On Tue, Sep 7, 2021 at 4:35 PM Nathan Chancellor [off-list ref] wrote:

Won't your example only fix the issue with CONFIG_CPUMASK_OFFSTACK=y

Yes, but..

or am I misreading the gigantic comment in include/linux/cpumask.h?

you're not misreading the comment, but you are missing this important fact:

  config NR_CPUS_RANGE_END
        int
        depends on X86_64
        default 8192 if  SMP && CPUMASK_OFFSTACK
        default  512 if  SMP && !CPUMASK_OFFSTACK
        default    1 if !SMP

so basically you can't choose more than 512 CPU's unless
CPUMASK_OFFSTACK is set.

Of course, we may have some bug in the Kconfig elsewhere, and I didn't
check other architectures. So maybe there's some way to work around
it.

But basically the rule is that CPUMASK_OFFSTACK and NR_CPUS are linked.

That linkage is admittedly a bit hidden and much too subtle. I think
the only real reason why it's done that way is because people wanted
to do test builds with CPUMASK_OFFSTACK even without having to have
some ludicrous number of NR_CPUS.

You'll notice that the question "CPUMASK_OFFSTACK" is only enabled if
DEBUG_PER_CPU_MAPS is true.

That whole "for debugging" reason made more sense a decade ago when
this was all new and fancy.

It might make more sense to do that very explicitly, and make
CPUMASK_OFFSTACK be just something like

  config NR_CPUS_RANGE_END
        def_bool NR_CPUS <= 512

and get rid of the subtlety and choice in the matter.

             Linus

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