linux-kernel - Re: [PATCH tip/core/rcu 8/9] nohz_full: Add full-system-idle state machine

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Date:	Sun, 8 Sep 2013 12:46:41 +0200
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Yann E. MORIN" <yann.morin.1998@...e.fr>
Cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Ingo Molnar <mingo@...e.hu>, laijs@...fujitsu.com,
	dipankar@...ibm.com, Andrew Morton <akpm@...ux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	josh@...htriplett.org, niv@...ibm.com,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	David Howells <dhowells@...hat.com>, edumazet@...gle.com,
	darren@...art.com, sbw@....edu,
	Linux-Arch <linux-arch@...r.kernel.org>,
	linux-kbuild <linux-kbuild@...r.kernel.org>
Subject: Re: [PATCH tip/core/rcu 8/9] nohz_full: Add full-system-idle state
 machine

On Sun, Sep 08, 2013 at 12:32:50PM +0200, Yann E. MORIN wrote:
> Paul, All,
> 
> On 2013-09-07 11:57 -0700, Paul E. McKenney spake thusly:
> > On Sat, Sep 07, 2013 at 11:13:48AM +0200, Yann E. MORIN wrote:
> [--SNIP--]
> > > I'll see if I can come up with a meaningfull construct that fixes your
> > > use-case. Don't hold your breath, though! ;-)
> > 
> > If not, we need to add NR_CPUS to the architectures lacking them...
> 
> Unfortunately, I was not able to come up with anything suitable.
> 
> I think your proposal to always define NR_CPUS=1 for architectures
> without SMP support is a good solution.
> 
> After all, if !SMP because the architecture does not support it, I
> believe it makes sense that NR_CPUS be defined to 1.
> 
> Unless NR_CPUS carries with it a hidden meaning about SMP being
> possible, that is, which would probably be wrong anyway, since we have
> SMP for this.

Right, if possible, the best would be to define CONFIG_NR_CPUS to a single
central place, like arch/Kconfig:

	config NR_CPUS
	default 1 if !SMP

It's a single int after all. Specific arch constraints can probably be overriden
from the arch.

> 
> Regards,
> Yann E. MORIN.
> 
> -- 
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