[PATCH] Hotplug CPUs: cpu_down()

Implement cpu_down(): uses stop_machine to freeze the machine, then
uses (arch-specific) __cpu_disable() and migrate_all_tasks().

Whole thing under CONFIG_HOTPLUG_CPU, so doesn't break archs which
don't define that.

include/linux/cpu.h

@@ -63,6 +63,7 @@ extern struct semaphore cpucontrol;
 	static struct notifier_block fn##_nb = { fn, pri };	\
 	register_cpu_notifier(&fn##_nb);			\
 }
+int cpu_down(unsigned int cpu);
 #define cpu_is_offline(cpu) unlikely(!cpu_online(cpu))
 #else
 #define lock_cpu_hotplug()	do { } while (0)

kernel/cpu.c

 /* CPU control.
- * (C) 2001 Rusty Russell
+ * (C) 2001, 2002, 2003, 2004 Rusty Russell
+ *
  * This code is licenced under the GPL.
  */
 #include <linux/proc_fs.h>
@@ -10,6 +11,9 @@
 #include <linux/unistd.h>
 #include <linux/cpu.h>
 #include <linux/module.h>
+#include <linux/kmod.h>		/* for hotplug_path */
+#include <linux/kthread.h>
+#include <linux/stop_machine.h>
 #include <asm/semaphore.h>
 
 /* This protects CPUs going up and down... */
@@ -38,6 +42,125 @@ void unregister_cpu_notifier(struct notifier_block *nb)
 }
 EXPORT_SYMBOL(unregister_cpu_notifier);
 
+#ifdef CONFIG_HOTPLUG_CPU
+static inline void check_for_tasks(int cpu, struct task_struct *k)
+{
+	struct task_struct *p;
+
+	write_lock_irq(&tasklist_lock);
+	for_each_process(p) {
+		if (task_cpu(p) == cpu && p != k)
+			printk(KERN_WARNING "Task %s is on cpu %d\n",
+				p->comm, cpu);
+	}
+	write_unlock_irq(&tasklist_lock);
+}
+
+/* Notify userspace when a cpu event occurs, by running '/sbin/hotplug
+ * cpu' with certain environment variables set.  */
+static int cpu_run_sbin_hotplug(unsigned int cpu, const char *action)
+{
+	char *argv[3], *envp[5], cpu_str[12], action_str[32];
+	int i;
+
+	sprintf(cpu_str, "CPU=%d", cpu);
+	sprintf(action_str, "ACTION=%s", action);
+	/* FIXME: Add DEVPATH. --RR */
+
+	i = 0;
+	argv[i++] = hotplug_path;
+	argv[i++] = "cpu";
+	argv[i] = NULL;
+
+	i = 0;
+	/* minimal command environment */
+	envp[i++] = "HOME=/";
+	envp[i++] = "PATH=/sbin:/bin:/usr/sbin:/usr/bin";
+	envp[i++] = cpu_str;
+	envp[i++] = action_str;
+	envp[i] = NULL;
+
+	return call_usermodehelper(argv[0], argv, envp, 0);
+}
+
+/* Take this CPU down. */
+static int take_cpu_down(void *unused)
+{
+	int err;
+
+	/* Take offline: makes arch_cpu_down somewhat easier. */
+	cpu_clear(smp_processor_id(), cpu_online_map);
+
+	/* Ensure this CPU doesn't handle any more interrupts. */
+	err = __cpu_disable();
+	if (err < 0)
+		cpu_set(smp_processor_id(), cpu_online_map);
+	else
+		/* Everyone else gets kicked off. */
+		migrate_all_tasks();
+
+	return err;
+}
+
+int cpu_down(unsigned int cpu)
+{
+	int err;
+	struct task_struct *p;
+
+	if ((err = lock_cpu_hotplug_interruptible()) != 0)
+		return err;
+
+	if (num_online_cpus() == 1) {
+		err = -EBUSY;
+		goto out;
+	}
+
+	if (!cpu_online(cpu)) {
+		err = -EINVAL;
+		goto out;
+	}
+
+	p = __stop_machine_run(take_cpu_down, NULL, cpu);
+	if (IS_ERR(p)) {
+		err = PTR_ERR(p);
+		goto out;
+	}
+
+	if (cpu_online(cpu))
+		goto out_thread;
+
+	check_for_tasks(cpu, p);
+
+	/* Wait for it to sleep (leaving idle task). */
+	while (!idle_cpu(cpu))
+		yield();
+
+	/* This actually kills the CPU. */
+	__cpu_die(cpu);
+
+	/* Move it here so it can run. */
+	kthread_bind(p, smp_processor_id());
+
+	/* CPU is completely dead: tell everyone.  Too late to complain. */
+	if (notifier_call_chain(&cpu_chain, CPU_DEAD, (void *)(long)cpu)
+	    == NOTIFY_BAD)
+		BUG();
+
+	cpu_run_sbin_hotplug(cpu, "offline");
+
+out_thread:
+	err = kthread_stop(p);
+out:
+	unlock_cpu_hotplug();
+	return err;
+}
+#else
+static inline int cpu_run_sbin_hotplug(unsigned int cpu, const char *action)
+{
+	return 0;
+}
+#endif /*CONFIG_HOTPLUG_CPU*/
+
 int __devinit cpu_up(unsigned int cpu)
 {
 	int ret;