Messages in this thread Patch in this message | | | Date | Tue, 18 Oct 2022 07:35:09 -0700 | From | Yury Norov <> | 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
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