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

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Message-ID: <20120614154556.GD2458@linux.vnet.ibm.com>
Date:	Thu, 14 Jun 2012 08:45:56 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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, 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?

							Thanx, Paul

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