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

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Date:	Fri, 6 Sep 2013 20:50:41 +0200
From:	Geert Uytterhoeven <geert@...ux-m68k.org>
To:	Paul McKenney <paulmck@...ux.vnet.ibm.com>
Cc:	"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,
	Frédéric Weisbecker <fweisbec@...il.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 Fri, Sep 6, 2013 at 7:30 PM, Paul E. McKenney
<paulmck@...ux.vnet.ibm.com> wrote:
>> Furthermore, it seems only hexagon, metag, mips, and x86 set NR_CPUS to 1
>> if !SMP. On other architectures, NR_CPUS is not defined and presumed to be 0.
>
> Would it make sense to require that NR_CPUS=1 for !SMP?

Yes, this looks reasonable to me.

> I tried creating a NR_CPUS_REALLY as follows:
>
> config NR_CPUS_REALLY
>         int "Fixed version of NR_CPUS"
>         default NR_CPUS if NR_CPUS
>         default 1 if !NR_CPUS
>
> But this still gave a warning on the first "default" even though it
> was not in effect.  I also tried using Kconfig "if":

IIRC, it tries to use the first default first, so the below may work
(the "if SMP" is probably not needed):

config NR_CPUS_REALLY
        int "Fixed version of NR_CPUS"
        default 1 if !SMP
        default NR_CPUS if SMP

> Defining NR_CPUS=1 if !SMP is looking pretty good to me just now.
> This would probably have other benefits -- I cannot be the only
> person who ever wanted this.  ;-)

Sure. I just didn't want to create patches for all architectures without
having a discussion first.

And it would be nice if it cuould be done in a central place, without
touching all architectures.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds
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