From: Yury Norov yury.norov@gmail.com
Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?i...
lib/cpumask: add FORCE_NR_CPUS config option
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 Signed-off-by: Zheng Zucheng zhengzucheng@huawei.com --- include/linux/cpumask.h | 10 +++++++--- kernel/smp.c | 2 +- lib/Kconfig | 9 +++++++++ 3 files changed, 17 insertions(+), 4 deletions(-)
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h index cb8f29fd0477..2e50162d9908 100644 --- a/include/linux/cpumask.h +++ b/include/linux/cpumask.h @@ -33,16 +33,20 @@ 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
static inline void set_nr_cpu_ids(unsigned int nr) { +#if (NR_CPUS == 1) || defined(CONFIG_FORCE_NR_CPUS) + WARN_ON(nr != nr_cpu_ids); +#else nr_cpu_ids = nr; -} #endif +}
/* Deprecated. Always use nr_cpu_ids. */ #define nr_cpumask_bits nr_cpu_ids diff --git a/kernel/smp.c b/kernel/smp.c index 843c7e46389f..4b13a7ef6a31 100644 --- a/kernel/smp.c +++ b/kernel/smp.c @@ -852,7 +852,7 @@ static int __init maxcpus(char *str)
early_param("maxcpus", maxcpus);
-#if (NR_CPUS > 1) +#if (NR_CPUS > 1) && !defined(CONFIG_FORCE_NR_CPUS) /* Setup number of possible processor ids */ unsigned int nr_cpu_ids __read_mostly = NR_CPUS; EXPORT_SYMBOL(nr_cpu_ids); diff --git a/lib/Kconfig b/lib/Kconfig index 36326864249d..8026964596fd 100644 --- a/lib/Kconfig +++ b/lib/Kconfig @@ -494,6 +494,15 @@ 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 + 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