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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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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