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-common.h"
  18 #include "qemu/queue.h"
  19 #include "qemu/thread.h"
  20 #include "qemu/osdep.h"
  21 #include "block/coroutine.h"
  22 #include "trace.h"
  23 #include "block/block_int.h"
  24 #include "qemu/event_notifier.h"
  25 #include "block/thread-pool.h"
  26
  27 static void do_spawn_thread(ThreadPool *pool);
  28
  29 typedef struct ThreadPoolElement ThreadPoolElement;
  30
  31 enum ThreadState {
  32     THREAD_QUEUED,
  33     THREAD_ACTIVE,
  34     THREAD_DONE,
  35     THREAD_CANCELED,
  36 };
  37
  38 struct ThreadPoolElement {
  39     BlockDriverAIOCB common;
  40     ThreadPool *pool;
  41     ThreadPoolFunc *func;
  42     void *arg;
  43
  44     /* Moving state out of THREAD_QUEUED is protected by lock.  After
  45      * that, only the worker thread can write to it.  Reads and writes
  46      * of state and ret are ordered with memory barriers.
  47      */
  48     enum ThreadState state;
  49     int ret;
  50
  51     /* Access to this list is protected by lock.  */
  52     QTAILQ_ENTRY(ThreadPoolElement) reqs;
  53
  54     /* Access to this list is protected by the global mutex.  */
  55     QLIST_ENTRY(ThreadPoolElement) all;
  56 };
  57
  58 struct ThreadPool {
  59     EventNotifier notifier;
  60     AioContext *ctx;
  61     QemuMutex lock;
  62     QemuCond check_cancel;
  63     QemuCond worker_stopped;
  64     QemuSemaphore sem;
  65     int max_threads;
  66     QEMUBH *new_thread_bh;
  67
  68     /* The following variables are only accessed from one AioContext. */
  69     QLIST_HEAD(, ThreadPoolElement) head;
  70
  71     /* The following variables are protected by lock.  */
  72     QTAILQ_HEAD(, ThreadPoolElement) request_list;
  73     int cur_threads;
  74     int idle_threads;
  75     int new_threads;     /* backlog of threads we need to create */
  76     int pending_threads; /* threads created but not running yet */
  77     int pending_cancellations; /* whether we need a cond_broadcast */
  78     bool stopping;
  79 };
  80
  81 /* Currently there is only one thread pool instance. */
  82 static ThreadPool global_pool;
  83
  84 static void *worker_thread(void *opaque)
  85 {
  86     ThreadPool *pool = opaque;
  87
  88     qemu_mutex_lock(&pool->lock);
  89     pool->pending_threads--;
  90     do_spawn_thread(pool);
  91
  92     while (!pool->stopping) {
  93         ThreadPoolElement *req;
  94         int ret;
  95
  96         do {
  97             pool->idle_threads++;
  98             qemu_mutex_unlock(&pool->lock);
  99             ret = qemu_sem_timedwait(&pool->sem, 10000);
 100             qemu_mutex_lock(&pool->lock);
 101             pool->idle_threads--;
 102         } while (ret == -1 && !QTAILQ_EMPTY(&pool->request_list));
 103         if (ret == -1 || pool->stopping) {
 104             break;
 105         }
 106
 107         req = QTAILQ_FIRST(&pool->request_list);
 108         QTAILQ_REMOVE(&pool->request_list, req, reqs);
 109         req->state = THREAD_ACTIVE;
 110         qemu_mutex_unlock(&pool->lock);
 111
 112         ret = req->func(req->arg);
 113
 114         req->ret = ret;
 115         /* Write ret before state.  */
 116         smp_wmb();
 117         req->state = THREAD_DONE;
 118
 119         qemu_mutex_lock(&pool->lock);
 120         if (pool->pending_cancellations) {
 121             qemu_cond_broadcast(&pool->check_cancel);
 122         }
 123
 124         event_notifier_set(&pool->notifier);
 125     }
 126
 127     pool->cur_threads--;
 128     qemu_cond_signal(&pool->worker_stopped);
 129     qemu_mutex_unlock(&pool->lock);
 130     return NULL;
 131 }
 132
 133 static void do_spawn_thread(ThreadPool *pool)
 134 {
 135     QemuThread t;
 136
 137     /* Runs with lock taken.  */
 138     if (!pool->new_threads) {
 139         return;
 140     }
 141
 142     pool->new_threads--;
 143     pool->pending_threads++;
 144
 145     qemu_thread_create(&t, worker_thread, pool, QEMU_THREAD_DETACHED);
 146 }
 147
 148 static void spawn_thread_bh_fn(void *opaque)
 149 {
 150     ThreadPool *pool = opaque;
 151
 152     qemu_mutex_lock(&pool->lock);
 153     do_spawn_thread(pool);
 154     qemu_mutex_unlock(&pool->lock);
 155 }
 156
 157 static void spawn_thread(ThreadPool *pool)
 158 {
 159     pool->cur_threads++;
 160     pool->new_threads++;
 161     /* If there are threads being created, they will spawn new workers, so
 162      * we don't spend time creating many threads in a loop holding a mutex or
 163      * starving the current vcpu.
 164      *
 165      * If there are no idle threads, ask the main thread to create one, so we
 166      * inherit the correct affinity instead of the vcpu affinity.
 167      */
 168     if (!pool->pending_threads) {
 169         qemu_bh_schedule(pool->new_thread_bh);
 170     }
 171 }
 172
 173 static void event_notifier_ready(EventNotifier *notifier)
 174 {
 175     ThreadPool *pool = container_of(notifier, ThreadPool, notifier);
 176     ThreadPoolElement *elem, *next;
 177
 178     event_notifier_test_and_clear(notifier);
 179 restart:
 180     QLIST_FOREACH_SAFE(elem, &pool->head, all, next) {
 181         if (elem->state != THREAD_CANCELED && elem->state != THREAD_DONE) {
 182             continue;
 183         }
 184         if (elem->state == THREAD_DONE) {
 185             trace_thread_pool_complete(pool, elem, elem->common.opaque,
 186                                        elem->ret);
 187         }
 188         if (elem->state == THREAD_DONE && elem->common.cb) {
 189             QLIST_REMOVE(elem, all);
 190             /* Read state before ret.  */
 191             smp_rmb();
 192             elem->common.cb(elem->common.opaque, elem->ret);
 193             qemu_aio_release(elem);
 194             goto restart;
 195         } else {
 196             /* remove the request */
 197             QLIST_REMOVE(elem, all);
 198             qemu_aio_release(elem);
 199         }
 200     }
 201 }
 202
 203 static int thread_pool_active(EventNotifier *notifier)
 204 {
 205     ThreadPool *pool = container_of(notifier, ThreadPool, notifier);
 206     return !QLIST_EMPTY(&pool->head);
 207 }
 208
 209 static void thread_pool_cancel(BlockDriverAIOCB *acb)
 210 {
 211     ThreadPoolElement *elem = (ThreadPoolElement *)acb;
 212     ThreadPool *pool = elem->pool;
 213
 214     trace_thread_pool_cancel(elem, elem->common.opaque);
 215
 216     qemu_mutex_lock(&pool->lock);
 217     if (elem->state == THREAD_QUEUED &&
 218         /* No thread has yet started working on elem. we can try to "steal"
 219          * the item from the worker if we can get a signal from the
 220          * semaphore.  Because this is non-blocking, we can do it with
 221          * the lock taken and ensure that elem will remain THREAD_QUEUED.
 222          */
 223         qemu_sem_timedwait(&pool->sem, 0) == 0) {
 224         QTAILQ_REMOVE(&pool->request_list, elem, reqs);
 225         elem->state = THREAD_CANCELED;
 226         event_notifier_set(&pool->notifier);
 227     } else {
 228         pool->pending_cancellations++;
 229         while (elem->state != THREAD_CANCELED && elem->state != THREAD_DONE) {
 230             qemu_cond_wait(&pool->check_cancel, &pool->lock);
 231         }
 232         pool->pending_cancellations--;
 233     }
 234     qemu_mutex_unlock(&pool->lock);
 235 }
 236
 237 static const AIOCBInfo thread_pool_aiocb_info = {
 238     .aiocb_size         = sizeof(ThreadPoolElement),
 239     .cancel             = thread_pool_cancel,
 240 };
 241
 242 BlockDriverAIOCB *thread_pool_submit_aio(ThreadPoolFunc *func, void *arg,
 243         BlockDriverCompletionFunc *cb, void *opaque)
 244 {
 245     ThreadPool *pool = &global_pool;
 246     ThreadPoolElement *req;
 247
 248     req = qemu_aio_get(&thread_pool_aiocb_info, NULL, cb, opaque);
 249     req->func = func;
 250     req->arg = arg;
 251     req->state = THREAD_QUEUED;
 252     req->pool = pool;
 253
 254     QLIST_INSERT_HEAD(&pool->head, req, all);
 255
 256     trace_thread_pool_submit(pool, req, arg);
 257
 258     qemu_mutex_lock(&pool->lock);
 259     if (pool->idle_threads == 0 && pool->cur_threads < pool->max_threads) {
 260         spawn_thread(pool);
 261     }
 262     QTAILQ_INSERT_TAIL(&pool->request_list, req, reqs);
 263     qemu_mutex_unlock(&pool->lock);
 264     qemu_sem_post(&pool->sem);
 265     return &req->common;
 266 }
 267
 268 typedef struct ThreadPoolCo {
 269     Coroutine *co;
 270     int ret;
 271 } ThreadPoolCo;
 272
 273 static void thread_pool_co_cb(void *opaque, int ret)
 274 {
 275     ThreadPoolCo *co = opaque;
 276
 277     co->ret = ret;
 278     qemu_coroutine_enter(co->co, NULL);
 279 }
 280
 281 int coroutine_fn thread_pool_submit_co(ThreadPoolFunc *func, void *arg)
 282 {
 283     ThreadPoolCo tpc = { .co = qemu_coroutine_self(), .ret = -EINPROGRESS };
 284     assert(qemu_in_coroutine());
 285     thread_pool_submit_aio(func, arg, thread_pool_co_cb, &tpc);
 286     qemu_coroutine_yield();
 287     return tpc.ret;
 288 }
 289
 290 void thread_pool_submit(ThreadPoolFunc *func, void *arg)
 291 {
 292     thread_pool_submit_aio(func, arg, NULL, NULL);
 293 }
 294
 295 static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
 296 {
 297     if (!ctx) {
 298         ctx = qemu_get_aio_context();
 299     }
 300
 301     memset(pool, 0, sizeof(*pool));
 302     event_notifier_init(&pool->notifier, false);
 303     pool->ctx = ctx;
 304     qemu_mutex_init(&pool->lock);
 305     qemu_cond_init(&pool->check_cancel);
 306     qemu_cond_init(&pool->worker_stopped);
 307     qemu_sem_init(&pool->sem, 0);
 308     pool->max_threads = 64;
 309     pool->new_thread_bh = aio_bh_new(ctx, spawn_thread_bh_fn, pool);
 310
 311     QLIST_INIT(&pool->head);
 312     QTAILQ_INIT(&pool->request_list);
 313
 314     aio_set_event_notifier(ctx, &pool->notifier, event_notifier_ready,
 315                            thread_pool_active);
 316 }
 317
 318 ThreadPool *thread_pool_new(AioContext *ctx)
 319 {
 320     ThreadPool *pool = g_new(ThreadPool, 1);
 321     thread_pool_init_one(pool, ctx);
 322     return pool;
 323 }
 324
 325 void thread_pool_free(ThreadPool *pool)
 326 {
 327     if (!pool) {
 328         return;
 329     }
 330
 331     assert(QLIST_EMPTY(&pool->head));
 332
 333     qemu_mutex_lock(&pool->lock);
 334
 335     /* Stop new threads from spawning */
 336     qemu_bh_delete(pool->new_thread_bh);
 337     pool->cur_threads -= pool->new_threads;
 338     pool->new_threads = 0;
 339
 340     /* Wait for worker threads to terminate */
 341     pool->stopping = true;
 342     while (pool->cur_threads > 0) {
 343         qemu_sem_post(&pool->sem);
 344         qemu_cond_wait(&pool->worker_stopped, &pool->lock);
 345     }
 346
 347     qemu_mutex_unlock(&pool->lock);
 348
 349     aio_set_event_notifier(pool->ctx, &pool->notifier, NULL, NULL);
 350     qemu_sem_destroy(&pool->sem);
 351     qemu_cond_destroy(&pool->check_cancel);
 352     qemu_cond_destroy(&pool->worker_stopped);
 353     qemu_mutex_destroy(&pool->lock);
 354     event_notifier_cleanup(&pool->notifier);
 355     g_free(pool);
 356 }
 357
 358 static void thread_pool_init(void)
 359 {
 360     thread_pool_init_one(&global_pool, NULL);
 361 }
 362
 363 block_init(thread_pool_init)