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

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Message-ID: <20120906203957.GW2448@linux.vnet.ibm.com>
Date:	Thu, 6 Sep 2012 13:39:57 -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,
	eric.dumazet@...il.com, darren@...art.com, fweisbec@...il.com,
	sbw@....edu, patches@...aro.org
Subject: Re: [PATCH RFC tip/core/rcu] Add callback-free CPUs

On Thu, Sep 06, 2012 at 08:21:50PM +0200, Peter Zijlstra wrote:
> On Thu, 2012-09-06 at 10:46 -0700, Paul E. McKenney wrote:
> > Getting these
> > CPUs into adaptive-tick mode more quickly reduces OS jitter, which is
> > one big expected benefit of adaptive-tick mode. 
> 
> I'm not sure I agree with that statement. Its a transition thing and
> therefore statistically irrelevant on the whole -- assuming we
> transition rarely.
> 
> And for those who cannot deal with the transition effects, a barrier was
> proposed which would wait until the system hit this state -- although
> the specific implementation of this isn't clear yet afaik.
> 
> I very much think we should focus on getting adaptive tick working as
> simple as possible and worry about transition effects later, if at all.

I expect that the RT folks will be interested as well, and they will
likely care about the transition effects because they degrade worst-case
behavior.

							Thanx, Paul

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