linux-kernel - Re: [PATCH tip/core/rcu 0/2] v2 Add callback-free CPUs

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Message-ID: <20121031074402.GV3027@linux.vnet.ibm.com>
Date:	Wed, 31 Oct 2012 00:44:02 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, laijs@...fujitsu.com,
	dipankar@...ibm.com, akpm@...ux-foundation.org,
	mathieu.desnoyers@...icios.com, josh@...htriplett.org,
	niv@...ibm.com, tglx@...utronix.de, rostedt@...dmis.org,
	Valdis.Kletnieks@...edu, dhowells@...hat.com, edumazet@...gle.com,
	darren@...art.com, fweisbec@...il.com, sbw@....edu,
	patches@...aro.org, joe.korty@...r.com
Subject: Re: [PATCH tip/core/rcu 0/2] v2 Add callback-free CPUs

On Wed, Oct 31, 2012 at 08:25:21AM +0100, Peter Zijlstra wrote:
> On Tue, 2012-10-30 at 20:45 -0700, Paul E. McKenney wrote:
> > This commit therefore adds the ability
> > for selected CPUs ("rcu_nocbs=" boot parameter) to have their
> > callbacks
> > offloaded to kthreads, inspired by Joe Korty's and Jim Houston's JRCU.
> > If the "rcu_nocb_poll" boot parameter is also specified, these
> > kthreads
> > will do polling, removing the need for the offloaded CPUs to do
> > wakeups. 
> 
> How much pain to make that dynamically configurable?

A fair amount of pain, though I do hope to make it runtime configurable
at some point.  Known problems include races between changes in the
nocb configuration, rcu_barrier(), grace-period initialization, and
CPU hotplug.  Should be solvable, but want to get the other bugs out of
the way first.

							Thanx, Paul

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