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    SubjectRe: [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@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



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