cpumask: Introduce DYING mask

Introduce a cpumask that indicates (for each CPU) what direction the
CPU hotplug is currently going. Notably, it tracks rollbacks. Eg. when
an up fails and we do a roll-back down, it will accurately reflect the
direction.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210310150109.151441252@infradead.org

include/linux/cpumask.h

@@ -91,10 +91,12 @@ extern struct cpumask __cpu_possible_mask;
 extern struct cpumask __cpu_online_mask;
 extern struct cpumask __cpu_present_mask;
 extern struct cpumask __cpu_active_mask;
+extern struct cpumask __cpu_dying_mask;
 #define cpu_possible_mask ((const struct cpumask *)&__cpu_possible_mask)
 #define cpu_online_mask   ((const struct cpumask *)&__cpu_online_mask)
 #define cpu_present_mask  ((const struct cpumask *)&__cpu_present_mask)
 #define cpu_active_mask   ((const struct cpumask *)&__cpu_active_mask)
+#define cpu_dying_mask    ((const struct cpumask *)&__cpu_dying_mask)
 
 extern atomic_t __num_online_cpus;
 
@@ -826,6 +828,14 @@ set_cpu_active(unsigned int cpu, bool active)
 		cpumask_clear_cpu(cpu, &__cpu_active_mask);
 }
 
+static inline void
+set_cpu_dying(unsigned int cpu, bool dying)
+{
+	if (dying)
+		cpumask_set_cpu(cpu, &__cpu_dying_mask);
+	else
+		cpumask_clear_cpu(cpu, &__cpu_dying_mask);
+}
 
 /**
  * to_cpumask - convert an NR_CPUS bitmap to a struct cpumask *
@@ -900,6 +910,11 @@ static inline bool cpu_active(unsigned int cpu)
 	return cpumask_test_cpu(cpu, cpu_active_mask);
 }
 
+static inline bool cpu_dying(unsigned int cpu)
+{
+	return cpumask_test_cpu(cpu, cpu_dying_mask);
+}
+
 #else
 
 #define num_online_cpus()	1U
@@ -927,6 +942,11 @@ static inline bool cpu_active(unsigned int cpu)
 	return cpu == 0;
 }
 
+static inline bool cpu_dying(unsigned int cpu)
+{
+	return false;
+}
+
 #endif /* NR_CPUS > 1 */
 
 #define cpu_is_offline(cpu)	unlikely(!cpu_online(cpu))

kernel/cpu.c

@@ -160,6 +160,9 @@ static int cpuhp_invoke_callback(unsigned int cpu, enum cpuhp_state state,
 	int (*cb)(unsigned int cpu);
 	int ret, cnt;
 
+	if (cpu_dying(cpu) != !bringup)
+		set_cpu_dying(cpu, !bringup);
+
 	if (st->fail == state) {
 		st->fail = CPUHP_INVALID;
 		return -EAGAIN;
@@ -2512,6 +2515,9 @@ EXPORT_SYMBOL(__cpu_present_mask);
 struct cpumask __cpu_active_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_active_mask);
 
+struct cpumask __cpu_dying_mask __read_mostly;
+EXPORT_SYMBOL(__cpu_dying_mask);
+
 atomic_t __num_online_cpus __read_mostly;
 EXPORT_SYMBOL(__num_online_cpus);