[PATCH 13/17] kernel/cpu: add num_possible_cpus counter

Yury Norov
2021-12-18T21:22:24Z

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

Suggested-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Yury Norov <yury.norov@gmail.com>
---
include/linux/cpumask.h | 30 ++++++++++++++++--------------
kernel/cpu.c | 22 ++++++++++++++++++++++
2 files changed, 38 insertions(+), 14 deletions(-)

diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index 1906e3225737..0be2504d8e4c 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -99,6 +99,7 @@ extern struct cpumask __cpu_dying_mask;
#define cpu_dying_mask ((const struct cpumask *)&__cpu_dying_mask)

extern atomic_t __num_online_cpus;
+extern atomic_t __num_possible_cpus;

extern cpumask_t cpus_booted_once_mask;

@@ -870,19 +871,8 @@ void init_cpu_present(const struct cpumask *src);
void init_cpu_possible(const struct cpumask *src);
void init_cpu_online(const struct cpumask *src);

-static inline void reset_cpu_possible_mask(void)
-{
- bitmap_zero(cpumask_bits(&__cpu_possible_mask), NR_CPUS);
-}
-
-static inline void
-set_cpu_possible(unsigned int cpu, bool possible)
-{
- if (possible)
- cpumask_set_cpu(cpu, &__cpu_possible_mask);
- else
- cpumask_clear_cpu(cpu, &__cpu_possible_mask);
-}
+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)
@@ -962,7 +952,19 @@ static inline unsigned int num_online_cpus(void)
{
return atomic_read(&__num_online_cpus);
}
-#define num_possible_cpus() cpumask_weight(cpu_possible_mask)
+
+/**
+ * num_possible_cpus() - Read the number of possible CPUs
+ *
+ * Despite the fact that __num_possible_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_possible_cpus(void)
+{
+ return atomic_read(&__num_possible_cpus);
+}
#define num_present_cpus() cpumask_weight(cpu_present_mask)
#define num_active_cpus() cpumask_weight(cpu_active_mask)

diff --git a/kernel/cpu.c b/kernel/cpu.c
index cd7605204d4d..a0a815911173 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2583,10 +2583,13 @@ EXPORT_SYMBOL(cpu_all_bits);
#ifdef CONFIG_INIT_ALL_POSSIBLE
struct cpumask __cpu_possible_mask __read_mostly
= {CPU_BITS_ALL};
+atomic_t __num_possible_cpus __read_mostly = ATOMIC_INIT(NR_CPUS);
#else
struct cpumask __cpu_possible_mask __read_mostly;
+atomic_t __num_possible_cpus __read_mostly;
#endif
EXPORT_SYMBOL(__cpu_possible_mask);
+EXPORT_SYMBOL(__num_possible_cpus);

struct cpumask __cpu_online_mask __read_mostly;
EXPORT_SYMBOL(__cpu_online_mask);
@@ -2611,6 +2614,7 @@ void init_cpu_present(const struct cpumask *src)
void init_cpu_possible(const struct cpumask *src)
{
cpumask_copy(&__cpu_possible_mask, src);
+ atomic_set(&__num_possible_cpus, cpumask_weight(cpu_possible_mask));
}

void init_cpu_online(const struct cpumask *src)
@@ -2640,6 +2644,24 @@ void set_cpu_online(unsigned int cpu, bool online)
}
}

+void reset_cpu_possible_mask(void)
+{
+ bitmap_zero(cpumask_bits(&__cpu_possible_mask), NR_CPUS);
+ atomic_set(&__num_possible_cpus, 0);
+}
+
+void set_cpu_possible(unsigned int cpu, bool possible)
+{
+ if (possible) {
+ if (!cpumask_test_and_set_cpu(cpu, &__cpu_possible_mask))
+ atomic_inc(&__num_possible_cpus);
+ } else {
+ if (cpumask_test_and_clear_cpu(cpu, &__cpu_possible_mask))
+ atomic_dec(&__num_possible_cpus);
+ }
+}
+EXPORT_SYMBOL(set_cpu_possible);
+
/*
* Activate the first processor.
*/
--
2.30.2