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

Paul E. McKenney
2013-09-06T19:32:15Z

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@linux.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