util/thread-pool.c

   1 /*
   2  * QEMU block layer thread pool
   3  *
   4  * Copyright IBM, Corp. 2008
   5  * Copyright Red Hat, Inc. 2012
   6  *
   7  * Authors:
   8  *  Anthony Liguori   <aliguori@us.ibm.com>
   9  *  Paolo Bonzini     <pbonzini@redhat.com>
  10  *
  11  * This work is licensed under the terms of the GNU GPL, version 2.  See
  12  * the COPYING file in the top-level directory.
  13  *
  14  * Contributions after 2012-01-13 are licensed under the terms of the
  15  * GNU GPL, version 2 or (at your option) any later version.
  16  */
  17 #include "qemu/osdep.h"
  18 #include "qemu/queue.h"
  19 #include "qemu/thread.h"
  20 #include "qemu/coroutine.h"
  21 #include "trace.h"
  22 #include "block/thread-pool.h"
  23 #include "qemu/main-loop.h"
  24
  25 static void do_spawn_thread(ThreadPool *pool);
  26
  27 typedef struct ThreadPoolElement ThreadPoolElement;
  28
  29 enum ThreadState {
  30     THREAD_QUEUED,
  31     THREAD_ACTIVE,
  32     THREAD_DONE,
  33 };
  34
  35 struct ThreadPoolElement {
  36     BlockAIOCB common;
  37     ThreadPool *pool;
  38     ThreadPoolFunc *func;
  39     void *arg;
  40
  41     /* Moving state out of THREAD_QUEUED is protected by lock.  After
  42      * that, only the worker thread can write to it.  Reads and writes
  43      * of state and ret are ordered with memory barriers.
  44      */
  45     enum ThreadState state;
  46     int ret;
  47
  48     /* Access to this list is protected by lock.  */
  49     QTAILQ_ENTRY(ThreadPoolElement) reqs;
  50
  51     /* Access to this list is protected by the global mutex.  */
  52     QLIST_ENTRY(ThreadPoolElement) all;
  53 };
  54
  55 struct ThreadPool {
  56     AioContext *ctx;
  57     QEMUBH *completion_bh;
  58     QemuMutex lock;
  59     QemuCond worker_stopped;
  60     QemuSemaphore sem;
  61     QEMUBH *new_thread_bh;
  62
  63     /* The following variables are only accessed from one AioContext. */
  64     QLIST_HEAD(, ThreadPoolElement) head;
  65
  66     /* The following variables are protected by lock.  */
  67     QTAILQ_HEAD(, ThreadPoolElement) request_list;
  68     int cur_threads;
  69     int idle_threads;
  70     int new_threads;     /* backlog of threads we need to create */
  71     int pending_threads; /* threads created but not running yet */
  72     bool stopping;
  73     int min_threads;
  74     int max_threads;
  75 };
  76
  77 static inline bool back_to_sleep(ThreadPool *pool, int ret)
  78 {
  79     /*
  80      * The semaphore timed out, we should exit the loop except when:
  81      *  - There is work to do, we raced with the signal.
  82      *  - The max threads threshold just changed, we raced with the signal.
  83      *  - The thread pool forces a minimum number of readily available threads.
  84      */
  85     if (ret == -1 && (!QTAILQ_EMPTY(&pool->request_list) ||
  86             pool->cur_threads > pool->max_threads ||
  87             pool->cur_threads <= pool->min_threads)) {
  88             return true;
  89     }
  90
  91     return false;
  92 }
  93
  94 static void *worker_thread(void *opaque)
  95 {
  96     ThreadPool *pool = opaque;
  97
  98     qemu_mutex_lock(&pool->lock);
  99     pool->pending_threads--;
 100     do_spawn_thread(pool);
 101
 102     while (!pool->stopping) {
 103         ThreadPoolElement *req;
 104         int ret;
 105
 106         do {
 107             pool->idle_threads++;
 108             qemu_mutex_unlock(&pool->lock);
 109             ret = qemu_sem_timedwait(&pool->sem, 10000);
 110             qemu_mutex_lock(&pool->lock);
 111             pool->idle_threads--;
 112         } while (back_to_sleep(pool, ret));
 113         if (ret == -1 || pool->stopping ||
 114             pool->cur_threads > pool->max_threads) {
 115             break;
 116         }
 117
 118         req = QTAILQ_FIRST(&pool->request_list);
 119         QTAILQ_REMOVE(&pool->request_list, req, reqs);
 120         req->state = THREAD_ACTIVE;
 121         qemu_mutex_unlock(&pool->lock);
 122
 123         ret = req->func(req->arg);
 124
 125         req->ret = ret;
 126         /* Write ret before state.  */
 127         smp_wmb();
 128         req->state = THREAD_DONE;
 129
 130         qemu_mutex_lock(&pool->lock);
 131
 132         qemu_bh_schedule(pool->completion_bh);
 133     }
 134
 135     pool->cur_threads--;
 136     qemu_cond_signal(&pool->worker_stopped);
 137     qemu_mutex_unlock(&pool->lock);
 138     return NULL;
 139 }
 140
 141 static void do_spawn_thread(ThreadPool *pool)
 142 {
 143     QemuThread t;
 144
 145     /* Runs with lock taken.  */
 146     if (!pool->new_threads) {
 147         return;
 148     }
 149
 150     pool->new_threads--;
 151     pool->pending_threads++;
 152
 153     qemu_thread_create(&t, "worker", worker_thread, pool, QEMU_THREAD_DETACHED);
 154 }
 155
 156 static void spawn_thread_bh_fn(void *opaque)
 157 {
 158     ThreadPool *pool = opaque;
 159
 160     qemu_mutex_lock(&pool->lock);
 161     do_spawn_thread(pool);
 162     qemu_mutex_unlock(&pool->lock);
 163 }
 164
 165 static void spawn_thread(ThreadPool *pool)
 166 {
 167     pool->cur_threads++;
 168     pool->new_threads++;
 169     /* If there are threads being created, they will spawn new workers, so
 170      * we don't spend time creating many threads in a loop holding a mutex or
 171      * starving the current vcpu.
 172      *
 173      * If there are no idle threads, ask the main thread to create one, so we
 174      * inherit the correct affinity instead of the vcpu affinity.
 175      */
 176     if (!pool->pending_threads) {
 177         qemu_bh_schedule(pool->new_thread_bh);
 178     }
 179 }
 180
 181 static void thread_pool_completion_bh(void *opaque)
 182 {
 183     ThreadPool *pool = opaque;
 184     ThreadPoolElement *elem, *next;
 185
 186     aio_context_acquire(pool->ctx);
 187 restart:
 188     QLIST_FOREACH_SAFE(elem, &pool->head, all, next) {
 189         if (elem->state != THREAD_DONE) {
 190             continue;
 191         }
 192
 193         trace_thread_pool_complete(pool, elem, elem->common.opaque,
 194                                    elem->ret);
 195         QLIST_REMOVE(elem, all);
 196
 197         if (elem->common.cb) {
 198             /* Read state before ret.  */
 199             smp_rmb();
 200
 201             /* Schedule ourselves in case elem->common.cb() calls aio_poll() to
 202              * wait for another request that completed at the same time.
 203              */
 204             qemu_bh_schedule(pool->completion_bh);
 205
 206             aio_context_release(pool->ctx);
 207             elem->common.cb(elem->common.opaque, elem->ret);
 208             aio_context_acquire(pool->ctx);
 209
 210             /* We can safely cancel the completion_bh here regardless of someone
 211              * else having scheduled it meanwhile because we reenter the
 212              * completion function anyway (goto restart).
 213              */
 214             qemu_bh_cancel(pool->completion_bh);
 215
 216             qemu_aio_unref(elem);
 217             goto restart;
 218         } else {
 219             qemu_aio_unref(elem);
 220         }
 221     }
 222     aio_context_release(pool->ctx);
 223 }
 224
 225 static void thread_pool_cancel(BlockAIOCB *acb)
 226 {
 227     ThreadPoolElement *elem = (ThreadPoolElement *)acb;
 228     ThreadPool *pool = elem->pool;
 229
 230     trace_thread_pool_cancel(elem, elem->common.opaque);
 231
 232     QEMU_LOCK_GUARD(&pool->lock);
 233     if (elem->state == THREAD_QUEUED &&
 234         /* No thread has yet started working on elem. we can try to "steal"
 235          * the item from the worker if we can get a signal from the
 236          * semaphore.  Because this is non-blocking, we can do it with
 237          * the lock taken and ensure that elem will remain THREAD_QUEUED.
 238          */
 239         qemu_sem_timedwait(&pool->sem, 0) == 0) {
 240         QTAILQ_REMOVE(&pool->request_list, elem, reqs);
 241         qemu_bh_schedule(pool->completion_bh);
 242
 243         elem->state = THREAD_DONE;
 244         elem->ret = -ECANCELED;
 245     }
 246
 247 }
 248
 249 static AioContext *thread_pool_get_aio_context(BlockAIOCB *acb)
 250 {
 251     ThreadPoolElement *elem = (ThreadPoolElement *)acb;
 252     ThreadPool *pool = elem->pool;
 253     return pool->ctx;
 254 }
 255
 256 static const AIOCBInfo thread_pool_aiocb_info = {
 257     .aiocb_size         = sizeof(ThreadPoolElement),
 258     .cancel_async       = thread_pool_cancel,
 259     .get_aio_context    = thread_pool_get_aio_context,
 260 };
 261
 262 BlockAIOCB *thread_pool_submit_aio(ThreadPool *pool,
 263         ThreadPoolFunc *func, void *arg,
 264         BlockCompletionFunc *cb, void *opaque)
 265 {
 266     ThreadPoolElement *req;
 267
 268     req = qemu_aio_get(&thread_pool_aiocb_info, NULL, cb, opaque);
 269     req->func = func;
 270     req->arg = arg;
 271     req->state = THREAD_QUEUED;
 272     req->pool = pool;
 273
 274     QLIST_INSERT_HEAD(&pool->head, req, all);
 275
 276     trace_thread_pool_submit(pool, req, arg);
 277
 278     qemu_mutex_lock(&pool->lock);
 279     if (pool->idle_threads == 0 && pool->cur_threads < pool->max_threads) {
 280         spawn_thread(pool);
 281     }
 282     QTAILQ_INSERT_TAIL(&pool->request_list, req, reqs);
 283     qemu_mutex_unlock(&pool->lock);
 284     qemu_sem_post(&pool->sem);
 285     return &req->common;
 286 }
 287
 288 typedef struct ThreadPoolCo {
 289     Coroutine *co;
 290     int ret;
 291 } ThreadPoolCo;
 292
 293 static void thread_pool_co_cb(void *opaque, int ret)
 294 {
 295     ThreadPoolCo *co = opaque;
 296
 297     co->ret = ret;
 298     aio_co_wake(co->co);
 299 }
 300
 301 int coroutine_fn thread_pool_submit_co(ThreadPool *pool, ThreadPoolFunc *func,
 302                                        void *arg)
 303 {
 304     ThreadPoolCo tpc = { .co = qemu_coroutine_self(), .ret = -EINPROGRESS };
 305     assert(qemu_in_coroutine());
 306     thread_pool_submit_aio(pool, func, arg, thread_pool_co_cb, &tpc);
 307     qemu_coroutine_yield();
 308     return tpc.ret;
 309 }
 310
 311 void thread_pool_submit(ThreadPool *pool, ThreadPoolFunc *func, void *arg)
 312 {
 313     thread_pool_submit_aio(pool, func, arg, NULL, NULL);
 314 }
 315
 316 void thread_pool_update_params(ThreadPool *pool, AioContext *ctx)
 317 {
 318     qemu_mutex_lock(&pool->lock);
 319
 320     pool->min_threads = ctx->thread_pool_min;
 321     pool->max_threads = ctx->thread_pool_max;
 322
 323     /*
 324      * We either have to:
 325      *  - Increase the number available of threads until over the min_threads
 326      *    threshold.
 327      *  - Decrease the number of available threads until under the max_threads
 328      *    threshold.
 329      *  - Do nothing. The current number of threads fall in between the min and
 330      *    max thresholds. We'll let the pool manage itself.
 331      */
 332     for (int i = pool->cur_threads; i < pool->min_threads; i++) {
 333         spawn_thread(pool);
 334     }
 335
 336     for (int i = pool->cur_threads; i > pool->max_threads; i--) {
 337         qemu_sem_post(&pool->sem);
 338     }
 339
 340     qemu_mutex_unlock(&pool->lock);
 341 }
 342
 343 static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
 344 {
 345     if (!ctx) {
 346         ctx = qemu_get_aio_context();
 347     }
 348
 349     memset(pool, 0, sizeof(*pool));
 350     pool->ctx = ctx;
 351     pool->completion_bh = aio_bh_new(ctx, thread_pool_completion_bh, pool);
 352     qemu_mutex_init(&pool->lock);
 353     qemu_cond_init(&pool->worker_stopped);
 354     qemu_sem_init(&pool->sem, 0);
 355     pool->new_thread_bh = aio_bh_new(ctx, spawn_thread_bh_fn, pool);
 356
 357     QLIST_INIT(&pool->head);
 358     QTAILQ_INIT(&pool->request_list);
 359
 360     thread_pool_update_params(pool, ctx);
 361 }
 362
 363 ThreadPool *thread_pool_new(AioContext *ctx)
 364 {
 365     ThreadPool *pool = g_new(ThreadPool, 1);
 366     thread_pool_init_one(pool, ctx);
 367     return pool;
 368 }
 369
 370 void thread_pool_free(ThreadPool *pool)
 371 {
 372     if (!pool) {
 373         return;
 374     }
 375
 376     assert(QLIST_EMPTY(&pool->head));
 377
 378     qemu_mutex_lock(&pool->lock);
 379
 380     /* Stop new threads from spawning */
 381     qemu_bh_delete(pool->new_thread_bh);
 382     pool->cur_threads -= pool->new_threads;
 383     pool->new_threads = 0;
 384
 385     /* Wait for worker threads to terminate */
 386     pool->stopping = true;
 387     while (pool->cur_threads > 0) {
 388         qemu_sem_post(&pool->sem);
 389         qemu_cond_wait(&pool->worker_stopped, &pool->lock);
 390     }
 391
 392     qemu_mutex_unlock(&pool->lock);
 393
 394     qemu_bh_delete(pool->completion_bh);
 395     qemu_sem_destroy(&pool->sem);
 396     qemu_cond_destroy(&pool->worker_stopped);
 397     qemu_mutex_destroy(&pool->lock);
 398     g_free(pool);
 399 }