[PATCH v2] cpu/hotplug: Create SMT sysfs interface for all arches

[Josh Poimboeuf]

Make the /sys/devices/system/cpu/smt/* files available on all arches, so
user space has a consistent way to detect whether SMT is enabled.

The 'control' file now shows 'notimplemented' for architectures which
don't yet have CONFIG_HOTPLUG_SMT.

Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
v2: add "notimplemented" pseudo-state [tglx]

 .../ABI/testing/sysfs-devices-system-cpu      | 10 +--
 include/linux/cpu.h                           |  2 +-
 kernel/cpu.c                                  | 72 +++++++++++--------
 3 files changed, 51 insertions(+), 33 deletions(-)

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index 9605dbd4b5b5..5eea46fefcb2 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -511,10 +511,12 @@ Description:	Control Symetric Multi Threading (SMT)
 		control: Read/write interface to control SMT. Possible
 			 values:
 
-			 "on"		SMT is enabled
-			 "off"		SMT is disabled
-			 "forceoff"	SMT is force disabled. Cannot be changed.
-			 "notsupported" SMT is not supported by the CPU
+			 "on"		  SMT is enabled
+			 "off"		  SMT is disabled
+			 "forceoff"	  SMT is force disabled. Cannot be changed.
+			 "notsupported"   SMT is not supported by the CPU
+			 "notimplemented" SMT runtime toggling is not
+					  implemented for the architecture
 
 			 If control status is "forceoff" or "notsupported" writes
 			 are rejected.
diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 5041357d0297..296753249343 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -182,7 +182,7 @@ extern enum cpuhp_smt_control cpu_smt_control;
 extern void cpu_smt_disable(bool force);
 extern void cpu_smt_check_topology(void);
 #else
-# define cpu_smt_control		(CPU_SMT_ENABLED)
+# define cpu_smt_control		(CPU_SMT_NOT_SUPPORTED)
 static inline void cpu_smt_disable(bool force) { }
 static inline void cpu_smt_check_topology(void) { }
 #endif
diff --git a/kernel/cpu.c b/kernel/cpu.c
index f69ba38573c2..b0614263963b 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -2019,19 +2019,6 @@ static const struct attribute_group cpuhp_cpu_root_attr_group = {
 
 #ifdef CONFIG_HOTPLUG_SMT
 
-static const char *smt_states[] = {
-	[CPU_SMT_ENABLED]		= "on",
-	[CPU_SMT_DISABLED]		= "off",
-	[CPU_SMT_FORCE_DISABLED]	= "forceoff",
-	[CPU_SMT_NOT_SUPPORTED]		= "notsupported",
-};
-
-static ssize_t
-show_smt_control(struct device *dev, struct device_attribute *attr, char *buf)
-{
-	return snprintf(buf, PAGE_SIZE - 2, "%s\n", smt_states[cpu_smt_control]);
-}
-
 static void cpuhp_offline_cpu_device(unsigned int cpu)
 {
 	struct device *dev = get_cpu_device(cpu);
@@ -2102,9 +2089,10 @@ static int cpuhp_smt_enable(void)
 	return ret;
 }
 
+
 static ssize_t
-store_smt_control(struct device *dev, struct device_attribute *attr,
-		  const char *buf, size_t count)
+__store_smt_control(struct device *dev, struct device_attribute *attr,
+		    const char *buf, size_t count)
 {
 	int ctrlval, ret;
 
@@ -2120,9 +2108,6 @@ store_smt_control(struct device *dev, struct device_attribute *attr,
 	if (cpu_smt_control == CPU_SMT_FORCE_DISABLED)
 		return -EPERM;
 
-	if (cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
-		return -ENODEV;
-
 	ret = lock_device_hotplug_sysfs();
 	if (ret)
 		return ret;
@@ -2142,14 +2127,49 @@ store_smt_control(struct device *dev, struct device_attribute *attr,
 	unlock_device_hotplug();
 	return ret ? ret : count;
 }
+
+#else /* !CONFIG_HOTPLUG_SMT */
+static ssize_t
+__store_smt_control(struct device *dev, struct device_attribute *attr,
+		    const char *buf, size_t count)
+{
+	return 0;
+}
+#endif /* CONFIG_HOTPLUG_SMT */
+
+static const char *smt_states[] = {
+	[CPU_SMT_ENABLED]		= "on",
+	[CPU_SMT_DISABLED]		= "off",
+	[CPU_SMT_FORCE_DISABLED]	= "forceoff",
+	[CPU_SMT_NOT_SUPPORTED]		= "notsupported",
+};
+
+static ssize_t
+show_smt_control(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	const char *state = IS_ENABLED(CONFIG_HOTPLUG_SMT) ?
+				smt_states[cpu_smt_control] :
+				"notimplemented";
+
+	return snprintf(buf, PAGE_SIZE - 2, "%s\n", state);
+}
+
+static ssize_t
+store_smt_control(struct device *dev, struct device_attribute *attr,
+		  const char *buf, size_t count)
+{
+
+	if (cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
+		return -ENODEV;
+
+	return __store_smt_control(dev, attr, buf, count);
+}
 static DEVICE_ATTR(control, 0644, show_smt_control, store_smt_control);
 
 static ssize_t
 show_smt_active(struct device *dev, struct device_attribute *attr, char *buf)
 {
-	bool active = topology_max_smt_threads() > 1;
-
-	return snprintf(buf, PAGE_SIZE - 2, "%d\n", active);
+	return snprintf(buf, PAGE_SIZE - 2, "%d\n", sched_smt_active());
 }
 static DEVICE_ATTR(active, 0444, show_smt_active, NULL);
 
@@ -2165,21 +2185,17 @@ static const struct attribute_group cpuhp_smt_attr_group = {
 	NULL
 };
 
-static int __init cpu_smt_state_init(void)
+static int __init cpu_smt_sysfs_init(void)
 {
 	return sysfs_create_group(&cpu_subsys.dev_root->kobj,
 				  &cpuhp_smt_attr_group);
 }
 
-#else
-static inline int cpu_smt_state_init(void) { return 0; }
-#endif
-
 static int __init cpuhp_sysfs_init(void)
 {
 	int cpu, ret;
 
-	ret = cpu_smt_state_init();
+	ret = cpu_smt_sysfs_init();
 	if (ret)
 		return ret;
 
@@ -2200,7 +2216,7 @@ static int __init cpuhp_sysfs_init(void)
 	return 0;
 }
 device_initcall(cpuhp_sysfs_init);
-#endif
+#endif /* CONFIG_SYSFS && CONFIG_HOTPLUG_CPU */
 
 /*
  * cpu_bit_bitmap[] is a special, "compressed" data structure that
-- 
2.17.2