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Subject[PATCH 1/4] cpu_stop: implement stop_cpu[s]()
Date
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.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Dimitri Sivanich <sivanich@sgi.com>
Cc: Peter Zijlstra <peterz@infradead.org>
---
include/linux/stop_machine.h | 39 ++++-
kernel/stop_machine.c | 367 +++++++++++++++++++++++++++++++++++++++++-
2 files changed, 397 insertions(+), 9 deletions(-)
diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
index baba3a2..efcbd6c 100644
--- a/include/linux/stop_machine.h
+++ b/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
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 9bb9fb1..3ae2485 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -1,17 +1,374 @@
-/* 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 <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)
+{
+ memset(work_buf, 0, sizeof(*work_buf));
+ work_buf->fn = fn;
+ work_buf->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) {
+ struct cpu_stop_done *done = work->done;
+
+ __set_current_state(TASK_RUNNING);
+
+ /* cpu stop callbacks are not allowed to sleep */
+ preempt_disable();
+
+ ret = work->fn(work->arg);
+ if (ret)
+ done->ret = ret;
+
+ /* restore preemption and check it's still balanced */
+ preempt_enable();
+ WARN_ON_ONCE(preempt_count());
+
+ 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 {
--
1.6.4.2


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