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Date:	Sun, 20 Jul 2014 02:35:39 +0200
From:	Frederic Weisbecker <>
To:	Peter Zijlstra <>
Cc:	"Paul E. McKenney" <>,
	LKML <>,
	Ingo Molnar <>,
	赖江山 <>,
	Dipankar Sarma <>,
	Andrew Morton <>,
	Mathieu Desnoyers <>,
	Josh Triplett <>,
	Thomas Gleixner <>,
	Steven Rostedt <>,
	David Howells <>,
	Eric Dumazet <>,
	Darren Hart <>,
	Oleg Nesterov <>,
	pranith kumar <>
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 <>:
> 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
( 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:
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