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Date:	Sun, 20 Jul 2014 02:35:39 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...nel.org>,
	赖江山 <laijs@...fujitsu.com>,
	Dipankar Sarma <dipankar@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Josh Triplett <josh@...htriplett.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>,
	Eric Dumazet <edumazet@...gle.com>,
	Darren Hart <dvhart@...ux.intel.com>,
	Oleg Nesterov <oleg@...hat.com>,
	pranith kumar <bobby.prani@...il.com>
Subject: Re: [PATCH tip/core/rcu] Do not keep timekeeping CPU tick running for
 non-nohz_full= CPUs

2014-07-19 20:28 GMT+02:00 Peter Zijlstra <peterz@...radead.org>:
> On Sat, Jul 19, 2014 at 08:01:24PM +0200, Frederic Weisbecker wrote:
>> This can be changed by allowing timekeeping duty from all non-nohz_full CPUs, that's
>> the initial direction I took, but it involved a lot of complications and scalability
>> issues.
>
> How so, currently any CPU can be timekeeper, how is any !nohz_full cpu
> different?

If timekeeping becomes a movable target in nohz full then we need to
make rcu_sys_is_idle() callable concurrently and we must send the
timekeeping-wakeup IPI to a possibly moving target. All that is a
predictable nightmare both in terms of complexity and scalability.

That's the direction I took initially
(https://lkml.org/lkml/2013/12/17/708) but I quickly resigned. The
changestat needed to be doubled to do it correctly. Moreover having
non-nohz-full CPUs other than CPU 0 is expected to be a corner case. A
corner case for a barely used feature (nohz full) as of today.

Also you might want to read tglx opinion on movable timekeepers in
nohz full: http://marc.info/?i=alpine.DEB.2.02.1405092358390.6261%40ionos.tec.linutronix.de
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