linux-kernel - Re: [PATCH v2 5/5] lib/cpumask: add FORCE_NR

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Date:   Tue, 18 Oct 2022 17:15:41 +0200
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        Andreas Schwab <schwab@...e.de>, linux-kernel@...r.kernel.org,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Sander Vanheule <sander@...nheule.net>,
        Alexey Klimov <klimov.linux@...il.com>,
        Eric Biggers <ebiggers@...gle.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>
Subject: Re: [PATCH v2 5/5] lib/cpumask: add FORCE_NR_CPUS config option

Hi Yuri,

On Tue, Oct 18, 2022 at 5:01 PM Yury Norov <yury.norov@...il.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@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@...ux-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