Subject: hotplug: Lightweight get online cpus
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed, 15 Jun 2011 12:36:06 +0200

get_online_cpus() is a heavy weight function which involves a global
mutex. migrate_disable() wants a simpler construct which prevents only
a CPU from going doing while a task is in a migrate disabled section.

Implement a per cpu lockless mechanism, which serializes only in the
real unplug case on a global mutex. That serialization affects only
tasks on the cpu which should be brought down.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
---
 include/linux/cpu.h |   12 +++-
 kernel/cpu.c        |  126 ++++++++++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 131 insertions(+), 7 deletions(-)

--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -1,15 +1,15 @@
 /*
  * include/linux/cpu.h - generic cpu definition
  *
- * This is mainly for topological representation. We define the 
- * basic 'struct cpu' here, which can be embedded in per-arch 
+ * This is mainly for topological representation. We define the
+ * basic 'struct cpu' here, which can be embedded in per-arch
  * definitions of processors.
  *
  * Basic handling of the devices is done in drivers/base/cpu.c
- * and system devices are handled in drivers/base/sys.c. 
+ * and system devices are handled in drivers/base/sys.c.
  *
  * CPUs are exported via sysfs in the class/cpu/devices/
- * directory. 
+ * directory.
  */
 #ifndef _LINUX_CPU_H_
 #define _LINUX_CPU_H_
@@ -177,6 +177,8 @@ extern void get_online_cpus(void);
 extern void put_online_cpus(void);
 extern void cpu_hotplug_disable(void);
 extern void cpu_hotplug_enable(void);
+extern void pin_current_cpu(void);
+extern void unpin_current_cpu(void);
 #define hotcpu_notifier(fn, pri)	cpu_notifier(fn, pri)
 #define register_hotcpu_notifier(nb)	register_cpu_notifier(nb)
 #define unregister_hotcpu_notifier(nb)	unregister_cpu_notifier(nb)
@@ -202,6 +204,8 @@ static inline void cpu_hotplug_driver_un
 #define put_online_cpus()	do { } while (0)
 #define cpu_hotplug_disable()	do { } while (0)
 #define cpu_hotplug_enable()	do { } while (0)
+static inline void pin_current_cpu(void) { }
+static inline void unpin_current_cpu(void) { }
 #define hotcpu_notifier(fn, pri)	do { (void)(fn); } while (0)
 /* These aren't inline functions due to a GCC bug. */
 #define register_hotcpu_notifier(nb)	({ (void)(nb); 0; })
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -63,6 +63,101 @@ static struct {
 	.refcount = 0,
 };
 
+struct hotplug_pcp {
+	struct task_struct *unplug;
+	int refcount;
+	struct completion synced;
+};
+
+static DEFINE_PER_CPU(struct hotplug_pcp, hotplug_pcp);
+
+/**
+ * pin_current_cpu - Prevent the current cpu from being unplugged
+ *
+ * Lightweight version of get_online_cpus() to prevent cpu from being
+ * unplugged when code runs in a migration disabled region.
+ *
+ * Must be called with preemption disabled (preempt_count = 1)!
+ */
+void pin_current_cpu(void)
+{
+	struct hotplug_pcp *hp = &__get_cpu_var(hotplug_pcp);
+
+retry:
+	if (!hp->unplug || hp->refcount || preempt_count() > 1 ||
+	    hp->unplug == current) {
+		hp->refcount++;
+		return;
+	}
+	preempt_enable();
+	mutex_lock(&cpu_hotplug.lock);
+	mutex_unlock(&cpu_hotplug.lock);
+	preempt_disable();
+	goto retry;
+}
+
+/**
+ * unpin_current_cpu - Allow unplug of current cpu
+ *
+ * Must be called with preemption or interrupts disabled!
+ */
+void unpin_current_cpu(void)
+{
+	struct hotplug_pcp *hp = &__get_cpu_var(hotplug_pcp);
+
+	WARN_ON(hp->refcount <= 0);
+
+	/* This is safe. sync_unplug_thread is pinned to this cpu */
+	if (!--hp->refcount && hp->unplug && hp->unplug != current)
+		wake_up_process(hp->unplug);
+}
+
+/*
+ * FIXME: Is this really correct under all circumstances ?
+ */
+static int sync_unplug_thread(void *data)
+{
+	struct hotplug_pcp *hp = data;
+
+	preempt_disable();
+	hp->unplug = current;
+	set_current_state(TASK_UNINTERRUPTIBLE);
+	while (hp->refcount) {
+		schedule_preempt_disabled();
+		set_current_state(TASK_UNINTERRUPTIBLE);
+	}
+	set_current_state(TASK_RUNNING);
+	preempt_enable();
+	complete(&hp->synced);
+	return 0;
+}
+
+/*
+ * Start the sync_unplug_thread on the target cpu and wait for it to
+ * complete.
+ */
+static int cpu_unplug_begin(unsigned int cpu)
+{
+	struct hotplug_pcp *hp = &per_cpu(hotplug_pcp, cpu);
+	struct task_struct *tsk;
+
+	init_completion(&hp->synced);
+	tsk = kthread_create(sync_unplug_thread, hp, "sync_unplug/%d\n", cpu);
+	if (IS_ERR(tsk))
+		return (PTR_ERR(tsk));
+	kthread_bind(tsk, cpu);
+	wake_up_process(tsk);
+	wait_for_completion(&hp->synced);
+	return 0;
+}
+
+static void cpu_unplug_done(unsigned int cpu)
+{
+	struct hotplug_pcp *hp = &per_cpu(hotplug_pcp, cpu);
+
+	hp->unplug = NULL;
+}
+
 void get_online_cpus(void)
 {
 	might_sleep();
@@ -285,13 +380,14 @@ static int __ref take_cpu_down(void *_pa
 /* Requires cpu_add_remove_lock to be held */
 static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
 {
-	int err, nr_calls = 0;
+	int mycpu, err, nr_calls = 0;
 	void *hcpu = (void *)(long)cpu;
 	unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
 	struct take_cpu_down_param tcd_param = {
 		.mod = mod,
 		.hcpu = hcpu,
 	};
+	cpumask_var_t cpumask;
 
 	if (num_online_cpus() == 1)
 		return -EBUSY;
@@ -299,7 +395,20 @@ static int __ref _cpu_down(unsigned int
 	if (!cpu_online(cpu))
 		return -EINVAL;
 
-	cpu_hotplug_begin();
+	/* Move the downtaker off the unplug cpu */
+	if (!alloc_cpumask_var(&cpumask, GFP_KERNEL))
+		return -ENOMEM;
+	cpumask_andnot(cpumask, cpu_online_mask, cpumask_of(cpu));
+	set_cpus_allowed_ptr(current, cpumask);
+	free_cpumask_var(cpumask);
+	preempt_disable();
+	mycpu = smp_processor_id();
+	if (mycpu == cpu) {
+		printk(KERN_ERR "Yuck! Still on unplug CPU\n!");
+		preempt_enable();
+		return -EBUSY;
+	}
+	preempt_enable();
 
 	err = __cpu_notify(CPU_DOWN_PREPARE | mod, hcpu, -1, &nr_calls);
 	if (err) {
@@ -307,7 +416,16 @@ static int __ref _cpu_down(unsigned int
 		__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
 		printk("%s: attempt to take down CPU %u failed\n",
 				__func__, cpu);
-		goto out_release;
+		goto out_cancel;
+	}
+
+	cpu_hotplug_begin();
+	err = cpu_unplug_begin(cpu);
+	if (err) {
+		nr_calls--;
+		__cpu_notify(CPU_DOWN_FAILED | mod, hcpu, nr_calls, NULL);
+		printk("cpu_unplug_begin(%d) failed\n", cpu);
+		goto out_cancel;
 	}
 	smpboot_park_threads(cpu);
 
@@ -339,6 +457,8 @@ static int __ref _cpu_down(unsigned int
 	check_for_tasks(cpu);
 
 out_release:
+	cpu_unplug_done(cpu);
+out_cancel:
 	cpu_hotplug_done();
 	if (!err)
 		cpu_notify_nofail(CPU_POST_DEAD | mod, hcpu);