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