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

["Paul E. McKenney" <paulmck@linux.vnet.ibm.com>]

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@linux.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