urcu/workqueue-fifo.h

   1 #ifndef _URCU_WORKQUEUE_FIFO_H
   2 #define _URCU_WORKQUEUE_FIFO_H
   3
   4 /*
   5  * urcu/workqueue-fifo.h
   6  *
   7  * Userspace RCU library - work queue scheme with FIFO semantic
   8  *
   9  * Copyright (c) 2014 Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
  10  *
  11  * This library is free software; you can redistribute it and/or
  12  * modify it under the terms of the GNU Lesser General Public
  13  * License as published by the Free Software Foundation; either
  14  * version 2.1 of the License, or (at your option) any later version.
  15  *
  16  * This library is distributed in the hope that it will be useful,
  17  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  18  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  19  * Lesser General Public License for more details.
  20  *
  21  * You should have received a copy of the GNU Lesser General Public
  22  * License along with this library; if not, write to the Free Software
  23  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  24  */
  25
  26 #include <urcu/uatomic.h>
  27 #include <urcu/wfstack.h>
  28 #include <urcu/waitqueue-lifo.h>
  29 #include <urcu/wfcqueue.h>
  30 #include <urcu/rculist.h>
  31 #include <pthread.h>
  32 #include <assert.h>
  33
  34 /*
  35  * We use RCU to steal work from siblings. Therefore, one of RCU flavors
  36  * need to be included before this header. All worker that participate
  37  * in stealing (initialized with the URCU_WORKER_STEAL flag) need to be
  38  * registered RCU readers threads.
  39  */
  40
  41 struct urcu_work {
  42         struct cds_wfcq_node node;
  43 };
  44
  45 struct urcu_workqueue {
  46         /* FIFO work queue */
  47         struct __cds_wfcq_head head;
  48         struct cds_wfcq_tail tail;
  49
  50         /* Associated wait queue for LIFO wait/wakeup */
  51         struct urcu_wait_queue waitqueue;
  52
  53         /* RCU linked list head of siblings for work stealing. */
  54         struct cds_list_head sibling_head;
  55         pthread_mutex_t sibling_lock;   /* Protect sibling list updates */
  56 };
  57
  58 struct urcu_worker {
  59         struct cds_wfcq_head head;
  60         struct cds_wfcq_tail tail;
  61
  62         struct urcu_wait_node wait_node;
  63         /* RCU linked list node of siblings for work stealing. */
  64         struct cds_list_head sibling_node;
  65         int flags;      /* enum urcu_worker_flags */
  66 };
  67
  68 enum urcu_worker_flags {
  69         URCU_WORKER_STEAL       = (1 << 0),
  70 };
  71
  72 static inline
  73 void urcu_workqueue_init(struct urcu_workqueue *queue)
  74 {
  75         __cds_wfcq_init(&queue->head, &queue->tail);
  76         urcu_wait_queue_init(&queue->waitqueue);
  77         CDS_INIT_LIST_HEAD(&queue->sibling_head);
  78 }
  79
  80 static inline
  81 void urcu_queue_work(struct urcu_workqueue *queue, struct urcu_work *work)
  82 {
  83         bool was_empty;
  84
  85         cds_wfcq_node_init(&work->node);
  86
  87         /* Enqueue work. */
  88         was_empty = !cds_wfcq_enqueue(&queue->head, &queue->tail,
  89                         &work->node);
  90         /*
  91          * If workqueue was previously empty, wakeup one worker thread.
  92          * It will eventually grab the entire content of the work-queue
  93          * (therefore grabbing a "work batch"). After having grabbed the
  94          * work batch, while that thread is running and taking care of
  95          * that work batch, when we enqueue more work, we will wake
  96          * another thread (if there is one waiting), which will
  97          * eventually grab the new batch, and so on. This scheme ensures
  98          * that contiguous batch of work are handled by the same thread
  99          * (for locality), and also ensures that we scale work to many
 100          * worker threads when threads are busy enough to still be
 101          * running when work is enqueued.
 102          */
 103         if (was_empty)
 104                 (void) urcu_dequeue_wake_single(&queue->waitqueue);
 105 }
 106
 107 static inline
 108 void urcu_workqueue_wakeup_all(struct urcu_workqueue *queue)
 109 {
 110         struct urcu_waiters waiters;
 111
 112         urcu_move_waiters(&waiters, &queue->waitqueue);
 113         (void) urcu_wake_all_waiters(&waiters);
 114 }
 115
 116 static inline
 117 void urcu_worker_init(struct urcu_worker *worker, int flags)
 118 {
 119         cds_wfcq_init(&worker->head, &worker->tail);
 120         worker->flags = flags;
 121         urcu_wait_node_init(&worker->wait_node, URCU_WAIT_RUNNING);
 122 }
 123
 124 static inline
 125 void urcu_worker_register(struct urcu_workqueue *queue,
 126                 struct urcu_worker *worker)
 127 {
 128         if (worker->flags & URCU_WORKER_STEAL) {
 129                 pthread_mutex_lock(&queue->sibling_lock);
 130                 cds_list_add_rcu(&worker->sibling_node, &queue->sibling_head);
 131                 pthread_mutex_unlock(&queue->sibling_lock);
 132         }
 133 }
 134
 135 static inline
 136 void urcu_worker_unregister(struct urcu_workqueue *queue,
 137                 struct urcu_worker *worker)
 138 {
 139         enum cds_wfcq_ret wfcq_ret;
 140
 141         if (worker->flags & URCU_WORKER_STEAL) {
 142                 pthread_mutex_lock(&queue->sibling_lock);
 143                 cds_list_del_rcu(&worker->sibling_node);
 144                 pthread_mutex_unlock(&queue->sibling_lock);
 145
 146                 /*
 147                  * Wait for grace period before freeing or reusing
 148                  * "worker" because used by RCU linked list.
 149                  */
 150                 synchronize_rcu();
 151         }
 152
 153         /*
 154          * Put any local work we still have back into the workqueue.
 155          */
 156         wfcq_ret = __cds_wfcq_splice_blocking(&queue->head,
 157                         &queue->tail,
 158                         &worker->head,
 159                         &worker->tail);
 160         if (wfcq_ret != CDS_WFCQ_RET_SRC_EMPTY
 161                         && wfcq_ret == CDS_WFCQ_RET_DEST_EMPTY) {
 162                 /*
 163                  * Wakeup worker thread if we have put work back into
 164                  * workqueue that was previously empty.
 165                  */
 166                 (void) urcu_dequeue_wake_single(&queue->waitqueue);
 167         }
 168 }
 169
 170 /*
 171  * Try stealing work from siblings when we have nothing to do.
 172  */
 173 static inline
 174 bool ___urcu_steal_work(struct urcu_worker *worker,
 175                 struct urcu_worker *sibling)
 176 {
 177         enum cds_wfcq_ret splice_ret;
 178
 179         /*
 180          * Don't bother grabbing the sibling queue lock if it is empty.
 181          */
 182         if (cds_wfcq_empty(&sibling->head, &sibling->tail))
 183                 return false;
 184         cds_wfcq_dequeue_lock(&sibling->head, &sibling->tail);
 185         splice_ret = __cds_wfcq_splice_blocking(&worker->head,
 186                         &worker->tail,
 187                         &sibling->head,
 188                         &sibling->tail);
 189         cds_wfcq_dequeue_unlock(&sibling->head, &sibling->tail);
 190         /* Ensure that we preserve FIFO work order. */
 191         assert(splice_ret != CDS_WFCQ_RET_DEST_NON_EMPTY);
 192         return splice_ret != CDS_WFCQ_RET_SRC_EMPTY;
 193 }
 194
 195 static inline
 196 bool __urcu_steal_work(struct urcu_workqueue *queue,
 197                 struct urcu_worker *worker)
 198 {
 199         struct urcu_worker *sibling_prev, *sibling_next;
 200         struct cds_list_head *sibling_node;
 201         bool steal_performed = 0;
 202
 203         if (!(worker->flags & URCU_WORKER_STEAL))
 204                 return false;
 205
 206         rcu_read_lock();
 207
 208         sibling_node = rcu_dereference(worker->sibling_node.next);
 209         if (sibling_node == &queue->sibling_head)
 210                 sibling_node = rcu_dereference(sibling_node->next);
 211         sibling_next = caa_container_of(sibling_node, struct urcu_worker,
 212                         sibling_node);
 213         if (sibling_next != worker)
 214                 steal_performed = ___urcu_steal_work(worker, sibling_next);
 215         if (steal_performed)
 216                 goto end;
 217
 218         sibling_node = rcu_dereference(worker->sibling_node.prev);
 219         if (sibling_node == &queue->sibling_head)
 220                 sibling_node = rcu_dereference(sibling_node->prev);
 221         sibling_prev = caa_container_of(sibling_node, struct urcu_worker,
 222                         sibling_node);
 223         if (sibling_prev != worker && sibling_prev != sibling_next)
 224                 steal_performed =  ___urcu_steal_work(worker, sibling_prev);
 225 end:
 226         rcu_read_unlock();
 227
 228         return steal_performed;
 229 }
 230
 231 static inline
 232 bool ___urcu_wakeup_sibling(struct urcu_worker *sibling)
 233 {
 234         return urcu_adaptative_wake_up(&sibling->wait_node);
 235 }
 236
 237 static inline
 238 bool __urcu_wakeup_siblings(struct urcu_workqueue *queue,
 239                 struct urcu_worker *worker)
 240 {
 241         struct urcu_worker *sibling_prev, *sibling_next;
 242         struct cds_list_head *sibling_node;
 243         bool wakeup_performed = 0;
 244
 245         if (!(worker->flags & URCU_WORKER_STEAL))
 246                 return;
 247
 248         /* Only wakeup siblings if we have work in our own queue. */
 249         if (cds_wfcq_empty(&worker->head, &worker->tail))
 250                 return;
 251
 252         rcu_read_lock();
 253
 254         sibling_node = rcu_dereference(worker->sibling_node.next);
 255         if (sibling_node == &queue->sibling_head)
 256                 sibling_node = rcu_dereference(sibling_node->next);
 257         sibling_next = caa_container_of(sibling_node, struct urcu_worker,
 258                         sibling_node);
 259         if (sibling_next != worker)
 260                 wakeup_performed = ___urcu_wakeup_sibling(sibling_next);
 261         if (wakeup_performed)
 262                 goto end;
 263
 264         sibling_node = rcu_dereference(worker->sibling_node.prev);
 265         if (sibling_node == &queue->sibling_head)
 266                 sibling_node = rcu_dereference(sibling_node->prev);
 267         sibling_prev = caa_container_of(sibling_node, struct urcu_worker,
 268                         sibling_node);
 269         if (sibling_prev != worker && sibling_prev != sibling_next)
 270                 wakeup_performed = ___urcu_wakeup_sibling(sibling_prev);
 271 end:
 272         rcu_read_unlock();
 273
 274         return wakeup_performed;
 275 }
 276
 277 static inline
 278 void urcu_accept_work(struct urcu_workqueue *queue,
 279                 struct urcu_worker *worker,
 280                 int blocking)
 281 {
 282         enum cds_wfcq_ret wfcq_ret;
 283
 284         wfcq_ret = __cds_wfcq_splice_blocking(&worker->head,
 285                         &worker->tail,
 286                         &queue->head,
 287                         &queue->tail);
 288         /* Don't wait if we have work to do. */
 289         if (wfcq_ret != CDS_WFCQ_RET_SRC_EMPTY
 290                         || !cds_wfcq_empty(&worker->head,
 291                                 &worker->tail))
 292                 goto do_work;
 293         /* Try to steal work from sibling instead of blocking */
 294         if (__urcu_steal_work(queue, worker))
 295                 goto do_work;
 296         if (!blocking)
 297                 return;
 298         urcu_wait_set_state(&worker->wait_node,
 299                         URCU_WAIT_WAITING);
 300         if (!CMM_LOAD_SHARED(worker->wait_node.node.next)) {
 301                 int was_empty;
 302
 303                 /*
 304                  * NULL next pointer. We are therefore not in
 305                  * the queue.
 306                  */
 307                 cds_wfs_node_init(&worker->wait_node.node);
 308                 was_empty = !urcu_wait_add(&queue->waitqueue,
 309                                 &worker->wait_node);
 310                 /*
 311                  * If the wait queue was empty, it means we are the
 312                  * first thread to be put back into an otherwise empty
 313                  * wait queue. Re-check if work queue is empty after
 314                  * adding ourself to wait queue, so we can wakeup the
 315                  * top of wait queue since new work have appeared, and
 316                  * work enqueuer may not have seen that it needed to do
 317                  * a wake up.
 318                  */
 319                 if (was_empty && !cds_wfcq_empty(&queue->head,
 320                                                 &queue->tail))
 321                         (void) urcu_dequeue_wake_single(&queue->waitqueue);
 322         } else {
 323                 /*
 324                  * Non-NULL next pointer. We are therefore in
 325                  * the queue, or the dispatcher just removed us
 326                  * from it (after we read the next pointer), and
 327                  * is therefore awakening us. The state will
 328                  * therefore have been changed from WAITING to
 329                  * some other state, which will let the busy
 330                  * wait pass through.
 331                  */
 332         }
 333         urcu_adaptative_busy_wait(&worker->wait_node);
 334         return;
 335
 336 do_work:
 337         /*
 338          * We will be busy handling the work batch, awaken siblings so
 339          * they can steal from us.
 340          */
 341         (void) __urcu_wakeup_siblings(queue, worker);
 342 }
 343
 344 static inline
 345 struct urcu_work *urcu_dequeue_work(struct urcu_worker *worker)
 346 {
 347         struct cds_wfcq_node *node;
 348
 349         /*
 350          * If we are registered for work stealing, we need to dequeue
 351          * safely against siblings.
 352          */
 353         if (worker->flags & URCU_WORKER_STEAL) {
 354                 /*
 355                  * Don't bother grabbing the worker queue lock if it is
 356                  * empty.
 357                  */
 358                 if (cds_wfcq_empty(&worker->head, &worker->tail))
 359                         return NULL;
 360                 node = cds_wfcq_dequeue_blocking(&worker->head,
 361                                 &worker->tail);
 362         } else {
 363                 node = ___cds_wfcq_dequeue_with_state(&worker->head,
 364                                 &worker->tail, NULL, 1, 0);
 365         }
 366         if (!node)
 367                 return NULL;
 368         return caa_container_of(node, struct urcu_work, node);
 369 }
 370
 371 #endif /* _URCU_WORKQUEUE_FIFO_H */