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

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Date:	Thu, 14 Jun 2012 17:07:30 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Peter Zijlstra <peterz@...radead.org>
cc:	paulmck@...ux.vnet.ibm.com, 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, 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?

softirqs are different. They loop ten times and then wake ksoftirqd
which runs with sched_other.

Though that's wonky, because if an interrupt arrives before ksoftirqd
can take over we loop another 10 times in irq_exit(). Rinse and
repeat.....

And the main difference is that the scheduler does not yell on the
softirq context, but it yells when an rt task monopolizes the cpu.

Thanks,

	tglx
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