[patch V3 09/20] cpumask: Cache num_possible_cpus()

Thomas Gleixner
2025-10-29T13:09:13Z

Reevaluating num_possible_cpus() over and over does not make sense. That
becomes a constant after init as cpu_possible_mask is marked ro_after_init.

Cache the value during initialization and provide that for consumption.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Yury Norov <yury.norov@gmail.com>
---
V2: New patch
---
include/linux/cpumask.h | 10 ++++++++--
kernel/cpu.c | 15 +++++++++++++++
2 files changed, 23 insertions(+), 2 deletions(-)

--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -126,6 +126,7 @@ extern struct cpumask __cpu_dying_mask;
#define cpu_dying_mask ((const struct cpumask *)&__cpu_dying_mask)

extern atomic_t __num_online_cpus;
+extern unsigned int __num_possible_cpus;

extern cpumask_t cpus_booted_once_mask;

@@ -1152,13 +1153,13 @@ void init_cpu_possible(const struct cpum
#define __assign_cpu(cpu, mask, val) \
__assign_bit(cpumask_check(cpu), cpumask_bits(mask), (val))

-#define set_cpu_possible(cpu, possible) assign_cpu((cpu), &__cpu_possible_mask, (possible))
#define set_cpu_enabled(cpu, enabled) assign_cpu((cpu), &__cpu_enabled_mask, (enabled))
#define set_cpu_present(cpu, present) assign_cpu((cpu), &__cpu_present_mask, (present))
#define set_cpu_active(cpu, active) assign_cpu((cpu), &__cpu_active_mask, (active))
#define set_cpu_dying(cpu, dying) assign_cpu((cpu), &__cpu_dying_mask, (dying))

void set_cpu_online(unsigned int cpu, bool online);
+void set_cpu_possible(unsigned int cpu, bool possible);

/**
* to_cpumask - convert a NR_CPUS bitmap to a struct cpumask *
@@ -1211,7 +1212,12 @@ static __always_inline unsigned int num_
{
return raw_atomic_read(&__num_online_cpus);
}
-#define num_possible_cpus() cpumask_weight(cpu_possible_mask)
+
+static __always_inline unsigned int num_possible_cpus(void)
+{
+ return __num_possible_cpus;
+}
+
#define num_enabled_cpus() cpumask_weight(cpu_enabled_mask)
#define num_present_cpus() cpumask_weight(cpu_present_mask)
#define num_active_cpus() cpumask_weight(cpu_active_mask)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -3108,6 +3108,9 @@ EXPORT_SYMBOL(__cpu_dying_mask);
atomic_t __num_online_cpus __read_mostly;
EXPORT_SYMBOL(__num_online_cpus);

+unsigned int __num_possible_cpus __ro_after_init = NR_CPUS;
+EXPORT_SYMBOL(__num_possible_cpus);
+
void init_cpu_present(const struct cpumask *src)
{
cpumask_copy(&__cpu_present_mask, src);
@@ -3116,6 +3119,7 @@ void init_cpu_present(const struct cpuma
void init_cpu_possible(const struct cpumask *src)
{
cpumask_copy(&__cpu_possible_mask, src);
+ __num_possible_cpus = cpumask_weight(&__cpu_possible_mask);
}

void set_cpu_online(unsigned int cpu, bool online)
@@ -3139,6 +3143,17 @@ void set_cpu_online(unsigned int cpu, bo
}
}

+void set_cpu_possible(unsigned int cpu, bool possible)
+{
+ if (possible) {
+ if (!cpumask_test_and_set_cpu(cpu, &__cpu_possible_mask))
+ __num_possible_cpus++;
+ } else {
+ if (cpumask_test_and_clear_cpu(cpu, &__cpu_possible_mask))
+ __num_possible_cpus--;
+ }
+}
+
/*
* Activate the first processor.
*/