Messages in this thread Patch in this message | | | From | Yury Norov <> | Subject | [PATCH 4/4] lib/cpumask: add FORCE_NR_CPUS config option | Date | Mon, 29 Aug 2022 09:57:48 -0700 |
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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> --- include/linux/cpumask.h | 4 ++-- include/linux/smp.h | 4 ++++ kernel/smp.c | 2 ++ lib/Kconfig | 10 ++++++++++ 4 files changed, 18 insertions(+), 2 deletions(-)
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h index b7b863cf8831..702c05329853 100644 --- a/include/linux/cpumask.h +++ b/include/linux/cpumask.h @@ -35,8 +35,8 @@ typedef struct cpumask { DECLARE_BITMAP(bits, NR_CPUS); } cpumask_t; */ #define cpumask_pr_args(maskp) nr_cpu_ids, cpumask_bits(maskp) -#if NR_CPUS == 1 -#define nr_cpu_ids 1U +#if (NR_CPUS == 1) || defined(CONFIG_FORCE_NR_CPUS) +#define nr_cpu_ids ((unsigned int)NR_CPUS) #else extern unsigned int nr_cpu_ids; #endif diff --git a/include/linux/smp.h b/include/linux/smp.h index c9e7a9abef2f..e9e74e08c886 100644 --- a/include/linux/smp.h +++ b/include/linux/smp.h @@ -183,7 +183,11 @@ static inline int get_boot_cpu_id(void) static inline void set_nr_cpu_ids(unsigned int nr) { +#ifdef CONFIG_FORCE_NR_CPUS + WARN_ON(nr_cpu_ids != nr); +#else nr_cpu_ids = nr; +#endif } #else /* !SMP */ diff --git a/kernel/smp.c b/kernel/smp.c index 27d4ed0aee55..9cd1d02221ba 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -1088,9 +1088,11 @@ static int __init maxcpus(char *str) early_param("maxcpus", maxcpus); +#ifndef CONFIG_FORCE_NR_CPUS /* Setup number of possible processor ids */ unsigned int nr_cpu_ids __read_mostly = NR_CPUS; EXPORT_SYMBOL(nr_cpu_ids); +#endif /* An arch may set nr_cpu_ids earlier if needed, so this would be redundant */ void __init setup_nr_cpu_ids(void) diff --git a/lib/Kconfig b/lib/Kconfig index dc1ab2ed1dc6..5de23382e5f3 100644 --- a/lib/Kconfig +++ b/lib/Kconfig @@ -527,6 +527,16 @@ config CPUMASK_OFFSTACK them on the stack. This is a bit more expensive, but avoids stack overflow. +config FORCE_NR_CPUS + bool "NR_CPUS is set to an actual number of CPUs" + depends on SMP + default n + 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. + config CPU_RMAP bool depends on SMP -- 2.34.1
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