linux-kernel - Re: [RT] rt priority losing

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Date:	Mon, 24 Jul 2006 18:44:49 +0200
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Esben Nielsen <nielsen.esben@...glemail.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	"Duetsch, Thomas LDE1" <thomas.duetsch@...mens.com>
Subject: Re: [RT] rt priority losing

At Mon, 24 Jul 2006 12:37:22 -0400,
Steven Rostedt wrote:
> > > So this can be a problem, if the softirq function holds a lock of a high
> > > priority task, and is running boosted.  If another timer goes off with a
> > > lower priority, we can lower the priority of the softirqd and lose the
> > > inherited priority that it was running at.
> > 
> > There is a check for that inside setscheduler():
> >  	p->prio = rt_mutex_getprio(p);
> 
> OK, you are right about this.  The PI chain should not be affected.  But
> this could still be a problem if the softirq was running at a high prio
> for a task when a lower prio callback needs to be made.  It looks like
> timer is removed from the base before the function runs.  So when the
> interrupt looks at the base to determine the priority to set it at, it
> might actually lower the priority of a running hrtimer thread.

The correct solution is to run the callback in the context of
the thread which will receive it and get rid of the softirq.

I experimented with this already, but its more than a saturday
afternoon project.

	tglx
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