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Date:   Tue, 18 Oct 2022 07:35:09 -0700
From:   Yury Norov <yury.norov@gmail.com>
To:     Geert Uytterhoeven <geert@linux-m68k.org>
Cc:     Andreas Schwab <schwab@suse.de>, linux-kernel@vger.kernel.org,
        Andy Shevchenko <andriy.shevchenko@linux.intel.com>,
        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>
Subject: Re: [PATCH v2 5/5] lib/cpumask: add FORCE_NR_CPUS config option
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On Tue, Oct 18, 2022 at 03:50:31PM +0200, Geert Uytterhoeven wrote:
> Hi Andreas,
> 
> On Tue, Oct 18, 2022 at 3:41 PM Andreas Schwab <schwab@suse.de> wrote:
> > On Okt 18 2022, Geert Uytterhoeven wrote:
> > > Moreover, this cannot be used on all systems.  E.g. on Icicle Kit with
> > > Microchip PolarFire SoC, CONFIG_NR_CPUS needs to be larger than 4,
> > > as the system has actually 5 CPU cores (1xE51 and 4xU54), but Linux
> > > runs only on 4 of them.  So you cannot use FORCE_NR_CPUS=y.
> >
> > But does Linux acually see the E51 core?  On the Hifive boards it is
> > disabled in the device tree, and the cpu probing just skips it,
> > effectively resulting in only four cpus.
> 
> The E51 is indeed disabled in DT.
> The CPU parts of arch/riscv/boot/dts/sifive/fu540-c000.dtsi and
> arch/riscv/boot/dts/microchip/mpfs.dtsi arre very similar.
> Do you get 4 CPUs on Hifive with CONFIG_NR_CPUS=4?
> 
> Gr{oetje,eeting}s,

Hi Geert, Andreas,

Thanks for pointing that. Indeed, it should be disabled in
allmodconfig.

Linus also asked to make this option depending on CONFIG_EXPERT.
So, I'm working on a patch.

From general considerations, NR_CPUS defines length of cpumasks,
per-cpu arrays etc. If it's impossible to boot Linux on a specific
core, we don't need to reserve memory for all that. In other words,
number of possible CPUs in your example should be equal to 4.

When FORCE_NR_CPUS=y, the boot code still parses DT/ACPI tables for
actual numbers of CPUs, and if it doesn't equal to NR_CPUS, prints
a message in syslog. 

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.

Thanks,
Yury
---
 lib/Kconfig | 20 +++++++++++++++++++-
 1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/lib/Kconfig b/lib/Kconfig
index 9bbf8a4b2108..578cee3593d7 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -528,15 +528,33 @@ config CPUMASK_OFFSTACK
 	  them on the stack.  This is a bit more expensive, but avoids
 	  stack overflow.
 
+choice
+	prompt "Number of CPUs detection"
+	default UNFORCE_NR_CPUS
+        depends on SMP && EXPERT
+	help
+	  Select between boot-time and compile-time detection of number
+	  of CPUs. If it's possible to provide exact number of CPUs at
+	  compile-time, kernel code may be optimized better.
+
+	  For general-purpose kernel, choose "boot time" option.
+
+config UNFORCE_NR_CPUS
+	bool "Detect number of CPUs at boot time"
+	help
+	  Choose it if you build general-purpose kernel and want to rely
+	  on kernel to detect actual number of CPUs.
+
 config FORCE_NR_CPUS
        bool "NR_CPUS is set to an actual number of CPUs"
-       depends on SMP
        help
          Say Yes if you have NR_CPUS set to an actual number of possible
          CPUs in your system, not to a default value. This forces the core
          code to rely on compile-time value and optimize kernel routines
          better.
 
+endchoice
+
 config CPU_RMAP
 	bool
 	depends on SMP
-- 
2.34.1