workqueue: implement unbound workqueue

This patch implements unbound workqueue which can be specified with
WQ_UNBOUND flag on creation.  An unbound workqueue has the following
properties.

* It uses a dedicated gcwq with a pseudo CPU number WORK_CPU_UNBOUND.
  This gcwq is always online and disassociated.

* Workers are not bound to any CPU and not concurrency managed.  Works
  are dispatched to workers as soon as possible and the only applied
  limitation is @max_active.  IOW, all unbound workqeueues are
  implicitly high priority.

Unbound workqueues can be used as simple execution context provider.
Contexts unbound to any cpu are served as soon as possible.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Arjan van de Ven <arjan@linux.intel.com>
Cc: David Howells <dhowells@redhat.com>

1 files changed, 159 insertions, 59 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index a105ddf55f79..4608563cdd63 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -53,9 +53,10 @@ enum {
         WORKER_ROGUE            = 1 << 4,       /* not bound to any cpu */
         WORKER_REBIND           = 1 << 5,       /* mom is home, come back */
         WORKER_CPU_INTENSIVE    = 1 << 6,       /* cpu intensive */
+        WORKER_UNBOUND          = 1 << 7,       /* worker is unbound */
 
         WORKER_NOT_RUNNING      = WORKER_PREP | WORKER_ROGUE | WORKER_REBIND |
-                                  WORKER_CPU_INTENSIVE,
+                                  WORKER_CPU_INTENSIVE | WORKER_UNBOUND,
 
         /* gcwq->trustee_state */
         TRUSTEE_START           = 0,            /* start */
@@ -96,7 +97,7 @@ enum {
  * X: During normal operation, modification requires gcwq->lock and
  *    should be done only from local cpu.  Either disabling preemption
  *    on local cpu or grabbing gcwq->lock is enough for read access.
- *    While trustee is in charge, it's identical to L.
+ *    If GCWQ_DISASSOCIATED is set, it's identical to L.
  *
  * F: wq->flush_mutex protected.
  *
@@ -220,14 +221,52 @@ struct workqueue_struct {
 struct workqueue_struct *system_wq __read_mostly;
 struct workqueue_struct *system_long_wq __read_mostly;
 struct workqueue_struct *system_nrt_wq __read_mostly;
+struct workqueue_struct *system_unbound_wq __read_mostly;
 EXPORT_SYMBOL_GPL(system_wq);
 EXPORT_SYMBOL_GPL(system_long_wq);
 EXPORT_SYMBOL_GPL(system_nrt_wq);
+EXPORT_SYMBOL_GPL(system_unbound_wq);
 
 #define for_each_busy_worker(worker, i, pos, gcwq)                      \
         for (i = 0; i < BUSY_WORKER_HASH_SIZE; i++)                     \
                 hlist_for_each_entry(worker, pos, &gcwq->busy_hash[i], hentry)
 
+static inline int __next_gcwq_cpu(int cpu, const struct cpumask *mask,
+                                  unsigned int sw)
+{
+        if (cpu < nr_cpu_ids) {
+                if (sw & 1) {
+                        cpu = cpumask_next(cpu, mask);
+                        if (cpu < nr_cpu_ids)
+                                return cpu;
+                }
+                if (sw & 2)
+                        return WORK_CPU_UNBOUND;
+        }
+        return WORK_CPU_NONE;
+}
+
+static inline int __next_wq_cpu(int cpu, const struct cpumask *mask,
+                                struct workqueue_struct *wq)
+{
+        return __next_gcwq_cpu(cpu, mask, !(wq->flags & WQ_UNBOUND) ? 1 : 2);
+}
+
+#define for_each_gcwq_cpu(cpu)                                          \
+        for ((cpu) = __next_gcwq_cpu(-1, cpu_possible_mask, 3);         \
+             (cpu) < WORK_CPU_NONE;                                     \
+             (cpu) = __next_gcwq_cpu((cpu), cpu_possible_mask, 3))
+
+#define for_each_online_gcwq_cpu(cpu)                                   \
+        for ((cpu) = __next_gcwq_cpu(-1, cpu_online_mask, 3);           \
+             (cpu) < WORK_CPU_NONE;                                     \
+             (cpu) = __next_gcwq_cpu((cpu), cpu_online_mask, 3))
+
+#define for_each_cwq_cpu(cpu, wq)                                       \
+        for ((cpu) = __next_wq_cpu(-1, cpu_possible_mask, (wq));        \
+             (cpu) < WORK_CPU_NONE;                                     \
+             (cpu) = __next_wq_cpu((cpu), cpu_possible_mask, (wq)))
+
 #ifdef CONFIG_DEBUG_OBJECTS_WORK
 
 static struct debug_obj_descr work_debug_descr;
@@ -351,26 +390,46 @@ static bool workqueue_freezing;		/* W: have wqs started freezing? */
 static DEFINE_PER_CPU(struct global_cwq, global_cwq);
 static DEFINE_PER_CPU_SHARED_ALIGNED(atomic_t, gcwq_nr_running);
 
+/*
+ * Global cpu workqueue and nr_running counter for unbound gcwq.  The
+ * gcwq is always online, has GCWQ_DISASSOCIATED set, and all its
+ * workers have WORKER_UNBOUND set.
+ */
+static struct global_cwq unbound_global_cwq;
+static atomic_t unbound_gcwq_nr_running = ATOMIC_INIT(0);       /* always 0 */
+
 static int worker_thread(void *__worker);
 
 static struct global_cwq *get_gcwq(unsigned int cpu)
 {
-        return &per_cpu(global_cwq, cpu);
+        if (cpu != WORK_CPU_UNBOUND)
+                return &per_cpu(global_cwq, cpu);
+        else
+                return &unbound_global_cwq;
 }
 
 static atomic_t *get_gcwq_nr_running(unsigned int cpu)
 {
-        return &per_cpu(gcwq_nr_running, cpu);
+        if (cpu != WORK_CPU_UNBOUND)
+                return &per_cpu(gcwq_nr_running, cpu);
+        else
+                return &unbound_gcwq_nr_running;
 }
 
 static struct cpu_workqueue_struct *get_cwq(unsigned int cpu,
                                             struct workqueue_struct *wq)
 {
-#ifndef CONFIG_SMP
-        return wq->cpu_wq.single;
+        if (!(wq->flags & WQ_UNBOUND)) {
+                if (likely(cpu < nr_cpu_ids)) {
+#ifdef CONFIG_SMP
+                        return per_cpu_ptr(wq->cpu_wq.pcpu, cpu);
 #else
-        return per_cpu_ptr(wq->cpu_wq.pcpu, cpu);
+                        return wq->cpu_wq.single;
 #endif
+                }
+        } else if (likely(cpu == WORK_CPU_UNBOUND))
+                return wq->cpu_wq.single;
+        return NULL;
 }
 
 static unsigned int work_color_to_flags(int color)
@@ -453,7 +512,7 @@ static struct global_cwq *get_work_gcwq(struct work_struct *work)
         if (cpu == WORK_CPU_NONE)
                 return NULL;
 
-        BUG_ON(cpu >= nr_cpu_ids);
+        BUG_ON(cpu >= nr_cpu_ids && cpu != WORK_CPU_UNBOUND);
         return get_gcwq(cpu);
 }
 
@@ -869,11 +928,14 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
 
         debug_work_activate(work);
 
+        if (unlikely(cpu == WORK_CPU_UNBOUND))
+                cpu = raw_smp_processor_id();
+
         /*
          * Determine gcwq to use.  SINGLE_CPU is inherently
          * NON_REENTRANT, so test it first.
          */
-        if (!(wq->flags & WQ_SINGLE_CPU)) {
+        if (!(wq->flags & (WQ_SINGLE_CPU | WQ_UNBOUND))) {
                 struct global_cwq *last_gcwq;
 
                 /*
@@ -900,7 +962,7 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
                         }
                 } else
                         spin_lock_irqsave(&gcwq->lock, flags);
-        } else {
+        } else if (!(wq->flags & WQ_UNBOUND)) {
                 unsigned int req_cpu = cpu;
 
                 /*
@@ -932,6 +994,9 @@ static void __queue_work(unsigned int cpu, struct workqueue_struct *wq,
                         spin_unlock_irqrestore(&gcwq->lock, flags);
                         goto retry;
                 }
+        } else {
+                gcwq = get_gcwq(WORK_CPU_UNBOUND);
+                spin_lock_irqsave(&gcwq->lock, flags);
         }
 
         /* gcwq determined, get cwq and queue */
@@ -1166,7 +1231,8 @@ static bool worker_maybe_bind_and_lock(struct worker *worker)
                  * it races with cpu hotunplug operation.  Verify
                  * against GCWQ_DISASSOCIATED.
                  */
-                set_cpus_allowed_ptr(task, get_cpu_mask(gcwq->cpu));
+                if (!(gcwq->flags & GCWQ_DISASSOCIATED))
+                        set_cpus_allowed_ptr(task, get_cpu_mask(gcwq->cpu));
 
                 spin_lock_irq(&gcwq->lock);
                 if (gcwq->flags & GCWQ_DISASSOCIATED)
@@ -1231,8 +1297,9 @@ static struct worker *alloc_worker(void)
  */
 static struct worker *create_worker(struct global_cwq *gcwq, bool bind)
 {
-        int id = -1;
+        bool on_unbound_cpu = gcwq->cpu == WORK_CPU_UNBOUND;
         struct worker *worker = NULL;
+        int id = -1;
 
         spin_lock_irq(&gcwq->lock);
         while (ida_get_new(&gcwq->worker_ida, &id)) {
@@ -1250,8 +1317,12 @@ static struct worker *create_worker(struct global_cwq *gcwq, bool bind)
         worker->gcwq = gcwq;
         worker->id = id;
 
-        worker->task = kthread_create(worker_thread, worker, "kworker/%u:%d",
-                                      gcwq->cpu, id);
+        if (!on_unbound_cpu)
+                worker->task = kthread_create(worker_thread, worker,
+                                              "kworker/%u:%d", gcwq->cpu, id);
+        else
+                worker->task = kthread_create(worker_thread, worker,
+                                              "kworker/u:%d", id);
         if (IS_ERR(worker->task))
                 goto fail;
 
@@ -1260,10 +1331,13 @@ static struct worker *create_worker(struct global_cwq *gcwq, bool bind)
          * online later on.  Make sure every worker has
          * PF_THREAD_BOUND set.
          */
-        if (bind)
+        if (bind && !on_unbound_cpu)
                 kthread_bind(worker->task, gcwq->cpu);
-        else
+        else {
                 worker->task->flags |= PF_THREAD_BOUND;
+                if (on_unbound_cpu)
+                        worker->flags |= WORKER_UNBOUND;
+        }
 
         return worker;
 fail:
@@ -1358,12 +1432,17 @@ static bool send_mayday(struct work_struct *work)
 {
         struct cpu_workqueue_struct *cwq = get_work_cwq(work);
         struct workqueue_struct *wq = cwq->wq;
+        unsigned int cpu;
 
         if (!(wq->flags & WQ_RESCUER))
                 return false;
 
         /* mayday mayday mayday */
-        if (!cpumask_test_and_set_cpu(cwq->gcwq->cpu, wq->mayday_mask))
+        cpu = cwq->gcwq->cpu;
+        /* WORK_CPU_UNBOUND can't be set in cpumask, use cpu 0 instead */
+        if (cpu == WORK_CPU_UNBOUND)
+                cpu = 0;
+        if (!cpumask_test_and_set_cpu(cpu, wq->mayday_mask))
                 wake_up_process(wq->rescuer->task);
         return true;
 }
@@ -1882,6 +1961,7 @@ static int rescuer_thread(void *__wq)
         struct workqueue_struct *wq = __wq;
         struct worker *rescuer = wq->rescuer;
         struct list_head *scheduled = &rescuer->scheduled;
+        bool is_unbound = wq->flags & WQ_UNBOUND;
         unsigned int cpu;
 
         set_user_nice(current, RESCUER_NICE_LEVEL);
@@ -1891,8 +1971,13 @@ repeat:
         if (kthread_should_stop())
                 return 0;
 
+        /*
+         * See whether any cpu is asking for help.  Unbounded
+         * workqueues use cpu 0 in mayday_mask for CPU_UNBOUND.
+         */
         for_each_cpu(cpu, wq->mayday_mask) {
-                struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
+                unsigned int tcpu = is_unbound ? WORK_CPU_UNBOUND : cpu;
+                struct cpu_workqueue_struct *cwq = get_cwq(tcpu, wq);
                 struct global_cwq *gcwq = cwq->gcwq;
                 struct work_struct *work, *n;
 
@@ -2034,7 +2119,7 @@ static bool flush_workqueue_prep_cwqs(struct workqueue_struct *wq,
                 atomic_set(&wq->nr_cwqs_to_flush, 1);
         }
 
-        for_each_possible_cpu(cpu) {
+        for_each_cwq_cpu(cpu, wq) {
                 struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
                 struct global_cwq *gcwq = cwq->gcwq;
 
@@ -2344,7 +2429,7 @@ static void wait_on_work(struct work_struct *work)
         lock_map_acquire(&work->lockdep_map);
         lock_map_release(&work->lockdep_map);
 
-        for_each_possible_cpu(cpu)
+        for_each_gcwq_cpu(cpu)
                 wait_on_cpu_work(get_gcwq(cpu), work);
 }
 
@@ -2590,23 +2675,25 @@ static int alloc_cwqs(struct workqueue_struct *wq)
         const size_t size = sizeof(struct cpu_workqueue_struct);
         const size_t align = max_t(size_t, 1 << WORK_STRUCT_FLAG_BITS,
                                    __alignof__(unsigned long long));
-#ifndef CONFIG_SMP
-        void *ptr;
 
-        /*
-         * Allocate enough room to align cwq and put an extra pointer
-         * at the end pointing back to the originally allocated
-         * pointer which will be used for free.
-         */
-        ptr = kzalloc(size + align + sizeof(void *), GFP_KERNEL);
-        if (ptr) {
-                wq->cpu_wq.single = PTR_ALIGN(ptr, align);
-                *(void **)(wq->cpu_wq.single + 1) = ptr;
+        if (CONFIG_SMP && !(wq->flags & WQ_UNBOUND)) {
+                /* on SMP, percpu allocator can align itself */
+                wq->cpu_wq.pcpu = __alloc_percpu(size, align);
+        } else {
+                void *ptr;
+
+                /*
+                 * Allocate enough room to align cwq and put an extra
+                 * pointer at the end pointing back to the originally
+                 * allocated pointer which will be used for free.
+                 */
+                ptr = kzalloc(size + align + sizeof(void *), GFP_KERNEL);
+                if (ptr) {
+                        wq->cpu_wq.single = PTR_ALIGN(ptr, align);
+                        *(void **)(wq->cpu_wq.single + 1) = ptr;
+                }
         }
-#else
-        /* On SMP, percpu allocator can align itself */
-        wq->cpu_wq.pcpu = __alloc_percpu(size, align);
-#endif
+
         /* just in case, make sure it's actually aligned */
         BUG_ON(!IS_ALIGNED(wq->cpu_wq.v, align));
         return wq->cpu_wq.v ? 0 : -ENOMEM;
@@ -2614,23 +2701,25 @@ static int alloc_cwqs(struct workqueue_struct *wq)
 
 static void free_cwqs(struct workqueue_struct *wq)
 {
-#ifndef CONFIG_SMP
-        /* on UP, the pointer to free is stored right after the cwq */
-        if (wq->cpu_wq.single)
+        if (CONFIG_SMP && !(wq->flags & WQ_UNBOUND))
+                free_percpu(wq->cpu_wq.pcpu);
+        else if (wq->cpu_wq.single) {
+                /* the pointer to free is stored right after the cwq */
                 kfree(*(void **)(wq->cpu_wq.single + 1));
-#else
-        free_percpu(wq->cpu_wq.pcpu);
-#endif
+        }
 }
 
-static int wq_clamp_max_active(int max_active, const char *name)
+static int wq_clamp_max_active(int max_active, unsigned int flags,
+                               const char *name)
 {
-        if (max_active < 1 || max_active > WQ_MAX_ACTIVE)
+        int lim = flags & WQ_UNBOUND ? WQ_UNBOUND_MAX_ACTIVE : WQ_MAX_ACTIVE;
+
+        if (max_active < 1 || max_active > lim)
                 printk(KERN_WARNING "workqueue: max_active %d requested for %s "
                        "is out of range, clamping between %d and %d\n",
-                       max_active, name, 1, WQ_MAX_ACTIVE);
+                       max_active, name, 1, lim);
 
-        return clamp_val(max_active, 1, WQ_MAX_ACTIVE);
+        return clamp_val(max_active, 1, lim);
 }
 
 struct workqueue_struct *__alloc_workqueue_key(const char *name,
@@ -2642,8 +2731,15 @@ struct workqueue_struct *__alloc_workqueue_key(const char *name,
         struct workqueue_struct *wq;
         unsigned int cpu;
 
+        /*
+         * Unbound workqueues aren't concurrency managed and should be
+         * dispatched to workers immediately.
+         */
+        if (flags & WQ_UNBOUND)
+                flags |= WQ_HIGHPRI;
+
         max_active = max_active ?: WQ_DFL_ACTIVE;
-        max_active = wq_clamp_max_active(max_active, name);
+        max_active = wq_clamp_max_active(max_active, flags, name);
 
         wq = kzalloc(sizeof(*wq), GFP_KERNEL);
         if (!wq)
@@ -2664,7 +2760,7 @@ struct workqueue_struct *__alloc_workqueue_key(const char *name,
         if (alloc_cwqs(wq) < 0)
                 goto err;
 
-        for_each_possible_cpu(cpu) {
+        for_each_cwq_cpu(cpu, wq) {
                 struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
                 struct global_cwq *gcwq = get_gcwq(cpu);
 
@@ -2703,7 +2799,7 @@ struct workqueue_struct *__alloc_workqueue_key(const char *name,
         spin_lock(&workqueue_lock);
 
         if (workqueue_freezing && wq->flags & WQ_FREEZEABLE)
-                for_each_possible_cpu(cpu)
+                for_each_cwq_cpu(cpu, wq)
                         get_cwq(cpu, wq)->max_active = 0;
 
         list_add(&wq->list, &workqueues);
@@ -2743,7 +2839,7 @@ void destroy_workqueue(struct workqueue_struct *wq)
         spin_unlock(&workqueue_lock);
 
         /* sanity check */
-        for_each_possible_cpu(cpu) {
+        for_each_cwq_cpu(cpu, wq) {
                 struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
                 int i;
 
@@ -2777,13 +2873,13 @@ void workqueue_set_max_active(struct workqueue_struct *wq, int max_active)
 {
         unsigned int cpu;
 
-        max_active = wq_clamp_max_active(max_active, wq->name);
+        max_active = wq_clamp_max_active(max_active, wq->flags, wq->name);
 
         spin_lock(&workqueue_lock);
 
         wq->saved_max_active = max_active;
 
-        for_each_possible_cpu(cpu) {
+        for_each_cwq_cpu(cpu, wq) {
                 struct global_cwq *gcwq = get_gcwq(cpu);
 
                 spin_lock_irq(&gcwq->lock);
@@ -3310,7 +3406,7 @@ void freeze_workqueues_begin(void)
         BUG_ON(workqueue_freezing);
         workqueue_freezing = true;
 
-        for_each_possible_cpu(cpu) {
+        for_each_gcwq_cpu(cpu) {
                 struct global_cwq *gcwq = get_gcwq(cpu);
                 struct workqueue_struct *wq;
 
@@ -3322,7 +3418,7 @@ void freeze_workqueues_begin(void)
                 list_for_each_entry(wq, &workqueues, list) {
                         struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
 
-                        if (wq->flags & WQ_FREEZEABLE)
+                        if (cwq && wq->flags & WQ_FREEZEABLE)
                                 cwq->max_active = 0;
                 }
 
@@ -3354,7 +3450,7 @@ bool freeze_workqueues_busy(void)
 
         BUG_ON(!workqueue_freezing);
 
-        for_each_possible_cpu(cpu) {
+        for_each_gcwq_cpu(cpu) {
                 struct workqueue_struct *wq;
                 /*
                  * nr_active is monotonically decreasing.  It's safe
@@ -3363,7 +3459,7 @@ bool freeze_workqueues_busy(void)
                 list_for_each_entry(wq, &workqueues, list) {
                         struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
 
-                        if (!(wq->flags & WQ_FREEZEABLE))
+                        if (!cwq || !(wq->flags & WQ_FREEZEABLE))
                                 continue;
 
                         BUG_ON(cwq->nr_active < 0);
@@ -3396,7 +3492,7 @@ void thaw_workqueues(void)
         if (!workqueue_freezing)
                 goto out_unlock;
 
-        for_each_possible_cpu(cpu) {
+        for_each_gcwq_cpu(cpu) {
                 struct global_cwq *gcwq = get_gcwq(cpu);
                 struct workqueue_struct *wq;
 
@@ -3408,7 +3504,7 @@ void thaw_workqueues(void)
                 list_for_each_entry(wq, &workqueues, list) {
                         struct cpu_workqueue_struct *cwq = get_cwq(cpu, wq);
 
-                        if (!(wq->flags & WQ_FREEZEABLE))
+                        if (!cwq || !(wq->flags & WQ_FREEZEABLE))
                                 continue;
 
                         /* restore max_active and repopulate worklist */
@@ -3451,12 +3547,14 @@ void __init init_workqueues(void)
         hotcpu_notifier(workqueue_cpu_callback, CPU_PRI_WORKQUEUE);
 
         /* initialize gcwqs */
-        for_each_possible_cpu(cpu) {
+        for_each_gcwq_cpu(cpu) {
                 struct global_cwq *gcwq = get_gcwq(cpu);
 
                 spin_lock_init(&gcwq->lock);
                 INIT_LIST_HEAD(&gcwq->worklist);
                 gcwq->cpu = cpu;
+                if (cpu == WORK_CPU_UNBOUND)
+                        gcwq->flags |= GCWQ_DISASSOCIATED;
 
                 INIT_LIST_HEAD(&gcwq->idle_list);
                 for (i = 0; i < BUSY_WORKER_HASH_SIZE; i++)
@@ -3476,7 +3574,7 @@ void __init init_workqueues(void)
         }
 
         /* create the initial worker */
-        for_each_online_cpu(cpu) {
+        for_each_online_gcwq_cpu(cpu) {
                 struct global_cwq *gcwq = get_gcwq(cpu);
                 struct worker *worker;
 
@@ -3490,5 +3588,7 @@ void __init init_workqueues(void)
         system_wq = alloc_workqueue("events", 0, 0);
         system_long_wq = alloc_workqueue("events_long", 0, 0);
         system_nrt_wq = alloc_workqueue("events_nrt", WQ_NON_REENTRANT, 0);
+        system_unbound_wq = alloc_workqueue("events_unbound", WQ_UNBOUND,
+                                            WQ_UNBOUND_MAX_ACTIVE);
         BUG_ON(!system_wq || !system_long_wq || !system_nrt_wq);
 }