Subject: Re: Very high CPU load when idle with 3.0-rc1
From: Peter Zijlstra <peterz@infradead.org>
To: paulmck@linux.vnet.ibm.com
Cc: Damien Wyart <damien.wyart@free.fr>, Ingo Molnar <mingo@elte.hu>,
        Mike Galbraith <efault@gmx.de>, linux-kernel@vger.kernel.org
In-Reply-To: <20110530212833.GS2668@linux.vnet.ibm.com>
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Date: Mon, 30 May 2011 23:33:39 +0200
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On Mon, 2011-05-30 at 14:28 -0700, Paul E. McKenney wrote:
> On Mon, May 30, 2011 at 07:19:49PM +0200, Peter Zijlstra wrote:
> > On Mon, 2011-05-30 at 09:23 -0700, Paul E. McKenney wrote:
> > >         sp.sched_priority = RCU_KTHREAD_PRIO;
> > >         sched_setscheduler_nocheck(t, SCHED_FIFO, &sp); 
> > 
> > Why are those things RT tasks anyway? The old ksoftirq runs as a regular
> > task. And once you start boosting things you can boost this into FIFO as
> > well...
> > 
> > just wondering..
> 
> Because priority boosting doesn't help unless the callbacks also run
> RT priority.
> 
> I could make it so that they ran as normal tasks if !RCU_BOOST, but
> they would still need to run as RT tasks for RCU_BOOST.  I figured
> running them the same way in both cases would be simpler.

Ah, I thought you'd boost the threads along with the waiters, to the
same prio so that they wouldn't disturb higher priority tasks for no
reason.
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