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 12:32:05 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Geert Uytterhoeven <geert@...ux-m68k.org>
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 06, 2013 at 08:50:41PM +0200, Geert Uytterhoeven wrote:
> 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

Seemed like a good idea, but I still get:

	make O=/tmp/e ARCH=m68k defconfig
	  GEN     /tmp/e/Makefile
	*** Default configuration is based on 'multi_defconfig'
	kernel/time/Kconfig:140:warning: 'NR_CPUS_REALLY': number is invalid
	#
	# configuration written to .config
	#

Diff below in case I messed something up.

> > 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.

Agreed, should it prove possible.  ;-)

							Thanx, Paul

------------------------------------------------------------------------

diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index 3381f09..cb7a932 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -134,6 +134,11 @@ config NO_HZ_FULL_ALL
 	 Note the boot CPU will still be kept outside the range to
 	 handle the timekeeping duty.
 
+config NR_CPUS_REALLY
+	int "Fixed version of NR_CPUS"
+	default 1 if !SMP
+	default NR_CPUS if SMP
+
 config NO_HZ_FULL_SYSIDLE
 	bool "Detect full-system idle state for full dynticks system"
 	depends on NO_HZ_FULL
@@ -160,7 +165,7 @@ config NO_HZ_FULL_SYSIDLE
 config NO_HZ_FULL_SYSIDLE_SMALL
 	int "Number of CPUs above which large-system approach is used"
 	depends on NO_HZ_FULL_SYSIDLE
-	range 1 NR_CPUS
+	range 1 NR_CPUS_REALLY
 	default 8
 	help
 	 The full-system idle detection mechanism takes a lazy approach

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