Messages in this thread | | | From | Geert Uytterhoeven <> | Date | Tue, 18 Oct 2022 15:15:32 +0200 | Subject | Re: [PATCH v2 5/5] lib/cpumask: add FORCE_NR_CPUS config option |
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Hi Yury,
On Tue, Oct 18, 2022 at 10:21 AM Geert Uytterhoeven <geert@linux-m68k.org> wrote: > On Tue, Sep 6, 2022 at 1:10 AM Yury Norov <yury.norov@gmail.com> wrote: > > The size of cpumasks is hard-limited by compile-time parameter NR_CPUS, > > but defined at boot-time when kernel parses ACPI/DT tables, and stored in > > nr_cpu_ids. In many practical cases, number of CPUs for a target is known > > at compile time, and can be provided with NR_CPUS. > > > > In that case, compiler may be instructed to rely on NR_CPUS as on actual > > number of CPUs, not an upper limit. It allows to optimize many cpumask > > routines and significantly shrink size of the kernel image. > > > > This patch adds FORCE_NR_CPUS option to teach the compiler to rely on > > NR_CPUS and enable corresponding optimizations. > > > > If FORCE_NR_CPUS=y, kernel will not set nr_cpu_ids at boot, but only check > > that the actual number of possible CPUs is equal to NR_CPUS, and WARN if > > that doesn't hold. > > > > The new option is especially useful in embedded applications because > > kernel configurations are unique for each SoC, the number of CPUs is > > constant and known well, and memory limitations are typically harder. > > > > For my 4-CPU ARM64 build with NR_CPUS=4, FORCE_NR_CPUS=y saves 46KB: > > add/remove: 3/4 grow/shrink: 46/729 up/down: 652/-46952 (-46300) > > > > Signed-off-by: Yury Norov <yury.norov@gmail.com> > > Thanks for your patch, which is now commit 6f9c07be9d020489 > ("lib/cpumask: add FORCE_NR_CPUS config option") in v6.1-rc1. > > FORCE_NR_CPUS is enabled for e.g. an allmodconfig kernel, which I > believe now makes it unsafe to boot such a kernel on any system that > does not have exactly CONFIG_NR_CPUS CPU cores? > > If my assumption is true, this really needs some protection to prevent > enabling this option inadvertently, as it is quite common to boot > allmodconfig kernels for testing.
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.
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
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