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
Date: Tue, 18 Oct 2022 07:35:09 -0700 [thread overview]
Message-ID: <Y065nbfVQwZhwt51@yury-laptop> (raw)
In-Reply-To: <CAMuHMdVeHPR_JH0dJ+WSddjd8_hax70JRzd4pqHLNvgbh+-znw@mail.gmail.com>
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
next prev parent reply other threads:[~2022-10-18 14:37 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-05 23:08 [PATCH v2 0/5] cpumask: cleanup nr_cpu_ids vs nr_cpumask_bits mess Yury Norov
2022-09-05 23:08 ` [PATCH v2 1/5] smp: don't declare nr_cpu_ids if NR_CPUS == 1 Yury Norov
2022-09-06 8:53 ` Peter Zijlstra
2022-09-06 14:06 ` Yury Norov
2022-09-06 14:36 ` Peter Zijlstra
2022-09-06 15:01 ` Andy Shevchenko
2022-09-06 17:53 ` Peter Zijlstra
2022-09-06 15:07 ` Yury Norov
2022-09-07 8:02 ` David Laight
2022-09-05 23:08 ` [PATCH v2 2/5] smp: add set_nr_cpu_ids() Yury Norov
2022-09-05 23:08 ` [PATCH v2 3/5] lib/cpumask: delete misleading comment Yury Norov
2022-09-05 23:08 ` [PATCH v2 4/5] lib/cpumask: deprecate nr_cpumask_bits Yury Norov
2022-09-05 23:08 ` [PATCH v2 5/5] lib/cpumask: add FORCE_NR_CPUS config option Yury Norov
2022-10-18 8:21 ` Geert Uytterhoeven
2022-10-18 13:15 ` Geert Uytterhoeven
2022-10-18 13:41 ` Andreas Schwab
2022-10-18 13:50 ` Geert Uytterhoeven
2022-10-18 14:35 ` Yury Norov [this message]
2022-10-18 14:44 ` Andy Shevchenko
2022-10-18 14:59 ` Yury Norov
2022-10-18 15:15 ` Geert Uytterhoeven
2022-10-18 16:16 ` Yury Norov
2022-10-19 6:58 ` Geert Uytterhoeven
2022-09-06 8:55 ` [PATCH v2 0/5] cpumask: cleanup nr_cpu_ids vs nr_cpumask_bits mess Peter Zijlstra
2022-09-06 12:06 ` Valentin Schneider
2022-09-06 14:38 ` Peter Zijlstra
2022-09-06 15:45 ` Valentin Schneider
2022-09-06 16:26 ` Yury Norov
2022-09-06 14:48 ` Yury Norov
2022-09-06 14:35 ` Yury Norov
2022-09-15 14:45 ` Yury Norov
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