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

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Message-ID: <20130906173047.GT3966@linux.vnet.ibm.com>
Date:	Fri, 6 Sep 2013 10:30:47 -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 10:08:22AM +0200, Geert Uytterhoeven wrote:
> On Tue, Aug 20, 2013 at 4:47 AM, Paul E. McKenney
> <paulmck@...ux.vnet.ibm.com> wrote:
> > --- a/kernel/time/Kconfig
> > +++ b/kernel/time/Kconfig
> > @@ -157,6 +157,33 @@ config NO_HZ_FULL_SYSIDLE
> >
> >          Say N if you are unsure.
> >
> > +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
> 
> This causes "kernel/time/Kconfig:162:warning: range is invalid" on m68k and
> all other architectures that do not support SMP.
> 
> How to reproduce:
>     make ARCH=m68k defconfig

OK, this does complain, but seems to give a reasonable .config file.
(From what I can tell.)  It would clearly be good to get rid of the
complaint.

> 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?

> Hence in non-interactive configs (e.g. "make defconfig"),
> NO_HZ_FULL_SYSIDLE_SMALL will end up as 0.
> In interactive configs (e.g. "make oldconfig") Kconfig suggest "0" as
> the default,
> but refuses to accept it as it doesn't fall within the range 1..0.
> 
> How to reproduce:
>     Remove the "depends on NO_HZ_FULL_SYSIDLE"
>     make ARCH=powerpc mpc83xx_defconfig
>     grep NO_HZ_FULL_SYSIDLE_SMALL .config
>     -> CONFIG_NO_HZ_FULL_SYSIDLE_SMALL=0
>     sed 's/CONFIG_NO_HZ_FULL_SYSIDLE_SMALL=0//g' -i .config
>     make ARCH=powerpc oldconfig
>     -> no value is accepted

If it turns out that there is some reason by NR_CPUS=1 is impossible,
there are a few things that I could do:

I could just leave the range off, which would allow people to give
nonsense values.  This would be harmless in the code, for example,
a negative value would simply disable small-system handling, while
a too-large value would similarly disable large-system handling.
Might be a bit obnoxious for the guy who typoed and then wasted a
kernel build/boot/test cycle, but it is an option.

I could use a small fixed range (say from 1 to 64), which would
provide at least some checking.  In the unlikely event that someone
really wants more than 64 CPUs handled with small-system handling,
we could revisit at that point.

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":

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

However, Kconfig complained about the use of NR_CPUS even though this
was under an "if" whose condition was not set.  Perhaps someone with
better Kconfig-fu than I have can come up with something.

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.  ;-)

							Thanx, Paul

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