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

[Yury Norov]

On Tue, Oct 18, 2022 at 05:15:41PM +0200, Geert Uytterhoeven wrote:
> 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.

...

> 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.

The problem is seemingly unrelated to CONFIG_FORCE_NR_CPUS...
If so, we don't need ARCH_UNFORCE_NR_CPUS. Is that right?

This all looks weird. RISCV hasn't an arch code to setup nr_cpu_ids,
and therefore should use generic setup_nr_cpu_ids(), which is:

  void __init setup_nr_cpu_ids(void)
  {
          set_nr_cpu_ids(find_last_bit(cpumask_bits(cpu_possible_mask), NR_CPUS) + 1);
  }

Where:

  static inline void set_nr_cpu_ids(unsigned int nr)
  {
  #if (NR_CPUS == 1) || defined(CONFIG_FORCE_NR_CPUS)
          WARN_ON(nr != nr_cpu_ids);
  #else
          nr_cpu_ids = nr;
  #endif
  }


As you can see, at this point cpu_possible_mask is initialized based
on DT, and even if arch has non-dense cpu_possible_mask, the logic
should still be correct.

Wish I could tell more, if I had an access to the hardware...

Thanks,
Yury