Re: [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@xxxxxxxxx> 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@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@xxxxxxxxxxxxxx

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