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     int max_threads;
  62     QEMUBH *new_thread_bh;
  63
  64     /* The following variables are only accessed from one AioContext. */
  65     QLIST_HEAD(, ThreadPoolElement) head;
  66
  67     /* The following variables are protected by lock.  */
  68     QTAILQ_HEAD(, ThreadPoolElement) request_list;
  69     int cur_threads;
  70     int idle_threads;
  71     int new_threads;     /* backlog of threads we need to create */
  72     int pending_threads; /* threads created but not running yet */
  73     bool stopping;
  74 };
  75
  76 static void *worker_thread(void *opaque)
  77 {
  78     ThreadPool *pool = opaque;
  79
  80     qemu_mutex_lock(&pool->lock);
  81     pool->pending_threads--;
  82     do_spawn_thread(pool);
  83
  84     while (!pool->stopping) {
  85         ThreadPoolElement *req;
  86         int ret;
  87
  88         do {
  89             pool->idle_threads++;
  90             qemu_mutex_unlock(&pool->lock);
  91             ret = qemu_sem_timedwait(&pool->sem, 10000);
  92             qemu_mutex_lock(&pool->lock);
  93             pool->idle_threads--;
  94         } while (ret == -1 && !QTAILQ_EMPTY(&pool->request_list));
  95         if (ret == -1 || pool->stopping) {
  96             break;
  97         }
  98
  99         req = QTAILQ_FIRST(&pool->request_list);
 100         QTAILQ_REMOVE(&pool->request_list, req, reqs);
 101         req->state = THREAD_ACTIVE;
 102         qemu_mutex_unlock(&pool->lock);
 103
 104         ret = req->func(req->arg);
 105
 106         req->ret = ret;
 107         /* Write ret before state.  */
 108         smp_wmb();
 109         req->state = THREAD_DONE;
 110
 111         qemu_mutex_lock(&pool->lock);
 112
 113         qemu_bh_schedule(pool->completion_bh);
 114     }
 115
 116     pool->cur_threads--;
 117     qemu_cond_signal(&pool->worker_stopped);
 118     qemu_mutex_unlock(&pool->lock);
 119     return NULL;
 120 }
 121
 122 static void do_spawn_thread(ThreadPool *pool)
 123 {
 124     QemuThread t;
 125
 126     /* Runs with lock taken.  */
 127     if (!pool->new_threads) {
 128         return;
 129     }
 130
 131     pool->new_threads--;
 132     pool->pending_threads++;
 133
 134     qemu_thread_create(&t, "worker", worker_thread, pool, QEMU_THREAD_DETACHED);
 135 }
 136
 137 static void spawn_thread_bh_fn(void *opaque)
 138 {
 139     ThreadPool *pool = opaque;
 140
 141     qemu_mutex_lock(&pool->lock);
 142     do_spawn_thread(pool);
 143     qemu_mutex_unlock(&pool->lock);
 144 }
 145
 146 static void spawn_thread(ThreadPool *pool)
 147 {
 148     pool->cur_threads++;
 149     pool->new_threads++;
 150     /* If there are threads being created, they will spawn new workers, so
 151      * we don't spend time creating many threads in a loop holding a mutex or
 152      * starving the current vcpu.
 153      *
 154      * If there are no idle threads, ask the main thread to create one, so we
 155      * inherit the correct affinity instead of the vcpu affinity.
 156      */
 157     if (!pool->pending_threads) {
 158         qemu_bh_schedule(pool->new_thread_bh);
 159     }
 160 }
 161
 162 static void thread_pool_completion_bh(void *opaque)
 163 {
 164     ThreadPool *pool = opaque;
 165     ThreadPoolElement *elem, *next;
 166
 167     aio_context_acquire(pool->ctx);
 168 restart:
 169     QLIST_FOREACH_SAFE(elem, &pool->head, all, next) {
 170         if (elem->state != THREAD_DONE) {
 171             continue;
 172         }
 173
 174         trace_thread_pool_complete(pool, elem, elem->common.opaque,
 175                                    elem->ret);
 176         QLIST_REMOVE(elem, all);
 177
 178         if (elem->common.cb) {
 179             /* Read state before ret.  */
 180             smp_rmb();
 181
 182             /* Schedule ourselves in case elem->common.cb() calls aio_poll() to
 183              * wait for another request that completed at the same time.
 184              */
 185             qemu_bh_schedule(pool->completion_bh);
 186
 187             aio_context_release(pool->ctx);
 188             elem->common.cb(elem->common.opaque, elem->ret);
 189             aio_context_acquire(pool->ctx);
 190
 191             /* We can safely cancel the completion_bh here regardless of someone
 192              * else having scheduled it meanwhile because we reenter the
 193              * completion function anyway (goto restart).
 194              */
 195             qemu_bh_cancel(pool->completion_bh);
 196
 197             qemu_aio_unref(elem);
 198             goto restart;
 199         } else {
 200             qemu_aio_unref(elem);
 201         }
 202     }
 203     aio_context_release(pool->ctx);
 204 }
 205
 206 static void thread_pool_cancel(BlockAIOCB *acb)
 207 {
 208     ThreadPoolElement *elem = (ThreadPoolElement *)acb;
 209     ThreadPool *pool = elem->pool;
 210
 211     trace_thread_pool_cancel(elem, elem->common.opaque);
 212
 213     qemu_mutex_lock(&pool->lock);
 214     if (elem->state == THREAD_QUEUED &&
 215         /* No thread has yet started working on elem. we can try to "steal"
 216          * the item from the worker if we can get a signal from the
 217          * semaphore.  Because this is non-blocking, we can do it with
 218          * the lock taken and ensure that elem will remain THREAD_QUEUED.
 219          */
 220         qemu_sem_timedwait(&pool->sem, 0) == 0) {
 221         QTAILQ_REMOVE(&pool->request_list, elem, reqs);
 222         qemu_bh_schedule(pool->completion_bh);
 223
 224         elem->state = THREAD_DONE;
 225         elem->ret = -ECANCELED;
 226     }
 227
 228     qemu_mutex_unlock(&pool->lock);
 229 }
 230
 231 static AioContext *thread_pool_get_aio_context(BlockAIOCB *acb)
 232 {
 233     ThreadPoolElement *elem = (ThreadPoolElement *)acb;
 234     ThreadPool *pool = elem->pool;
 235     return pool->ctx;
 236 }
 237
 238 static const AIOCBInfo thread_pool_aiocb_info = {
 239     .aiocb_size         = sizeof(ThreadPoolElement),
 240     .cancel_async       = thread_pool_cancel,
 241     .get_aio_context    = thread_pool_get_aio_context,
 242 };
 243
 244 BlockAIOCB *thread_pool_submit_aio(ThreadPool *pool,
 245         ThreadPoolFunc *func, void *arg,
 246         BlockCompletionFunc *cb, void *opaque)
 247 {
 248     ThreadPoolElement *req;
 249
 250     req = qemu_aio_get(&thread_pool_aiocb_info, NULL, cb, opaque);
 251     req->func = func;
 252     req->arg = arg;
 253     req->state = THREAD_QUEUED;
 254     req->pool = pool;
 255
 256     QLIST_INSERT_HEAD(&pool->head, req, all);
 257
 258     trace_thread_pool_submit(pool, req, arg);
 259
 260     qemu_mutex_lock(&pool->lock);
 261     if (pool->idle_threads == 0 && pool->cur_threads < pool->max_threads) {
 262         spawn_thread(pool);
 263     }
 264     QTAILQ_INSERT_TAIL(&pool->request_list, req, reqs);
 265     qemu_mutex_unlock(&pool->lock);
 266     qemu_sem_post(&pool->sem);
 267     return &req->common;
 268 }
 269
 270 typedef struct ThreadPoolCo {
 271     Coroutine *co;
 272     int ret;
 273 } ThreadPoolCo;
 274
 275 static void thread_pool_co_cb(void *opaque, int ret)
 276 {
 277     ThreadPoolCo *co = opaque;
 278
 279     co->ret = ret;
 280     aio_co_wake(co->co);
 281 }
 282
 283 int coroutine_fn thread_pool_submit_co(ThreadPool *pool, ThreadPoolFunc *func,
 284                                        void *arg)
 285 {
 286     ThreadPoolCo tpc = { .co = qemu_coroutine_self(), .ret = -EINPROGRESS };
 287     assert(qemu_in_coroutine());
 288     thread_pool_submit_aio(pool, func, arg, thread_pool_co_cb, &tpc);
 289     qemu_coroutine_yield();
 290     return tpc.ret;
 291 }
 292
 293 void thread_pool_submit(ThreadPool *pool, ThreadPoolFunc *func, void *arg)
 294 {
 295     thread_pool_submit_aio(pool, func, arg, NULL, NULL);
 296 }
 297
 298 static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
 299 {
 300     if (!ctx) {
 301         ctx = qemu_get_aio_context();
 302     }
 303
 304     memset(pool, 0, sizeof(*pool));
 305     pool->ctx = ctx;
 306     pool->completion_bh = aio_bh_new(ctx, thread_pool_completion_bh, pool);
 307     qemu_mutex_init(&pool->lock);
 308     qemu_cond_init(&pool->worker_stopped);
 309     qemu_sem_init(&pool->sem, 0);
 310     pool->max_threads = 64;
 311     pool->new_thread_bh = aio_bh_new(ctx, spawn_thread_bh_fn, pool);
 312
 313     QLIST_INIT(&pool->head);
 314     QTAILQ_INIT(&pool->request_list);
 315 }
 316
 317 ThreadPool *thread_pool_new(AioContext *ctx)
 318 {
 319     ThreadPool *pool = g_new(ThreadPool, 1);
 320     thread_pool_init_one(pool, ctx);
 321     return pool;
 322 }
 323
 324 void thread_pool_free(ThreadPool *pool)
 325 {
 326     if (!pool) {
 327         return;
 328     }
 329
 330     assert(QLIST_EMPTY(&pool->head));
 331
 332     qemu_mutex_lock(&pool->lock);
 333
 334     /* Stop new threads from spawning */
 335     qemu_bh_delete(pool->new_thread_bh);
 336     pool->cur_threads -= pool->new_threads;
 337     pool->new_threads = 0;
 338
 339     /* Wait for worker threads to terminate */
 340     pool->stopping = true;
 341     while (pool->cur_threads > 0) {
 342         qemu_sem_post(&pool->sem);
 343         qemu_cond_wait(&pool->worker_stopped, &pool->lock);
 344     }
 345
 346     qemu_mutex_unlock(&pool->lock);
 347
 348     qemu_bh_delete(pool->completion_bh);
 349     qemu_sem_destroy(&pool->sem);
 350     qemu_cond_destroy(&pool->worker_stopped);
 351     qemu_mutex_destroy(&pool->lock);
 352     g_free(pool);
 353 }