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