Messages in this thread | | | Date | Mon, 29 Aug 2022 20:02:24 +0300 | From | Andy Shevchenko <> | Subject | Re: [PATCH 4/4] lib/cpumask: add FORCE_NR_CPUS config option |
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On Mon, Aug 29, 2022 at 09:57:48AM -0700, Yury Norov 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)
...
> + default n
Redundant. 'n' _is_ default.
-- With Best Regards, Andy Shevchenko
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