[PATCH v2 5/5] lib/cpumask: add FORCE_NR_CPUS config option

[Geert Uytterhoeven]

Hi Yuri,

On Tue, Oct 18, 2022 at 5:01 PM Yury Norov <yury.norov at gmail.com> wrote:
> On Tue, Oct 18, 2022 at 05:44:09PM +0300, Andy Shevchenko wrote:
> > On Tue, Oct 18, 2022 at 07:35:09AM -0700, Yury Norov wrote:
> > > On Tue, Oct 18, 2022 at 03:50:31PM +0200, Geert Uytterhoeven wrote:
> >
> > ...
> >
> > > For those who choose FORCE_NR_CPUS, it's required to set NR_CPUS
> > > to a value that matches to what's parsed from DT.
> > >
> > > Can you please look at the draft below that disables FORCE_NR_CPUS
> > > in allmodconfig? If it's OK with you, I'll send a patch. If you think
> > > that there are architectures where it's not possible to set correct
> > > NR_CPUS at compile time for some reason, I'll add ARCH_UNFORCE_NR_CPUS
> > > option.
> >
> > Instead you may simply add
> >
> >       depends on CONFIG_$ARCH/$MACHINE=n
> >
> > and so on to the FORCE_NR_CPUS, no?
>
> Yes, if there's just one machine like that. If there's many of them, the
> 'depends' list would be too long.
>
> I hope there's no such a weird machines, and we don't need that at
> all. Let's see what Geert will say.

I haven't tried the patch from your other email yet, but I did try
CONFIG_NR_CPUS=4 and CONFIG_FORCE_NR_CPUS=y on
Icicle earlier today.

There was no warning, as the number of CPUs did match, but the
fourth CPU (cpu at 4, i.e. the fifth core in DT) failed to come online:

    CPU3: failed to come online
    smp: Brought up 1 node, 3 CPUs

BTW, it behaves the same with CONFIG_FORCE_NR_CPUS=n.
Increasing CONFIG_NR_CPUS (before I used 8) makes the fourth
CPU core come online again.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds