cpu/SMT: Allow enabling partial SMT states via sysfs

Add support to the /sys/devices/system/cpu/smt/control interface for
enabling a specified number of SMT threads per core, including partial
SMT states where not all threads are brought online.

The current interface accepts "on" and "off", to enable either 1 or all
SMT threads per core.

This commit allows writing an integer, between 1 and the number of SMT
threads supported by the machine. Writing 1 is a synonym for "off", 2 or
more enables SMT with the specified number of threads.

When reading the file, if all threads are online "on" is returned, to
avoid changing behaviour for existing users. If some other number of
threads is online then the integer value is returned.

Architectures like x86 only supporting 1 thread or all threads, should not
define CONFIG_SMT_NUM_THREADS_DYNAMIC. Architecture supporting partial SMT
states, like PowerPC, should define it.

[ ldufour: Slightly reword the commit's description ]
[ ldufour: Remove switch() in __store_smt_control() ]
[ ldufour: Rix build issue in control_show() ]
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Laurent Dufour <ldufour@linux.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20230705145143.40545-8-ldufour@linux.ibm.com

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -555,6 +555,7 @@ Description:	Control Symmetric Multi Threading (SMT)
 			 ================ =========================================
 			 "on"		  SMT is enabled
 			 "off"		  SMT is disabled
+			 "<N>"		  SMT is enabled with N threads per core.
 			 "forceoff"	  SMT is force disabled. Cannot be changed.
 			 "notsupported"   SMT is not supported by the CPU
 			 "notimplemented" SMT runtime toggling is not

kernel/cpu.c

@@ -2876,11 +2876,19 @@ static const struct attribute_group cpuhp_cpu_root_attr_group = {
 
 #ifdef CONFIG_HOTPLUG_SMT
 
+static bool cpu_smt_num_threads_valid(unsigned int threads)
+{
+	if (IS_ENABLED(CONFIG_SMT_NUM_THREADS_DYNAMIC))
+		return threads >= 1 && threads <= cpu_smt_max_threads;
+	return threads == 1 || threads == cpu_smt_max_threads;
+}
+
 static ssize_t
 __store_smt_control(struct device *dev, struct device_attribute *attr,
 		    const char *buf, size_t count)
 {
-	int ctrlval, ret;
+	int ctrlval, ret, num_threads, orig_threads;
+	bool force_off;
 
 	if (cpu_smt_control == CPU_SMT_FORCE_DISABLED)
 		return -EPERM;
@@ -2888,30 +2896,39 @@ __store_smt_control(struct device *dev, struct device_attribute *attr,
 	if (cpu_smt_control == CPU_SMT_NOT_SUPPORTED)
 		return -ENODEV;
 
-	if (sysfs_streq(buf, "on"))
+	if (sysfs_streq(buf, "on")) {
 		ctrlval = CPU_SMT_ENABLED;
-	else if (sysfs_streq(buf, "off"))
+		num_threads = cpu_smt_max_threads;
+	} else if (sysfs_streq(buf, "off")) {
 		ctrlval = CPU_SMT_DISABLED;
-	else if (sysfs_streq(buf, "forceoff"))
+		num_threads = 1;
+	} else if (sysfs_streq(buf, "forceoff")) {
 		ctrlval = CPU_SMT_FORCE_DISABLED;
-	else
+		num_threads = 1;
+	} else if (kstrtoint(buf, 10, &num_threads) == 0) {
+		if (num_threads == 1)
+			ctrlval = CPU_SMT_DISABLED;
+		else if (cpu_smt_num_threads_valid(num_threads))
+			ctrlval = CPU_SMT_ENABLED;
+		else
+			return -EINVAL;
+	} else {
 		return -EINVAL;
+	}
 
 	ret = lock_device_hotplug_sysfs();
 	if (ret)
 		return ret;
 
-	if (ctrlval != cpu_smt_control) {
-		switch (ctrlval) {
-		case CPU_SMT_ENABLED:
-			ret = cpuhp_smt_enable();
-			break;
-		case CPU_SMT_DISABLED:
-		case CPU_SMT_FORCE_DISABLED:
-			ret = cpuhp_smt_disable(ctrlval);
-			break;
-		}
-	}
+	orig_threads = cpu_smt_num_threads;
+	cpu_smt_num_threads = num_threads;
+
+	force_off = ctrlval != cpu_smt_control && ctrlval == CPU_SMT_FORCE_DISABLED;
+
+	if (num_threads > orig_threads)
+		ret = cpuhp_smt_enable();
+	else if (num_threads < orig_threads || force_off)
+		ret = cpuhp_smt_disable(ctrlval);
 
 	unlock_device_hotplug();
 	return ret ? ret : count;
@@ -2939,6 +2956,17 @@ static ssize_t control_show(struct device *dev,
 {
 	const char *state = smt_states[cpu_smt_control];
 
+#ifdef CONFIG_HOTPLUG_SMT
+	/*
+	 * If SMT is enabled but not all threads are enabled then show the
+	 * number of threads. If all threads are enabled show "on". Otherwise
+	 * show the state name.
+	 */
+	if (cpu_smt_control == CPU_SMT_ENABLED &&
+	    cpu_smt_num_threads != cpu_smt_max_threads)
+		return sysfs_emit(buf, "%d\n", cpu_smt_num_threads);
+#endif
+
 	return snprintf(buf, PAGE_SIZE - 2, "%s\n", state);
 }