linux-kernel - Re: [RFC patch 2/5] smpboot: Provide infrastructure for percpu hotplug threads

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Date:	Thu, 14 Jun 2012 18:20:39 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Tejun Heo <tj@...nel.org>
Subject: Re: [RFC patch 2/5] smpboot: Provide infrastructure for percpu
 hotplug threads

On Thu, 14 Jun 2012, Paul E. McKenney wrote:

> On Thu, Jun 14, 2012 at 04:56:29PM +0200, Peter Zijlstra wrote:
> > On Thu, 2012-06-14 at 07:47 -0700, Paul E. McKenney wrote:
> > > RCU callback processing consumes the entire CPU in RCU_BOOST case where
> > > processing runs at real-time priority.  This is analogous to RT throttling
> > > in the scheduler. 
> > 
> > But previously we can in non-preemptible softirq context, why would if
> > behave differently when done from a RT task?
> 
> In -rt, yes, but in mainline, ksoftirqd does not run at RT prio, right?
> 
> > Also, no its not quite like the throttling, that really idles the cpu
> > even if there's no SCHED_OTHER tasks to run.
> 
> Agreed, not -exactly- like throttling, but it has a broadly similar
> goal, namely to prevent a given type of processing from starving
> everything else in the system.
> 
> That said, why does throttling idle the CPU even if there is no other
> SCHED_OTHER tasks to run?

For simplicity reasons :)

Thanks,

	tglx
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