[RFC PATCH 32/32] cpumask: Add enabled cpumask for present CPUs that can be brought online

[James Morse]

The 'offline' file in sysfs shows all offline CPUs, including those
that aren't present. User-space is expected to remove not-present CPUs
from thie list to learn which CPUs could be brought online.

CPUs can be present but not-enabled. These CPUs can't be brought online
until the firmware policy changes, which comes with an ACPI notification
that will register the CPUs.

With only the offline and present files, user-space is unable to
determine which CPUs it can try to bring online. Add a new CPU mask
that shows this based on all the registered CPUs.

Signed-off-by: James Morse <james.morse@xxxxxxx>
---
 drivers/base/cpu.c      | 10 ++++++++++
 include/linux/cpumask.h | 25 +++++++++++++++++++++++++
 kernel/cpu.c            |  3 +++
 3 files changed, 38 insertions(+)

diff --git a/drivers/base/cpu.c b/drivers/base/cpu.c
index bc2ce8c7f383..3cc06f9fd230 100644
--- a/drivers/base/cpu.c
+++ b/drivers/base/cpu.c
@@ -76,6 +76,7 @@ void unregister_cpu(struct cpu *cpu)
 {
 	int logical_cpu = cpu->dev.id;
 
+	set_cpu_enabled(logical_cpu, false);
 	unregister_cpu_under_node(logical_cpu, cpu_to_node(logical_cpu));
 
 	device_unregister(&cpu->dev);
@@ -265,6 +266,13 @@ static ssize_t print_cpus_offline(struct device *dev,
 }
 static DEVICE_ATTR(offline, 0444, print_cpus_offline, NULL);
 
+static ssize_t print_cpus_enabled(struct device *dev,
+				  struct device_attribute *attr, char *buf)
+{
+	return sysfs_emit(buf, "%*pbl\n", cpumask_pr_args(cpu_enabled_mask));
+}
+static DEVICE_ATTR(enabled, 0444, print_cpus_enabled, NULL);
+
 static ssize_t print_cpus_isolated(struct device *dev,
 				  struct device_attribute *attr, char *buf)
 {
@@ -384,6 +392,7 @@ int register_cpu(struct cpu *cpu, int num)
 	register_cpu_under_node(num, cpu_to_node(num));
 	dev_pm_qos_expose_latency_limit(&cpu->dev,
 					PM_QOS_RESUME_LATENCY_NO_CONSTRAINT);
+	set_cpu_enabled(num, true);
 
 	return 0;
 }
@@ -465,6 +474,7 @@ static struct attribute *cpu_root_attrs[] = {
 	&cpu_attrs[2].attr.attr,
 	&dev_attr_kernel_max.attr,
 	&dev_attr_offline.attr,
+	&dev_attr_enabled.attr,
 	&dev_attr_isolated.attr,
 #ifdef CONFIG_NO_HZ_FULL
 	&dev_attr_nohz_full.attr,
diff --git a/include/linux/cpumask.h b/include/linux/cpumask.h
index c2aa0aa26b45..1513bb00dee4 100644
--- a/include/linux/cpumask.h
+++ b/include/linux/cpumask.h
@@ -59,6 +59,7 @@ static inline void set_nr_cpu_ids(unsigned int nr)
  *
  *     cpu_possible_mask- has bit 'cpu' set iff cpu is populatable
  *     cpu_present_mask - has bit 'cpu' set iff cpu is populated
+ *     cpu_enabled_mask  - has bit 'cpu' set iff cpu can be brought online
  *     cpu_online_mask  - has bit 'cpu' set iff cpu available to scheduler
  *     cpu_active_mask  - has bit 'cpu' set iff cpu available to migration
  *
@@ -91,11 +92,13 @@ static inline void set_nr_cpu_ids(unsigned int nr)
 
 extern struct cpumask __cpu_possible_mask;
 extern struct cpumask __cpu_online_mask;
+extern struct cpumask __cpu_enabled_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_enabled_mask   ((const struct cpumask *)&__cpu_enabled_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)
@@ -921,6 +924,7 @@ extern const DECLARE_BITMAP(cpu_all_bits, NR_CPUS);
 #else
 #define for_each_possible_cpu(cpu) for_each_cpu((cpu), cpu_possible_mask)
 #define for_each_online_cpu(cpu)   for_each_cpu((cpu), cpu_online_mask)
+#define for_each_enabled_cpu(cpu)   for_each_cpu((cpu), cpu_enabled_mask)
 #define for_each_present_cpu(cpu)  for_each_cpu((cpu), cpu_present_mask)
 #endif
 
@@ -943,6 +947,15 @@ set_cpu_possible(unsigned int cpu, bool possible)
 		cpumask_clear_cpu(cpu, &__cpu_possible_mask);
 }
 
+static inline void
+set_cpu_enabled(unsigned int cpu, bool can_be_onlined)
+{
+	if (can_be_onlined)
+		cpumask_set_cpu(cpu, &__cpu_enabled_mask);
+	else
+		cpumask_clear_cpu(cpu, &__cpu_enabled_mask);
+}
+
 static inline void
 set_cpu_present(unsigned int cpu, bool present)
 {
@@ -1022,6 +1035,7 @@ static inline unsigned int num_online_cpus(void)
 	return atomic_read(&__num_online_cpus);
 }
 #define num_possible_cpus()	cpumask_weight(cpu_possible_mask)
+#define num_enabled_cpus()	cpumask_weight(cpu_enabled_mask)
 #define num_present_cpus()	cpumask_weight(cpu_present_mask)
 #define num_active_cpus()	cpumask_weight(cpu_active_mask)
 
@@ -1030,6 +1044,11 @@ static inline bool cpu_online(unsigned int cpu)
 	return cpumask_test_cpu(cpu, cpu_online_mask);
 }
 
+static inline bool cpu_enabled(unsigned int cpu)
+{
+	return cpumask_test_cpu(cpu, cpu_enabled_mask);
+}
+
 static inline bool cpu_possible(unsigned int cpu)
 {
 	return cpumask_test_cpu(cpu, cpu_possible_mask);
@@ -1054,6 +1073,7 @@ static inline bool cpu_dying(unsigned int cpu)
 
 #define num_online_cpus()	1U
 #define num_possible_cpus()	1U
+#define num_enabled_cpus()	1U
 #define num_present_cpus()	1U
 #define num_active_cpus()	1U
 
@@ -1067,6 +1087,11 @@ static inline bool cpu_possible(unsigned int cpu)
 	return cpu == 0;
 }
 
+static inline bool cpu_enabled(unsigned int cpu)
+{
+	return cpu == 0;
+}
+
 static inline bool cpu_present(unsigned int cpu)
 {
 	return cpu == 0;
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 6c0a92ca6bb5..6f4febf0142e 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2639,6 +2639,9 @@ EXPORT_SYMBOL(__cpu_possible_mask);
 struct cpumask __cpu_online_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_online_mask);
 
+struct cpumask __cpu_enabled_mask __read_mostly;
+EXPORT_SYMBOL(__cpu_enabled_mask);
+
 struct cpumask __cpu_present_mask __read_mostly;
 EXPORT_SYMBOL(__cpu_present_mask);
 
-- 
2.30.2