kernel/cpu: add num_active_cpu counter

Similarly to the online cpus, the cpu_active_mask is actively used
in the kernel. This patch adds a counter for active cpus, so that
users that call num_active_cpus() would know the result immediately,
instead of calling the bitmap_weight for the mask. The time of execution
would be significant for many-cpu machines.

Signed-off-by: Yury Norov <yury.norov@gmail.com>

include/linux/cpumask.h

@@ -101,6 +101,7 @@ 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 atomic_t __num_active_cpus;
 
 extern cpumask_t cpus_booted_once_mask;
 
@@ -879,17 +880,8 @@ void init_cpu_online(const struct cpumask *src);
 void set_cpu_possible(unsigned int cpu, bool possible);
 void reset_cpu_possible_mask(void);
 void set_cpu_present(unsigned int cpu, bool present);
-
 void set_cpu_online(unsigned int cpu, bool online);
-
-static inline void
-set_cpu_active(unsigned int cpu, bool active)
-{
-	if (active)
-		cpumask_set_cpu(cpu, &__cpu_active_mask);
-	else
-		cpumask_clear_cpu(cpu, &__cpu_active_mask);
-}
+void set_cpu_active(unsigned int cpu, bool active);
 
 static inline void
 set_cpu_dying(unsigned int cpu, bool dying)
@@ -975,7 +967,19 @@ static inline unsigned int num_present_cpus(void)
 {
 	return atomic_read(&__num_present_cpus);
 }
-#define num_active_cpus()	cpumask_weight(cpu_active_mask)
+
+/**
+ * num_active_cpus() - Read the number of online CPUs
+ *
+ * Despite the fact that __num_active_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_active_cpus(void)
+{
+	return atomic_read(&__num_active_cpus);
+}
 
 static inline bool cpu_online(unsigned int cpu)
 {

kernel/cpu.c

@@ -2603,6 +2603,9 @@ EXPORT_SYMBOL(__num_present_cpus);
 struct cpumask __cpu_active_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_active_mask);
 
+atomic_t __num_active_cpus __read_mostly;
+EXPORT_SYMBOL(__num_active_cpus);
+
 struct cpumask __cpu_dying_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_dying_mask);
 
@@ -2675,6 +2678,17 @@ void set_cpu_present(unsigned int cpu, bool present)
 	}
 }
 
+void set_cpu_active(unsigned int cpu, bool active)
+{
+	if (active) {
+		if (!cpumask_test_and_set_cpu(cpu, &__cpu_active_mask))
+			atomic_inc(&__num_active_cpus);
+	} else {
+		if (cpumask_test_and_clear_cpu(cpu, &__cpu_active_mask))
+			atomic_dec(&__num_active_cpus);
+	}
+}
+
 /*
  * Activate the first processor.
  */