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From: Thomas Gleixner <tglx@linutronix.de>
Subject: Re: Setting the priority of an IRQ thread
Date: Wed, 17 Jun 2009 11:17:03 +0200 (CEST)
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Cc: Martin Shepherd <mcs@astro.caltech.edu>,
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On Wed, 17 Jun 2009, Thomas Gleixner wrote:
> On Tue, 16 Jun 2009, Uwe Kleine-König wrote:
> > On Tue, Jun 16, 2009 at 07:59:28AM +0000, Martin Shepherd wrote:
> > > Suresh Kumar SHUKLA <suresh.shukla <at> st.com> writes:
> > > > 	// get IRQ descriptor from IRQ, it contains pid
> > > > 	desc = &irq_desc[IRQ_BASIC_TIMER_3_1];
> > > 
> > > That's a very useful clue. I was thinking that I would have to walk the process
> > > tree to find the thread by name. Following up on this clue, it appears as though
> > > in kernel 2.6.29.4 I should be able to use desc=irq_to_desc(irq) to look up the
> > > IRQ descriptor, then use pid=get_task_pid(desc->thread), to get the PID of the
> > > IRQ thread, then use sys_sched_setscheduler(pid,...) to change its priority.
> > > I'll try that out in the morning.
> > I wouldn't recommend calling sys_sched_setscheduler from kernel space.
> > That's the userspace API and you need to pass a __user pointer as third
> > argument.
> > 
> > The right way is to do this change from userspace.
> 
> What a nonsense. Care to look at the code ?
> 
> The irq/softirq threads call sys_sched_setscheduler() already to
> change their priorities.

More awake, it should be changed for correctness sake. :)

> The right way to do this is to have an interface
> set_irq_thread_prio(irq, prio)

This still stands, if we need a way to change the prio from the
driver.

But the more I think about it should be done from user space.

Thanks,

	tglx
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