[patch V2 05/12] cpu/hotplug: Provide knob to control SMT

[Thomas Gleixner <tglx@linutronix.de>]

Subject: [patch V2 05/12] cpu/hotplug: Provide knob to control SMT
From: Thomas Gleixner <tglx@linutronix.de>

Proide a command line and a sysfs knob to control SMT.

Switching SMT control off, offlines all online CPUs which are secondary
threads of a physical core and prevents onlining of secondary threads after
that point.

Switching it back on lifts the restrictions again, but does not online the
offlined secondary siblings.

It also can be set to force off, which is an irreversible operation and
later changes will add support for x86 to discard secondary threads in the
MP table and ACPI/MADT.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 Documentation/ABI/testing/sysfs-devices-system-cpu |   16 ++
 Documentation/admin-guide/kernel-parameters.txt    |    4 
 arch/Kconfig                                       |    3 
 arch/x86/Kconfig                                   |    1 
 include/linux/cpu.h                                |   12 +
 kernel/cpu.c                                       |  132 +++++++++++++++++++++
 6 files changed, 168 insertions(+)

--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -490,3 +490,19 @@ Description:	Information about CPU vulne
 		"Not affected"	  CPU is not affected by the vulnerability
 		"Vulnerable"	  CPU is affected and no mitigation in effect
 		"Mitigation: $M"  CPU is affected and mitigation $M is in effect
+
+What:		/sys/devices/system/cpu/smt
+		/sys/devices/system/cpu/smt/active
+		/sys/devices/system/cpu/smt/control
+Date:		June 2018
+Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
+Description:	Control Symetric Multi Threading (SMT)
+
+		active:  Tells whether SMT is active (enabled and available)
+
+		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.
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -2687,6 +2687,10 @@
 	nosmt		[KNL,S390] Disable symmetric multithreading (SMT).
 			Equivalent to smt=1.
 
+			[KNL,x86] Disable symmetric multithreading (SMT).
+			nosmt=force: Force disable SMT, similar to disabling
+				     it in the BIOS.
+
 	nospectre_v2	[X86] Disable all mitigations for the Spectre variant 2
 			(indirect branch prediction) vulnerability. System may
 			allow data leaks with this option, which is equivalent
--- a/arch/Kconfig
+++ b/arch/Kconfig
@@ -13,6 +13,9 @@ config KEXEC_CORE
 config HAVE_IMA_KEXEC
 	bool
 
+config HOTPLUG_SMT
+	bool
+
 config OPROFILE
 	tristate "OProfile system profiling"
 	depends on PROFILING
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -185,6 +185,7 @@ config X86
 	select HAVE_SYSCALL_TRACEPOINTS
 	select HAVE_UNSTABLE_SCHED_CLOCK
 	select HAVE_USER_RETURN_NOTIFIER
+	select HOTPLUG_SMT			if SMP
 	select IRQ_FORCED_THREADING
 	select NEED_SG_DMA_LENGTH
 	select PCI_LOCKLESS_CONFIG
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -166,4 +166,16 @@ void cpuhp_report_idle_dead(void);
 static inline void cpuhp_report_idle_dead(void) { }
 #endif /* #ifdef CONFIG_HOTPLUG_CPU */
 
+enum {
+	CPU_SMT_ENABLED,
+	CPU_SMT_DISABLED,
+	CPU_SMT_FORCE_DISABLED,
+};
+
+#if defined(CONFIG_SMP) && defined(CONFIG_HOTPLUG_SMT)
+extern int cpu_smt_control;
+#else
+# define cpu_smt_control		(CPU_SMT_ENABLED)
+#endif
+
 #endif /* _LINUX_CPU_H_ */
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -933,6 +933,29 @@ EXPORT_SYMBOL(cpu_down);
 #define takedown_cpu		NULL
 #endif /*CONFIG_HOTPLUG_CPU*/
 
+#ifdef CONFIG_HOTPLUG_SMT
+int cpu_smt_control __read_mostly = CPU_SMT_ENABLED;
+
+static int __init smt_cmdline_disable(char *str)
+{
+	cpu_smt_control = CPU_SMT_DISABLED;
+	if (str && !strcmp(str, "force")) {
+		pr_info("SMT: Force disabled\n");
+		cpu_smt_control = CPU_SMT_FORCE_DISABLED;
+	}
+	return 0;
+}
+early_param("nosmt", smt_cmdline_disable);
+
+static inline bool cpu_smt_blocked(unsigned int cpu)
+{
+	return cpu_smt_control != CPU_SMT_ENABLED &&
+		!topology_is_primary_thread(cpu);
+}
+#else
+static inline bool cpu_smt_blocked(unsigned int cpu) { return false; }
+#endif
+
 /**
  * notify_cpu_starting(cpu) - Invoke the callbacks on the starting CPU
  * @cpu: cpu that just started
@@ -1056,6 +1079,10 @@ static int do_cpu_up(unsigned int cpu, e
 		err = -EBUSY;
 		goto out;
 	}
+	if (cpu_smt_blocked(cpu)) {
+		err = -EPERM;
+		goto out;
+	}
 
 	err = _cpu_up(cpu, 0, target);
 out:
@@ -1904,10 +1931,115 @@ static const struct attribute_group cpuh
 	NULL
 };
 
+#ifdef CONFIG_HOTPLUG_SMT
+
+static const char *smt_states[] = {
+	"on",
+	"off",
+	"forceoff",
+};
+
+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 int cpuhp_smt_disable(int val)
+{
+	int cpu, ret = 0;
+
+	cpu_maps_update_begin();
+	for_each_online_cpu(cpu) {
+		if (topology_is_primary_thread(cpu))
+			continue;
+		ret = cpu_down_maps_locked(cpu, CPUHP_OFFLINE);
+		if (ret)
+			break;
+	}
+	if (!ret)
+		cpu_smt_control = val;
+	cpu_maps_update_done();
+	return ret;
+}
+
+static int cpuhp_smt_enable(void)
+{
+	cpu_maps_update_begin();
+	cpu_smt_control = CPU_SMT_ENABLED;
+	cpu_maps_update_done();
+	return 0;
+}
+
+static ssize_t
+store_smt_control(struct device *dev, struct device_attribute *attr,
+		  const char *buf, size_t count)
+{
+	int val, ret;
+
+	if (sysfs_streq(buf, "on"))
+		val = CPU_SMT_ENABLED;
+	else if (sysfs_streq(buf, "off"))
+		val = CPU_SMT_DISABLED;
+	else if (sysfs_streq(buf, "forceoff"))
+		val = CPU_SMT_FORCE_DISABLED;
+	else
+		return -EINVAL;
+
+	if (cpu_smt_control == CPU_SMT_FORCE_DISABLED)
+		return -EPERM;
+
+	ret = lock_device_hotplug_sysfs();
+	if (ret)
+		return ret;
+
+	if (val != cpu_smt_control)
+		ret = val ? cpuhp_smt_disable(val) : cpuhp_smt_enable();
+
+	unlock_device_hotplug();
+	return ret ? ret : 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);
+}
+static DEVICE_ATTR(active, 0444, show_smt_active, NULL);
+
+static struct attribute *cpuhp_smt_attrs[] = {
+	&dev_attr_control.attr,
+	&dev_attr_active.attr,
+	NULL
+};
+
+static const struct attribute_group cpuhp_smt_attr_group = {
+	.attrs = cpuhp_smt_attrs,
+	.name = "smt",
+	NULL
+};
+
+static int __init cpu_smt_state_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();
+	if (ret)
+		return ret;
+
 	ret = sysfs_create_group(&cpu_subsys.dev_root->kobj,
 				 &cpuhp_cpu_root_attr_group);
 	if (ret)