[PATCH 14/17] kernel/cpu: add num_present_cpu counter

Yury Norov
2021-12-18T21:22:17Z

Similarly to the online cpus, the cpu_present_mask is actively used
in the kernel. This patch adds a counter for present cpus, so that
users that call num_present_cpus() would know the result immediately,
instead of calling the bitmap_weight for the mask.

Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
include/linux/cpumask.h | 25 +++++++++++++++----------
kernel/cpu.c | 16 ++++++++++++++++
2 files changed, 31 insertions(+), 10 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 0be2504d8e4c..c2a9d15e2cbd 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -100,6 +100,7 @@ extern struct cpumask __cpu_dying_mask;

extern atomic_t __num_online_cpus;
extern atomic_t __num_possible_cpus;
+extern atomic_t __num_present_cpus;

extern cpumask_t cpus_booted_once_mask;

@@ -873,15 +874,7 @@ void init_cpu_online(const struct cpumask *src);

void set_cpu_possible(unsigned int cpu, bool possible);
void reset_cpu_possible_mask(void);
-
-static inline void
-set_cpu_present(unsigned int cpu, bool present)
-{
- if (present)
- cpumask_set_cpu(cpu, &__cpu_present_mask);
- else
- cpumask_clear_cpu(cpu, &__cpu_present_mask);
-}
+void set_cpu_present(unsigned int cpu, bool present);

void set_cpu_online(unsigned int cpu, bool online);

@@ -965,7 +958,19 @@ static inline unsigned int num_possible_cpus(void)
{
return atomic_read(&__num_possible_cpus);
}
-#define num_present_cpus() cpumask_weight(cpu_present_mask)
+
+/**
+ * num_present_cpus() - Read the number of present CPUs
+ *
+ * Despite the fact that __num_present_cpus is of type atomic_t, this
+ * interface gives only a momentary snapshot and is not protected against
+ * concurrent CPU hotplug operations unless invoked from a cpuhp_lock held
+ * region.
+ */
+static inline unsigned int num_present_cpus(void)
+{
+ return atomic_read(&__num_present_cpus);
+}
#define num_active_cpus() cpumask_weight(cpu_active_mask)

static inline bool cpu_online(unsigned int cpu)
diff --git a/kernel/cpu.c b/kernel/cpu.c
index a0a815911173..1f7ea1bdde1a 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2597,6 +2597,9 @@ EXPORT_SYMBOL(__cpu_online_mask);
struct cpumask __cpu_present_mask __read_mostly;
EXPORT_SYMBOL(__cpu_present_mask);

+atomic_t __num_present_cpus __read_mostly;
+EXPORT_SYMBOL(__num_present_cpus);
+
struct cpumask __cpu_active_mask __read_mostly;
EXPORT_SYMBOL(__cpu_active_mask);

@@ -2609,6 +2612,7 @@ EXPORT_SYMBOL(__num_online_cpus);
void init_cpu_present(const struct cpumask *src)
{
cpumask_copy(&__cpu_present_mask, src);
+ atomic_set(&__num_present_cpus, cpumask_weight(cpu_present_mask));
}

void init_cpu_possible(const struct cpumask *src)
@@ -2662,6 +2666,18 @@ void set_cpu_possible(unsigned int cpu, bool possible)
}
EXPORT_SYMBOL(set_cpu_possible);

+void set_cpu_present(unsigned int cpu, bool present)
+{
+ if (present) {
+ if (!cpumask_test_and_set_cpu(cpu, &__cpu_present_mask))
+ atomic_inc(&__num_present_cpus);
+ } else {
+ if (cpumask_test_and_clear_cpu(cpu, &__cpu_present_mask))
+ atomic_dec(&__num_present_cpus);
+ }
+}
+EXPORT_SYMBOL(set_cpu_present);
+
/*
* Activate the first processor.