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From:   Geert Uytterhoeven <geert@linux-m68k.org>
Date:   Wed, 19 Oct 2022 08:58:53 +0200
Message-ID: <CAMuHMdX3S2GS_YW4KP63TSWN2MEyusrvC6fh1ep7KMjU2OYHcg@mail.gmail.com>
Subject: Re: [PATCH v2 5/5] lib/cpumask: add FORCE_NR_CPUS config option
To:     Yury Norov <yury.norov@gmail.com>
Cc:     Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        Andreas Schwab <schwab@suse.de>, linux-kernel@vger.kernel.org,
        Rasmus Villemoes <linux@rasmusvillemoes.dk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Stephen Rothwell <sfr@canb.auug.org.au>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        "Paul E . McKenney" <paulmck@kernel.org>,
        Vlastimil Babka <vbabka@suse.cz>,
        Dmitry Vyukov <dvyukov@google.com>,
        Valentin Schneider <vschneid@redhat.com>,
        Sander Vanheule <sander@svanheule.net>,
        Alexey Klimov <klimov.linux@gmail.com>,
        Eric Biggers <ebiggers@google.com>,
        linux-riscv <linux-riscv@lists.infradead.org>
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Hi Yury,

On Tue, Oct 18, 2022 at 6:19 PM Yury Norov <yury.norov@gmail.com> wrote:
> On Tue, Oct 18, 2022 at 05:15:41PM +0200, Geert Uytterhoeven wrote:
> > On Tue, Oct 18, 2022 at 5:01 PM Yury Norov <yury.norov@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.
>
> ...
>
> > 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.

Quite possible this is just an issue with the RISC-V CPU sparse hart ID
handling code.  E.g. arm64 works fine with cpu@{0,1,2,3,100,101,102,103}
and CONFIG_NR_CPUS=8.  NR_CPUS in arch/riscv/Kconfig has always
defaulted to at least 8, while all upstream DTS files describe only
4 Linux-capable CPU cores (+ 1 management core).

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@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