexes which are available in the RT preempt patch
(if you build with CONFIG_PREEMPT_RT).  This should be usable for hard
realtime applications.

http://people.redhat.com/mingo/realtime-preempt

If you just need very good soft realtime performance I recommend
PREEMPT_DESKTOP.

Lee

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