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

[Yury Norov]

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 at gmail.com>
Signed-off-by: Yury Norov <yury.norov at 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.
  */
-- 
2.30.2