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