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     QemuCond request_cond;
  61     QEMUBH *new_thread_bh;
  62
  63     /* The following variables are only accessed from one AioContext. */
  64     QLIST_HEAD(, ThreadPoolElement) head;
  65
  66     /* The following variables are protected by lock.  */
  67     QTAILQ_HEAD(, ThreadPoolElement) request_list;
  68     int cur_threads;
  69     int idle_threads;
  70     int new_threads;     /* backlog of threads we need to create */
  71     int pending_threads; /* threads created but not running yet */
  72     int min_threads;
  73     int max_threads;
  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->cur_threads <= pool->max_threads) {
  85         ThreadPoolElement *req;
  86         int ret;
  87
  88         if (QTAILQ_EMPTY(&pool->request_list)) {
  89             pool->idle_threads++;
  90             ret = qemu_cond_timedwait(&pool->request_cond, &pool->lock, 10000);
  91             pool->idle_threads--;
  92             if (ret == 0 &&
  93                 QTAILQ_EMPTY(&pool->request_list) &&
  94                 pool->cur_threads > pool->min_threads) {
  95                 /* Timed out + no work to do + no need for warm threads = exit.  */
  96                 break;
  97             }
  98             /*
  99              * Even if there was some work to do, check if there aren't
 100              * too many worker threads before picking it up.
 101              */
 102             continue;
 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_bh_schedule(pool->completion_bh);
 118         qemu_mutex_lock(&pool->lock);
 119     }
 120
 121     pool->cur_threads--;
 122     qemu_cond_signal(&pool->worker_stopped);
 123     qemu_mutex_unlock(&pool->lock);
 124
 125     /*
 126      * Wake up another thread, in case we got a wakeup but decided
 127      * to exit due to pool->cur_threads > pool->max_threads.
 128      */
 129     qemu_cond_signal(&pool->request_cond);
 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", 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 thread_pool_completion_bh(void *opaque)
 174 {
 175     ThreadPool *pool = opaque;
 176     ThreadPoolElement *elem, *next;
 177
 178     aio_context_acquire(pool->ctx);
 179 restart:
 180     QLIST_FOREACH_SAFE(elem, &pool->head, all, next) {
 181         if (elem->state != THREAD_DONE) {
 182             continue;
 183         }
 184
 185         trace_thread_pool_complete(pool, elem, elem->common.opaque,
 186                                    elem->ret);
 187         QLIST_REMOVE(elem, all);
 188
 189         if (elem->common.cb) {
 190             /* Read state before ret.  */
 191             smp_rmb();
 192
 193             /* Schedule ourselves in case elem->common.cb() calls aio_poll() to
 194              * wait for another request that completed at the same time.
 195              */
 196             qemu_bh_schedule(pool->completion_bh);
 197
 198             aio_context_release(pool->ctx);
 199             elem->common.cb(elem->common.opaque, elem->ret);
 200             aio_context_acquire(pool->ctx);
 201
 202             /* We can safely cancel the completion_bh here regardless of someone
 203              * else having scheduled it meanwhile because we reenter the
 204              * completion function anyway (goto restart).
 205              */
 206             qemu_bh_cancel(pool->completion_bh);
 207
 208             qemu_aio_unref(elem);
 209             goto restart;
 210         } else {
 211             qemu_aio_unref(elem);
 212         }
 213     }
 214     aio_context_release(pool->ctx);
 215 }
 216
 217 static void thread_pool_cancel(BlockAIOCB *acb)
 218 {
 219     ThreadPoolElement *elem = (ThreadPoolElement *)acb;
 220     ThreadPool *pool = elem->pool;
 221
 222     trace_thread_pool_cancel(elem, elem->common.opaque);
 223
 224     QEMU_LOCK_GUARD(&pool->lock);
 225     if (elem->state == THREAD_QUEUED) {
 226         QTAILQ_REMOVE(&pool->request_list, elem, reqs);
 227         qemu_bh_schedule(pool->completion_bh);
 228
 229         elem->state = THREAD_DONE;
 230         elem->ret = -ECANCELED;
 231     }
 232
 233 }
 234
 235 static AioContext *thread_pool_get_aio_context(BlockAIOCB *acb)
 236 {
 237     ThreadPoolElement *elem = (ThreadPoolElement *)acb;
 238     ThreadPool *pool = elem->pool;
 239     return pool->ctx;
 240 }
 241
 242 static const AIOCBInfo thread_pool_aiocb_info = {
 243     .aiocb_size         = sizeof(ThreadPoolElement),
 244     .cancel_async       = thread_pool_cancel,
 245     .get_aio_context    = thread_pool_get_aio_context,
 246 };
 247
 248 BlockAIOCB *thread_pool_submit_aio(ThreadPool *pool,
 249         ThreadPoolFunc *func, void *arg,
 250         BlockCompletionFunc *cb, void *opaque)
 251 {
 252     ThreadPoolElement *req;
 253
 254     req = qemu_aio_get(&thread_pool_aiocb_info, NULL, cb, opaque);
 255     req->func = func;
 256     req->arg = arg;
 257     req->state = THREAD_QUEUED;
 258     req->pool = pool;
 259
 260     QLIST_INSERT_HEAD(&pool->head, req, all);
 261
 262     trace_thread_pool_submit(pool, req, arg);
 263
 264     qemu_mutex_lock(&pool->lock);
 265     if (pool->idle_threads == 0 && pool->cur_threads < pool->max_threads) {
 266         spawn_thread(pool);
 267     }
 268     QTAILQ_INSERT_TAIL(&pool->request_list, req, reqs);
 269     qemu_mutex_unlock(&pool->lock);
 270     qemu_cond_signal(&pool->request_cond);
 271     return &req->common;
 272 }
 273
 274 typedef struct ThreadPoolCo {
 275     Coroutine *co;
 276     int ret;
 277 } ThreadPoolCo;
 278
 279 static void thread_pool_co_cb(void *opaque, int ret)
 280 {
 281     ThreadPoolCo *co = opaque;
 282
 283     co->ret = ret;
 284     aio_co_wake(co->co);
 285 }
 286
 287 int coroutine_fn thread_pool_submit_co(ThreadPool *pool, ThreadPoolFunc *func,
 288                                        void *arg)
 289 {
 290     ThreadPoolCo tpc = { .co = qemu_coroutine_self(), .ret = -EINPROGRESS };
 291     assert(qemu_in_coroutine());
 292     thread_pool_submit_aio(pool, func, arg, thread_pool_co_cb, &tpc);
 293     qemu_coroutine_yield();
 294     return tpc.ret;
 295 }
 296
 297 void thread_pool_submit(ThreadPool *pool, ThreadPoolFunc *func, void *arg)
 298 {
 299     thread_pool_submit_aio(pool, func, arg, NULL, NULL);
 300 }
 301
 302 void thread_pool_update_params(ThreadPool *pool, AioContext *ctx)
 303 {
 304     qemu_mutex_lock(&pool->lock);
 305
 306     pool->min_threads = ctx->thread_pool_min;
 307     pool->max_threads = ctx->thread_pool_max;
 308
 309     /*
 310      * We either have to:
 311      *  - Increase the number available of threads until over the min_threads
 312      *    threshold.
 313      *  - Bump the worker threads so that they exit, until under the max_threads
 314      *    threshold.
 315      *  - Do nothing. The current number of threads fall in between the min and
 316      *    max thresholds. We'll let the pool manage itself.
 317      */
 318     for (int i = pool->cur_threads; i < pool->min_threads; i++) {
 319         spawn_thread(pool);
 320     }
 321
 322     for (int i = pool->cur_threads; i > pool->max_threads; i--) {
 323         qemu_cond_signal(&pool->request_cond);
 324     }
 325
 326     qemu_mutex_unlock(&pool->lock);
 327 }
 328
 329 static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
 330 {
 331     if (!ctx) {
 332         ctx = qemu_get_aio_context();
 333     }
 334
 335     memset(pool, 0, sizeof(*pool));
 336     pool->ctx = ctx;
 337     pool->completion_bh = aio_bh_new(ctx, thread_pool_completion_bh, pool);
 338     qemu_mutex_init(&pool->lock);
 339     qemu_cond_init(&pool->worker_stopped);
 340     qemu_cond_init(&pool->request_cond);
 341     pool->new_thread_bh = aio_bh_new(ctx, spawn_thread_bh_fn, pool);
 342
 343     QLIST_INIT(&pool->head);
 344     QTAILQ_INIT(&pool->request_list);
 345
 346     thread_pool_update_params(pool, ctx);
 347 }
 348
 349 ThreadPool *thread_pool_new(AioContext *ctx)
 350 {
 351     ThreadPool *pool = g_new(ThreadPool, 1);
 352     thread_pool_init_one(pool, ctx);
 353     return pool;
 354 }
 355
 356 void thread_pool_free(ThreadPool *pool)
 357 {
 358     if (!pool) {
 359         return;
 360     }
 361
 362     assert(QLIST_EMPTY(&pool->head));
 363
 364     qemu_mutex_lock(&pool->lock);
 365
 366     /* Stop new threads from spawning */
 367     qemu_bh_delete(pool->new_thread_bh);
 368     pool->cur_threads -= pool->new_threads;
 369     pool->new_threads = 0;
 370
 371     /* Wait for worker threads to terminate */
 372     pool->max_threads = 0;
 373     qemu_cond_broadcast(&pool->request_cond);
 374     while (pool->cur_threads > 0) {
 375         qemu_cond_wait(&pool->worker_stopped, &pool->lock);
 376     }
 377
 378     qemu_mutex_unlock(&pool->lock);
 379
 380     qemu_bh_delete(pool->completion_bh);
 381     qemu_cond_destroy(&pool->request_cond);
 382     qemu_cond_destroy(&pool->worker_stopped);
 383     qemu_mutex_destroy(&pool->lock);
 384     g_free(pool);
 385 }