cpu_stop: implement stop_cpu[s]()

Implement a simplistic per-cpu maximum priority cpu monopolization
mechanism.  A non-sleeping callback can be scheduled to run on one or
multiple cpus with maximum priority monopolozing those cpus.  This is
primarily to replace and unify RT workqueue usage in stop_machine and
scheduler migration_thread which currently is serving multiple
purposes.

Four functions are provided - stop_one_cpu(), stop_one_cpu_nowait(),
stop_cpus() and try_stop_cpus().

This is to allow clean sharing of resources among stop_cpu and all the
migration thread users.  One stopper thread per cpu is created which
is currently named "stopper/CPU".  This will eventually replace the
migration thread and take on its name.

* This facility was originally named cpuhog and lived in separate
  files but Peter Zijlstra nacked the name and thus got renamed to
  cpu_stop and moved into stop_machine.c.

* Better reporting of preemption leak as per Peter's suggestion.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Dimitri Sivanich <sivanich@sgi.com>

include/linux/stop_machine.h

@@ -1,15 +1,46 @@
 #ifndef _LINUX_STOP_MACHINE
 #define _LINUX_STOP_MACHINE
-/* "Bogolock": stop the entire machine, disable interrupts.  This is a
-   very heavy lock, which is equivalent to grabbing every spinlock
-   (and more).  So the "read" side to such a lock is anything which
-   disables preeempt. */
+
 #include <linux/cpu.h>
 #include <linux/cpumask.h>
+#include <linux/list.h>
 #include <asm/system.h>
 
 #if defined(CONFIG_STOP_MACHINE) && defined(CONFIG_SMP)
 
+/*
+ * stop_cpu[s]() is simplistic per-cpu maximum priority cpu
+ * monopolization mechanism.  The caller can specify a non-sleeping
+ * function to be executed on a single or multiple cpus preempting all
+ * other processes and monopolizing those cpus until it finishes.
+ *
+ * Resources for this mechanism are preallocated when a cpu is brought
+ * up and requests are guaranteed to be served as long as the target
+ * cpus are online.
+ */
+
+typedef int (*cpu_stop_fn_t)(void *arg);
+
+struct cpu_stop_work {
+	struct list_head	list;		/* cpu_stopper->works */
+	cpu_stop_fn_t		fn;
+	void			*arg;
+	struct cpu_stop_done	*done;
+};
+
+int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg);
+void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
+			 struct cpu_stop_work *work_buf);
+int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);
+int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);
+
+/*
+ * stop_machine "Bogolock": stop the entire machine, disable
+ * interrupts.  This is a very heavy lock, which is equivalent to
+ * grabbing every spinlock (and more).  So the "read" side to such a
+ * lock is anything which disables preeempt.
+ */
+
 /**
  * stop_machine: freeze the machine on all CPUs and run this function
  * @fn: the function to run

kernel/stop_machine.c

@@ -1,17 +1,379 @@
-/* Copyright 2008, 2005 Rusty Russell rusty@rustcorp.com.au IBM Corporation.
- * GPL v2 and any later version.
+/*
+ * kernel/stop_machine.c
+ *
+ * Copyright (C) 2008, 2005	IBM Corporation.
+ * Copyright (C) 2008, 2005	Rusty Russell rusty@rustcorp.com.au
+ * Copyright (C) 2010		SUSE Linux Products GmbH
+ * Copyright (C) 2010		Tejun Heo <tj@kernel.org>
+ *
+ * This file is released under the GPLv2 and any later version.
  */
+#include <linux/completion.h>
 #include <linux/cpu.h>
-#include <linux/err.h>
+#include <linux/init.h>
 #include <linux/kthread.h>
 #include <linux/module.h>
+#include <linux/percpu.h>
 #include <linux/sched.h>
 #include <linux/stop_machine.h>
-#include <linux/syscalls.h>
 #include <linux/interrupt.h>
+#include <linux/kallsyms.h>
 
 #include <asm/atomic.h>
-#include <asm/uaccess.h>
+
+/*
+ * Structure to determine completion condition and record errors.  May
+ * be shared by works on different cpus.
+ */
+struct cpu_stop_done {
+	atomic_t		nr_todo;	/* nr left to execute */
+	bool			executed;	/* actually executed? */
+	int			ret;		/* collected return value */
+	struct completion	completion;	/* fired if nr_todo reaches 0 */
+};
+
+/* the actual stopper, one per every possible cpu, enabled on online cpus */
+struct cpu_stopper {
+	spinlock_t		lock;
+	struct list_head	works;		/* list of pending works */
+	struct task_struct	*thread;	/* stopper thread */
+	bool			enabled;	/* is this stopper enabled? */
+};
+
+static DEFINE_PER_CPU(struct cpu_stopper, cpu_stopper);
+
+static void cpu_stop_init_done(struct cpu_stop_done *done, unsigned int nr_todo)
+{
+	memset(done, 0, sizeof(*done));
+	atomic_set(&done->nr_todo, nr_todo);
+	init_completion(&done->completion);
+}
+
+/* signal completion unless @done is NULL */
+static void cpu_stop_signal_done(struct cpu_stop_done *done, bool executed)
+{
+	if (done) {
+		if (executed)
+			done->executed = true;
+		if (atomic_dec_and_test(&done->nr_todo))
+			complete(&done->completion);
+	}
+}
+
+/* queue @work to @stopper.  if offline, @work is completed immediately */
+static void cpu_stop_queue_work(struct cpu_stopper *stopper,
+				struct cpu_stop_work *work)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&stopper->lock, flags);
+
+	if (stopper->enabled) {
+		list_add_tail(&work->list, &stopper->works);
+		wake_up_process(stopper->thread);
+	} else
+		cpu_stop_signal_done(work->done, false);
+
+	spin_unlock_irqrestore(&stopper->lock, flags);
+}
+
+/**
+ * stop_one_cpu - stop a cpu
+ * @cpu: cpu to stop
+ * @fn: function to execute
+ * @arg: argument to @fn
+ *
+ * Execute @fn(@arg) on @cpu.  @fn is run in a process context with
+ * the highest priority preempting any task on the cpu and
+ * monopolizing it.  This function returns after the execution is
+ * complete.
+ *
+ * This function doesn't guarantee @cpu stays online till @fn
+ * completes.  If @cpu goes down in the middle, execution may happen
+ * partially or fully on different cpus.  @fn should either be ready
+ * for that or the caller should ensure that @cpu stays online until
+ * this function completes.
+ *
+ * CONTEXT:
+ * Might sleep.
+ *
+ * RETURNS:
+ * -ENOENT if @fn(@arg) was not executed because @cpu was offline;
+ * otherwise, the return value of @fn.
+ */
+int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg)
+{
+	struct cpu_stop_done done;
+	struct cpu_stop_work work = { .fn = fn, .arg = arg, .done = &done };
+
+	cpu_stop_init_done(&done, 1);
+	cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu), &work);
+	wait_for_completion(&done.completion);
+	return done.executed ? done.ret : -ENOENT;
+}
+
+/**
+ * stop_one_cpu_nowait - stop a cpu but don't wait for completion
+ * @cpu: cpu to stop
+ * @fn: function to execute
+ * @arg: argument to @fn
+ *
+ * Similar to stop_one_cpu() but doesn't wait for completion.  The
+ * caller is responsible for ensuring @work_buf is currently unused
+ * and will remain untouched until stopper starts executing @fn.
+ *
+ * CONTEXT:
+ * Don't care.
+ */
+void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
+			struct cpu_stop_work *work_buf)
+{
+	*work_buf = (struct cpu_stop_work){ .fn = fn, .arg = arg, };
+	cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu), work_buf);
+}
+
+/* static data for stop_cpus */
+static DEFINE_MUTEX(stop_cpus_mutex);
+static DEFINE_PER_CPU(struct cpu_stop_work, stop_cpus_work);
+
+int __stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
+{
+	struct cpu_stop_work *work;
+	struct cpu_stop_done done;
+	unsigned int cpu;
+
+	/* initialize works and done */
+	for_each_cpu(cpu, cpumask) {
+		work = &per_cpu(stop_cpus_work, cpu);
+		work->fn = fn;
+		work->arg = arg;
+		work->done = &done;
+	}
+	cpu_stop_init_done(&done, cpumask_weight(cpumask));
+
+	/*
+	 * Disable preemption while queueing to avoid getting
+	 * preempted by a stopper which might wait for other stoppers
+	 * to enter @fn which can lead to deadlock.
+	 */
+	preempt_disable();
+	for_each_cpu(cpu, cpumask)
+		cpu_stop_queue_work(&per_cpu(cpu_stopper, cpu),
+				    &per_cpu(stop_cpus_work, cpu));
+	preempt_enable();
+
+	wait_for_completion(&done.completion);
+	return done.executed ? done.ret : -ENOENT;
+}
+
+/**
+ * stop_cpus - stop multiple cpus
+ * @cpumask: cpus to stop
+ * @fn: function to execute
+ * @arg: argument to @fn
+ *
+ * Execute @fn(@arg) on online cpus in @cpumask.  On each target cpu,
+ * @fn is run in a process context with the highest priority
+ * preempting any task on the cpu and monopolizing it.  This function
+ * returns after all executions are complete.
+ *
+ * This function doesn't guarantee the cpus in @cpumask stay online
+ * till @fn completes.  If some cpus go down in the middle, execution
+ * on the cpu may happen partially or fully on different cpus.  @fn
+ * should either be ready for that or the caller should ensure that
+ * the cpus stay online until this function completes.
+ *
+ * All stop_cpus() calls are serialized making it safe for @fn to wait
+ * for all cpus to start executing it.
+ *
+ * CONTEXT:
+ * Might sleep.
+ *
+ * RETURNS:
+ * -ENOENT if @fn(@arg) was not executed at all because all cpus in
+ * @cpumask were offline; otherwise, 0 if all executions of @fn
+ * returned 0, any non zero return value if any returned non zero.
+ */
+int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
+{
+	int ret;
+
+	/* static works are used, process one request at a time */
+	mutex_lock(&stop_cpus_mutex);
+	ret = __stop_cpus(cpumask, fn, arg);
+	mutex_unlock(&stop_cpus_mutex);
+	return ret;
+}
+
+/**
+ * try_stop_cpus - try to stop multiple cpus
+ * @cpumask: cpus to stop
+ * @fn: function to execute
+ * @arg: argument to @fn
+ *
+ * Identical to stop_cpus() except that it fails with -EAGAIN if
+ * someone else is already using the facility.
+ *
+ * CONTEXT:
+ * Might sleep.
+ *
+ * RETURNS:
+ * -EAGAIN if someone else is already stopping cpus, -ENOENT if
+ * @fn(@arg) was not executed at all because all cpus in @cpumask were
+ * offline; otherwise, 0 if all executions of @fn returned 0, any non
+ * zero return value if any returned non zero.
+ */
+int try_stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg)
+{
+	int ret;
+
+	/* static works are used, process one request at a time */
+	if (!mutex_trylock(&stop_cpus_mutex))
+		return -EAGAIN;
+	ret = __stop_cpus(cpumask, fn, arg);
+	mutex_unlock(&stop_cpus_mutex);
+	return ret;
+}
+
+static int cpu_stopper_thread(void *data)
+{
+	struct cpu_stopper *stopper = data;
+	struct cpu_stop_work *work;
+	int ret;
+
+repeat:
+	set_current_state(TASK_INTERRUPTIBLE);	/* mb paired w/ kthread_stop */
+
+	if (kthread_should_stop()) {
+		__set_current_state(TASK_RUNNING);
+		return 0;
+	}
+
+	work = NULL;
+	spin_lock_irq(&stopper->lock);
+	if (!list_empty(&stopper->works)) {
+		work = list_first_entry(&stopper->works,
+					struct cpu_stop_work, list);
+		list_del_init(&work->list);
+	}
+	spin_unlock_irq(&stopper->lock);
+
+	if (work) {
+		cpu_stop_fn_t fn = work->fn;
+		void *arg = work->arg;
+		struct cpu_stop_done *done = work->done;
+		char ksym_buf[KSYM_NAME_LEN];
+
+		__set_current_state(TASK_RUNNING);
+
+		/* cpu stop callbacks are not allowed to sleep */
+		preempt_disable();
+
+		ret = fn(arg);
+		if (ret)
+			done->ret = ret;
+
+		/* restore preemption and check it's still balanced */
+		preempt_enable();
+		WARN_ONCE(preempt_count(),
+			  "cpu_stop: %s(%p) leaked preempt count\n",
+			  kallsyms_lookup((unsigned long)fn, NULL, NULL, NULL,
+					  ksym_buf), arg);
+
+		cpu_stop_signal_done(done, true);
+	} else
+		schedule();
+
+	goto repeat;
+}
+
+/* manage stopper for a cpu, mostly lifted from sched migration thread mgmt */
+static int __cpuinit cpu_stop_cpu_callback(struct notifier_block *nfb,
+					   unsigned long action, void *hcpu)
+{
+	struct sched_param param = { .sched_priority = MAX_RT_PRIO - 1 };
+	unsigned int cpu = (unsigned long)hcpu;
+	struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
+	struct cpu_stop_work *work;
+	struct task_struct *p;
+
+	switch (action & ~CPU_TASKS_FROZEN) {
+	case CPU_UP_PREPARE:
+		BUG_ON(stopper->thread || stopper->enabled ||
+		       !list_empty(&stopper->works));
+		p = kthread_create(cpu_stopper_thread, stopper, "stopper/%d",
+				   cpu);
+		if (IS_ERR(p))
+			return NOTIFY_BAD;
+		sched_setscheduler_nocheck(p, SCHED_FIFO, &param);
+		get_task_struct(p);
+		stopper->thread = p;
+		break;
+
+	case CPU_ONLINE:
+		kthread_bind(stopper->thread, cpu);
+		/* strictly unnecessary, as first user will wake it */
+		wake_up_process(stopper->thread);
+		/* mark enabled */
+		spin_lock_irq(&stopper->lock);
+		stopper->enabled = true;
+		spin_unlock_irq(&stopper->lock);
+		break;
+
+#ifdef CONFIG_HOTPLUG_CPU
+	case CPU_UP_CANCELED:
+	case CPU_DEAD:
+		/* kill the stopper */
+		kthread_stop(stopper->thread);
+		/* drain remaining works */
+		spin_lock_irq(&stopper->lock);
+		list_for_each_entry(work, &stopper->works, list)
+			cpu_stop_signal_done(work->done, false);
+		stopper->enabled = false;
+		spin_unlock_irq(&stopper->lock);
+		/* release the stopper */
+		put_task_struct(stopper->thread);
+		stopper->thread = NULL;
+		break;
+#endif
+	}
+
+	return NOTIFY_OK;
+}
+
+/*
+ * Give it a higher priority so that cpu stopper is available to other
+ * cpu notifiers.  It currently shares the same priority as sched
+ * migration_notifier.
+ */
+static struct notifier_block __cpuinitdata cpu_stop_cpu_notifier = {
+	.notifier_call	= cpu_stop_cpu_callback,
+	.priority	= 10,
+};
+
+static int __init cpu_stop_init(void)
+{
+	void *bcpu = (void *)(long)smp_processor_id();
+	unsigned int cpu;
+	int err;
+
+	for_each_possible_cpu(cpu) {
+		struct cpu_stopper *stopper = &per_cpu(cpu_stopper, cpu);
+
+		spin_lock_init(&stopper->lock);
+		INIT_LIST_HEAD(&stopper->works);
+	}
+
+	/* start one for the boot cpu */
+	err = cpu_stop_cpu_callback(&cpu_stop_cpu_notifier, CPU_UP_PREPARE,
+				    bcpu);
+	BUG_ON(err == NOTIFY_BAD);
+	cpu_stop_cpu_callback(&cpu_stop_cpu_notifier, CPU_ONLINE, bcpu);
+	register_cpu_notifier(&cpu_stop_cpu_notifier);
+
+	return 0;
+}
+early_initcall(cpu_stop_init);
 
 /* This controls the threads on each CPU. */
 enum stopmachine_state {