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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>
This commit is contained in:
Yury Norov 2022-09-05 16:08:20 -07:00
parent 546a073d62
commit 6f9c07be9d
3 changed files with 17 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

10
include/linux/cpumask.h
View File

@ -35,16 +35,20 @@ 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

2
kernel/smp.c
View File

@ -1088,7 +1088,7 @@ 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);

9
lib/Kconfig
View File

@ -527,6 +527,15 @@ 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