present_cpu

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

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;
 
@@ -877,15 +878,7 @@ 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);
 
@@ -969,7 +962,19 @@ 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 online 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)

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)
@@ -2660,6 +2664,17 @@ void set_cpu_possible(unsigned int cpu, bool 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);
+	}
+}
+
 /*
  * Activate the first processor.
  */