linux-kernel - Re: [PATCH 0/6] support "dataplane" mode for nohz

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Message-ID: <55510218.9090104@ezchip.com>
Date:	Mon, 11 May 2015 15:25:12 -0400
From:	Chris Metcalf <cmetcalf@...hip.com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>
CC:	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gilad Ben Yossef <giladb@...hip.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>, Tejun Heo <tj@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Christoph Lameter <cl@...ux.com>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	<linux-doc@...r.kernel.org>, <linux-api@...r.kernel.org>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/6] support "dataplane" mode for nohz_full

On 05/11/2015 03:19 PM, Mike Galbraith wrote:
> I really shouldn't have acked nohz_full -> isolcpus.  Beside the fact
> that old static isolcpus was_supposed_  to crawl off and die, I know
> beyond doubt that having isolated a cpu as well as you can definitely
> does NOT imply that said cpu should become tickless.

True, at a high level, I agree that it would be better to have a
top-level concept like Frederic's proposed ISOLATION that includes
isolcpus and nohz_cpu (and other stuff as needed).

That said, what you wrote above is wrong; even with the patch you
acked, setting isolcpus does not automatically turn on nohz_full for
a given cpu.  The patch made it true the other way around: when
you say nohz_full, you automatically get isolcpus on that cpu too.
That does, at least, make sense for the semantics of nohz_full.

-- 
Chris Metcalf, EZChip Semiconductor
http://www.ezchip.com

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