workqueue: don't migrate pending works from the dead CPU

Currently CPU_DEAD uses kthread_stop() to stop cwq->thread and then
transfers cwq->worklist to another CPU.  However, it is very unlikely that
worker_thread() will notice kthread_should_stop() before flushing
cwq->worklist.  It is only possible if worker_thread() was preempted after
run_workqueue(cwq), a new work_struct was added, and CPU_DEAD happened
before cwq->thread has a chance to run.

This means that take_over_work() mostly adds unneeded complications.  Note
also that kthread_stop() is not good per se, wake_up_process() may confuse
work->func() if it sleeps waiting for some event.

Remove take_over_work() and migrate_sequence complications.  CPU_DEAD sets
the cwq->should_stop flag (introduced by this patch) and waits for
cwq->thread to flush cwq->worklist and exit.  Because the dead CPU is not
on cpu_online_map, no more works can be added to that cwq.

cpu_populated_map was introduced to optimize for_each_possible_cpu(), it is
not strictly needed, and it is more a documentation in fact.

Saves 418 bytes.

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: Srivatsa Vaddagiri <vatsa@in.ibm.com>
Cc: "Pallipadi, Venkatesh" <venkatesh.pallipadi@intel.com>
Cc: Gautham shenoy <ego@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 211 insertions, 219 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 398c34ff6a54..a981add58fb9 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -43,10 +43,11 @@ struct cpu_workqueue_struct {
 
         struct list_head worklist;
         wait_queue_head_t more_work;
+        struct work_struct *current_work;
 
         struct workqueue_struct *wq;
         struct task_struct *thread;
-        struct work_struct *current_work;
+        int should_stop;
 
         int run_depth;          /* Detect run_workqueue() recursion depth */
 } ____cacheline_aligned;
@@ -64,11 +65,12 @@ struct workqueue_struct {
 
 /* All the per-cpu workqueues on the system, for hotplug cpu to add/remove
    threads to each one as cpus come/go. */
-static long migrate_sequence __read_mostly;
 static DEFINE_MUTEX(workqueue_mutex);
 static LIST_HEAD(workqueues);
 
-static int singlethread_cpu;
+static int singlethread_cpu __read_mostly;
+/* optimization, we could use cpu_possible_map */
+static cpumask_t cpu_populated_map __read_mostly;
 
 /* If it's single threaded, it isn't in the list of workqueues. */
 static inline int is_single_threaded(struct workqueue_struct *wq)
@@ -344,10 +346,28 @@ static void run_workqueue(struct cpu_workqueue_struct *cwq)
         spin_unlock_irqrestore(&cwq->lock, flags);
 }
 
+/*
+ * NOTE: the caller must not touch *cwq if this func returns true
+ */
+static int cwq_should_stop(struct cpu_workqueue_struct *cwq)
+{
+        int should_stop = cwq->should_stop;
+
+        if (unlikely(should_stop)) {
+                spin_lock_irq(&cwq->lock);
+                should_stop = cwq->should_stop && list_empty(&cwq->worklist);
+                if (should_stop)
+                        cwq->thread = NULL;
+                spin_unlock_irq(&cwq->lock);
+        }
+
+        return should_stop;
+}
+
 static int worker_thread(void *__cwq)
 {
         struct cpu_workqueue_struct *cwq = __cwq;
-        DECLARE_WAITQUEUE(wait, current);
+        DEFINE_WAIT(wait);
         struct k_sigaction sa;
         sigset_t blocked;
 
@@ -373,23 +393,21 @@ static int worker_thread(void *__cwq)
         siginitset(&sa.sa.sa_mask, sigmask(SIGCHLD));
         do_sigaction(SIGCHLD, &sa, (struct k_sigaction *)0);
 
-        set_current_state(TASK_INTERRUPTIBLE);
-        while (!kthread_should_stop()) {
+        for (;;) {
                 if (cwq->wq->freezeable)
                         try_to_freeze();
 
-                add_wait_queue(&cwq->more_work, &wait);
-                if (list_empty(&cwq->worklist))
+                prepare_to_wait(&cwq->more_work, &wait, TASK_INTERRUPTIBLE);
+                if (!cwq->should_stop && list_empty(&cwq->worklist))
                         schedule();
-                else
-                        __set_current_state(TASK_RUNNING);
-                remove_wait_queue(&cwq->more_work, &wait);
+                finish_wait(&cwq->more_work, &wait);
+
+                if (cwq_should_stop(cwq))
+                        break;
 
-                if (!list_empty(&cwq->worklist))
-                        run_workqueue(cwq);
-                set_current_state(TASK_INTERRUPTIBLE);
+                run_workqueue(cwq);
         }
-        __set_current_state(TASK_RUNNING);
+
         return 0;
 }
 
@@ -454,20 +472,13 @@ static void flush_cpu_workqueue(struct cpu_workqueue_struct *cwq)
  */
 void fastcall flush_workqueue(struct workqueue_struct *wq)
 {
-        if (is_single_threaded(wq)) {
-                /* Always use first cpu's area. */
+        if (is_single_threaded(wq))
                 flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, singlethread_cpu));
-        } else {
-                long sequence;
+        else {
                 int cpu;
-again:
-                sequence = migrate_sequence;
 
-                for_each_possible_cpu(cpu)
+                for_each_cpu_mask(cpu, cpu_populated_map)
                         flush_cpu_workqueue(per_cpu_ptr(wq->cpu_wq, cpu));
-
-                if (unlikely(sequence != migrate_sequence))
-                        goto again;
         }
 }
 EXPORT_SYMBOL_GPL(flush_workqueue);
@@ -485,11 +496,8 @@ static void wait_on_work(struct cpu_workqueue_struct *cwq,
         }
         spin_unlock_irq(&cwq->lock);
 
-        if (unlikely(running)) {
-                mutex_unlock(&workqueue_mutex);
+        if (unlikely(running))
                 wait_for_completion(&barr.done);
-                mutex_lock(&workqueue_mutex);
-        }
 }
 
 /**
@@ -510,155 +518,31 @@ void flush_work(struct workqueue_struct *wq, struct work_struct *work)
 {
         struct cpu_workqueue_struct *cwq;
 
-        mutex_lock(&workqueue_mutex);
         cwq = get_wq_data(work);
         /* Was it ever queued ? */
         if (!cwq)
-                goto out;
+                return;
 
         /*
-         * This work can't be re-queued, and the lock above protects us
-         * from take_over_work(), no need to re-check that get_wq_data()
-         * is still the same when we take cwq->lock.
+         * This work can't be re-queued, no need to re-check that
+         * get_wq_data() is still the same when we take cwq->lock.
          */
         spin_lock_irq(&cwq->lock);
         list_del_init(&work->entry);
         work_release(work);
         spin_unlock_irq(&cwq->lock);
 
-        if (is_single_threaded(wq)) {
-                /* Always use first cpu's area. */
+        if (is_single_threaded(wq))
                 wait_on_work(per_cpu_ptr(wq->cpu_wq, singlethread_cpu), work);
-        } else {
+        else {
                 int cpu;
 
-                for_each_online_cpu(cpu)
+                for_each_cpu_mask(cpu, cpu_populated_map)
                         wait_on_work(per_cpu_ptr(wq->cpu_wq, cpu), work);
         }
-out:
-        mutex_unlock(&workqueue_mutex);
 }
 EXPORT_SYMBOL_GPL(flush_work);
 
-static void init_cpu_workqueue(struct workqueue_struct *wq, int cpu)
-{
-        struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
-
-        cwq->wq = wq;
-        spin_lock_init(&cwq->lock);
-        INIT_LIST_HEAD(&cwq->worklist);
-        init_waitqueue_head(&cwq->more_work);
-}
-
-static struct task_struct *create_workqueue_thread(struct workqueue_struct *wq,
-                                                        int cpu)
-{
-        struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
-        struct task_struct *p;
-
-        if (is_single_threaded(wq))
-                p = kthread_create(worker_thread, cwq, "%s", wq->name);
-        else
-                p = kthread_create(worker_thread, cwq, "%s/%d", wq->name, cpu);
-        if (IS_ERR(p))
-                return NULL;
-        cwq->thread = p;
-        return p;
-}
-
-struct workqueue_struct *__create_workqueue(const char *name,
-                                            int singlethread, int freezeable)
-{
-        int cpu, destroy = 0;
-        struct workqueue_struct *wq;
-        struct task_struct *p;
-
-        wq = kzalloc(sizeof(*wq), GFP_KERNEL);
-        if (!wq)
-                return NULL;
-
-        wq->cpu_wq = alloc_percpu(struct cpu_workqueue_struct);
-        if (!wq->cpu_wq) {
-                kfree(wq);
-                return NULL;
-        }
-
-        wq->name = name;
-        wq->freezeable = freezeable;
-
-        mutex_lock(&workqueue_mutex);
-        if (singlethread) {
-                INIT_LIST_HEAD(&wq->list);
-                init_cpu_workqueue(wq, singlethread_cpu);
-                p = create_workqueue_thread(wq, singlethread_cpu);
-                if (!p)
-                        destroy = 1;
-                else
-                        wake_up_process(p);
-        } else {
-                list_add(&wq->list, &workqueues);
-                for_each_possible_cpu(cpu) {
-                        init_cpu_workqueue(wq, cpu);
-                        if (!cpu_online(cpu))
-                                continue;
-
-                        p = create_workqueue_thread(wq, cpu);
-                        if (p) {
-                                kthread_bind(p, cpu);
-                                wake_up_process(p);
-                        } else
-                                destroy = 1;
-                }
-        }
-        mutex_unlock(&workqueue_mutex);
-
-        /*
-         * Was there any error during startup? If yes then clean up:
-         */
-        if (destroy) {
-                destroy_workqueue(wq);
-                wq = NULL;
-        }
-        return wq;
-}
-EXPORT_SYMBOL_GPL(__create_workqueue);
-
-static void cleanup_workqueue_thread(struct workqueue_struct *wq, int cpu)
-{
-        struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
-
-        if (cwq->thread) {
-                kthread_stop(cwq->thread);
-                cwq->thread = NULL;
-        }
-}
-
-/**
- * destroy_workqueue - safely terminate a workqueue
- * @wq: target workqueue
- *
- * Safely destroy a workqueue. All work currently pending will be done first.
- */
-void destroy_workqueue(struct workqueue_struct *wq)
-{
-        int cpu;
-
-        flush_workqueue(wq);
-
-        /* We don't need the distraction of CPUs appearing and vanishing. */
-        mutex_lock(&workqueue_mutex);
-        if (is_single_threaded(wq))
-                cleanup_workqueue_thread(wq, singlethread_cpu);
-        else {
-                for_each_online_cpu(cpu)
-                        cleanup_workqueue_thread(wq, cpu);
-                list_del(&wq->list);
-        }
-        mutex_unlock(&workqueue_mutex);
-        free_percpu(wq->cpu_wq);
-        kfree(wq);
-}
-EXPORT_SYMBOL_GPL(destroy_workqueue);
 
 static struct workqueue_struct *keventd_wq;
 
@@ -822,85 +706,193 @@ int current_is_keventd(void)
 
 }
 
-/* Take the work from this (downed) CPU. */
-static void take_over_work(struct workqueue_struct *wq, unsigned int cpu)
+static struct cpu_workqueue_struct *
+init_cpu_workqueue(struct workqueue_struct *wq, int cpu)
 {
         struct cpu_workqueue_struct *cwq = per_cpu_ptr(wq->cpu_wq, cpu);
-        struct list_head list;
-        struct work_struct *work;
 
-        spin_lock_irq(&cwq->lock);
-        list_replace_init(&cwq->worklist, &list);
-        migrate_sequence++;
-
-        while (!list_empty(&list)) {
-                printk("Taking work for %s\n", wq->name);
-                work = list_entry(list.next,struct work_struct,entry);
-                list_del(&work->entry);
-                __queue_work(per_cpu_ptr(wq->cpu_wq, smp_processor_id()), work);
-        }
-        spin_unlock_irq(&cwq->lock);
+        cwq->wq = wq;
+        spin_lock_init(&cwq->lock);
+        INIT_LIST_HEAD(&cwq->worklist);
+        init_waitqueue_head(&cwq->more_work);
+
+        return cwq;
 }
 
-/* We're holding the cpucontrol mutex here */
-static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
-                                  unsigned long action,
-                                  void *hcpu)
+static int create_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu)
+{
+        struct workqueue_struct *wq = cwq->wq;
+        const char *fmt = is_single_threaded(wq) ? "%s" : "%s/%d";
+        struct task_struct *p;
+
+        p = kthread_create(worker_thread, cwq, fmt, wq->name, cpu);
+        /*
+         * Nobody can add the work_struct to this cwq,
+         *      if (caller is __create_workqueue)
+         *              nobody should see this wq
+         *      else // caller is CPU_UP_PREPARE
+         *              cpu is not on cpu_online_map
+         * so we can abort safely.
+         */
+        if (IS_ERR(p))
+                return PTR_ERR(p);
+
+        cwq->thread = p;
+        cwq->should_stop = 0;
+        if (!is_single_threaded(wq))
+                kthread_bind(p, cpu);
+
+        if (is_single_threaded(wq) || cpu_online(cpu))
+                wake_up_process(p);
+
+        return 0;
+}
+
+struct workqueue_struct *__create_workqueue(const char *name,
+                                            int singlethread, int freezeable)
 {
-        unsigned int hotcpu = (unsigned long)hcpu;
         struct workqueue_struct *wq;
+        struct cpu_workqueue_struct *cwq;
+        int err = 0, cpu;
 
-        switch (action) {
-        case CPU_UP_PREPARE:
+        wq = kzalloc(sizeof(*wq), GFP_KERNEL);
+        if (!wq)
+                return NULL;
+
+        wq->cpu_wq = alloc_percpu(struct cpu_workqueue_struct);
+        if (!wq->cpu_wq) {
+                kfree(wq);
+                return NULL;
+        }
+
+        wq->name = name;
+        wq->freezeable = freezeable;
+
+        if (singlethread) {
+                INIT_LIST_HEAD(&wq->list);
+                cwq = init_cpu_workqueue(wq, singlethread_cpu);
+                err = create_workqueue_thread(cwq, singlethread_cpu);
+        } else {
                 mutex_lock(&workqueue_mutex);
-                /* Create a new workqueue thread for it. */
-                list_for_each_entry(wq, &workqueues, list) {
-                        if (!create_workqueue_thread(wq, hotcpu)) {
-                                printk("workqueue for %i failed\n", hotcpu);
-                                return NOTIFY_BAD;
-                        }
+                list_add(&wq->list, &workqueues);
+
+                for_each_possible_cpu(cpu) {
+                        cwq = init_cpu_workqueue(wq, cpu);
+                        if (err || !cpu_online(cpu))
+                                continue;
+                        err = create_workqueue_thread(cwq, cpu);
                 }
-                break;
+                mutex_unlock(&workqueue_mutex);
+        }
+
+        if (err) {
+                destroy_workqueue(wq);
+                wq = NULL;
+        }
+        return wq;
+}
+EXPORT_SYMBOL_GPL(__create_workqueue);
 
-        case CPU_ONLINE:
-                /* Kick off worker threads. */
-                list_for_each_entry(wq, &workqueues, list) {
-                        struct cpu_workqueue_struct *cwq;
+static void cleanup_workqueue_thread(struct cpu_workqueue_struct *cwq, int cpu)
+{
+        struct wq_barrier barr;
+        int alive = 0;
 
-                        cwq = per_cpu_ptr(wq->cpu_wq, hotcpu);
-                        kthread_bind(cwq->thread, hotcpu);
-                        wake_up_process(cwq->thread);
-                }
+        spin_lock_irq(&cwq->lock);
+        if (cwq->thread != NULL) {
+                insert_wq_barrier(cwq, &barr, 1);
+                cwq->should_stop = 1;
+                alive = 1;
+        }
+        spin_unlock_irq(&cwq->lock);
+
+        if (alive) {
+                wait_for_completion(&barr.done);
+
+                while (unlikely(cwq->thread != NULL))
+                        cpu_relax();
+                /*
+                 * Wait until cwq->thread unlocks cwq->lock,
+                 * it won't touch *cwq after that.
+                 */
+                smp_rmb();
+                spin_unlock_wait(&cwq->lock);
+        }
+}
+
+/**
+ * destroy_workqueue - safely terminate a workqueue
+ * @wq: target workqueue
+ *
+ * Safely destroy a workqueue. All work currently pending will be done first.
+ */
+void destroy_workqueue(struct workqueue_struct *wq)
+{
+        struct cpu_workqueue_struct *cwq;
+
+        if (is_single_threaded(wq)) {
+                cwq = per_cpu_ptr(wq->cpu_wq, singlethread_cpu);
+                cleanup_workqueue_thread(cwq, singlethread_cpu);
+        } else {
+                int cpu;
+
+                mutex_lock(&workqueue_mutex);
+                list_del(&wq->list);
                 mutex_unlock(&workqueue_mutex);
-                break;
 
-        case CPU_UP_CANCELED:
-                list_for_each_entry(wq, &workqueues, list) {
-                        if (!per_cpu_ptr(wq->cpu_wq, hotcpu)->thread)
-                                continue;
-                        /* Unbind so it can run. */
-                        kthread_bind(per_cpu_ptr(wq->cpu_wq, hotcpu)->thread,
-                                     any_online_cpu(cpu_online_map));
-                        cleanup_workqueue_thread(wq, hotcpu);
+                for_each_cpu_mask(cpu, cpu_populated_map) {
+                        cwq = per_cpu_ptr(wq->cpu_wq, cpu);
+                        cleanup_workqueue_thread(cwq, cpu);
                 }
-                mutex_unlock(&workqueue_mutex);
-                break;
+        }
 
-        case CPU_DOWN_PREPARE:
+        free_percpu(wq->cpu_wq);
+        kfree(wq);
+}
+EXPORT_SYMBOL_GPL(destroy_workqueue);
+
+static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
+                                                unsigned long action,
+                                                void *hcpu)
+{
+        unsigned int cpu = (unsigned long)hcpu;
+        struct cpu_workqueue_struct *cwq;
+        struct workqueue_struct *wq;
+
+        switch (action) {
+        case CPU_LOCK_ACQUIRE:
                 mutex_lock(&workqueue_mutex);
-                break;
+                return NOTIFY_OK;
 
-        case CPU_DOWN_FAILED:
+        case CPU_LOCK_RELEASE:
                 mutex_unlock(&workqueue_mutex);
-                break;
+                return NOTIFY_OK;
 
-        case CPU_DEAD:
-                list_for_each_entry(wq, &workqueues, list)
-                        cleanup_workqueue_thread(wq, hotcpu);
-                list_for_each_entry(wq, &workqueues, list)
-                        take_over_work(wq, hotcpu);
-                mutex_unlock(&workqueue_mutex);
-                break;
+        case CPU_UP_PREPARE:
+                cpu_set(cpu, cpu_populated_map);
+        }
+
+        list_for_each_entry(wq, &workqueues, list) {
+                cwq = per_cpu_ptr(wq->cpu_wq, cpu);
+
+                switch (action) {
+                case CPU_UP_PREPARE:
+                        if (!create_workqueue_thread(cwq, cpu))
+                                break;
+                        printk(KERN_ERR "workqueue for %i failed\n", cpu);
+                        return NOTIFY_BAD;
+
+                case CPU_ONLINE:
+                        wake_up_process(cwq->thread);
+                        break;
+
+                case CPU_UP_CANCELED:
+                        if (cwq->thread)
+                                wake_up_process(cwq->thread);
+                case CPU_DEAD:
+                        cleanup_workqueue_thread(cwq, cpu);
+                        break;
+                }
         }
 
         return NOTIFY_OK;
@@ -908,9 +900,9 @@ static int __devinit workqueue_cpu_callback(struct notifier_block *nfb,
 
 void init_workqueues(void)
 {
+        cpu_populated_map = cpu_online_map;
         singlethread_cpu = first_cpu(cpu_possible_map);
         hotcpu_notifier(workqueue_cpu_callback, 0);
         keventd_wq = create_workqueue("events");
         BUG_ON(!keventd_wq);
 }
-