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

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Date:	Tue, 12 May 2015 03:47:55 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Chris Metcalf <cmetcalf@...hip.com>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	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 Mon, 2015-05-11 at 15:25 -0400, Chris Metcalf wrote:
> 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.

I didn't write that, I wrote nohz_full implies (spelled '->') isolcpus.
Yes, with nohz_full currently being static, the old allegedly dying but
also static isolcpus scheduler off switch is a convenient thing to wire
the nohz_full CPU SET (<- hint;) property to.

	-Mike


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