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     /* This list is only written by the thread pool's mother thread.  */
  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
 124     /*
 125      * Wake up another thread, in case we got a wakeup but decided
 126      * to exit due to pool->cur_threads > pool->max_threads.
 127      */
 128     qemu_cond_signal(&pool->request_cond);
 129     qemu_mutex_unlock(&pool->lock);
 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 restart:
 179     QLIST_FOREACH_SAFE(elem, &pool->head, all, next) {
 180         if (elem->state != THREAD_DONE) {
 181             continue;
 182         }
 183
 184         trace_thread_pool_complete(pool, elem, elem->common.opaque,
 185                                    elem->ret);
 186         QLIST_REMOVE(elem, all);
 187
 188         if (elem->common.cb) {
 189             /* Read state before ret.  */
 190             smp_rmb();
 191
 192             /* Schedule ourselves in case elem->common.cb() calls aio_poll() to
 193              * wait for another request that completed at the same time.
 194              */
 195             qemu_bh_schedule(pool->completion_bh);
 196
 197             elem->common.cb(elem->common.opaque, elem->ret);
 198
 199             /* We can safely cancel the completion_bh here regardless of someone
 200              * else having scheduled it meanwhile because we reenter the
 201              * completion function anyway (goto restart).
 202              */
 203             qemu_bh_cancel(pool->completion_bh);
 204
 205             qemu_aio_unref(elem);
 206             goto restart;
 207         } else {
 208             qemu_aio_unref(elem);
 209         }
 210     }
 211 }
 212
 213 static void thread_pool_cancel(BlockAIOCB *acb)
 214 {
 215     ThreadPoolElement *elem = (ThreadPoolElement *)acb;
 216     ThreadPool *pool = elem->pool;
 217
 218     trace_thread_pool_cancel(elem, elem->common.opaque);
 219
 220     QEMU_LOCK_GUARD(&pool->lock);
 221     if (elem->state == THREAD_QUEUED) {
 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 }
 230
 231 static const AIOCBInfo thread_pool_aiocb_info = {
 232     .aiocb_size         = sizeof(ThreadPoolElement),
 233     .cancel_async       = thread_pool_cancel,
 234 };
 235
 236 BlockAIOCB *thread_pool_submit_aio(ThreadPoolFunc *func, void *arg,
 237                                    BlockCompletionFunc *cb, void *opaque)
 238 {
 239     ThreadPoolElement *req;
 240     AioContext *ctx = qemu_get_current_aio_context();
 241     ThreadPool *pool = aio_get_thread_pool(ctx);
 242
 243     /* Assert that the thread submitting work is the same running the pool */
 244     assert(pool->ctx == qemu_get_current_aio_context());
 245
 246     req = qemu_aio_get(&thread_pool_aiocb_info, NULL, cb, opaque);
 247     req->func = func;
 248     req->arg = arg;
 249     req->state = THREAD_QUEUED;
 250     req->pool = pool;
 251
 252     QLIST_INSERT_HEAD(&pool->head, req, all);
 253
 254     trace_thread_pool_submit(pool, req, arg);
 255
 256     qemu_mutex_lock(&pool->lock);
 257     if (pool->idle_threads == 0 && pool->cur_threads < pool->max_threads) {
 258         spawn_thread(pool);
 259     }
 260     QTAILQ_INSERT_TAIL(&pool->request_list, req, reqs);
 261     qemu_mutex_unlock(&pool->lock);
 262     qemu_cond_signal(&pool->request_cond);
 263     return &req->common;
 264 }
 265
 266 typedef struct ThreadPoolCo {
 267     Coroutine *co;
 268     int ret;
 269 } ThreadPoolCo;
 270
 271 static void thread_pool_co_cb(void *opaque, int ret)
 272 {
 273     ThreadPoolCo *co = opaque;
 274
 275     co->ret = ret;
 276     aio_co_wake(co->co);
 277 }
 278
 279 int coroutine_fn thread_pool_submit_co(ThreadPoolFunc *func, void *arg)
 280 {
 281     ThreadPoolCo tpc = { .co = qemu_coroutine_self(), .ret = -EINPROGRESS };
 282     assert(qemu_in_coroutine());
 283     thread_pool_submit_aio(func, arg, thread_pool_co_cb, &tpc);
 284     qemu_coroutine_yield();
 285     return tpc.ret;
 286 }
 287
 288 void thread_pool_submit(ThreadPoolFunc *func, void *arg)
 289 {
 290     thread_pool_submit_aio(func, arg, NULL, NULL);
 291 }
 292
 293 void thread_pool_update_params(ThreadPool *pool, AioContext *ctx)
 294 {
 295     qemu_mutex_lock(&pool->lock);
 296
 297     pool->min_threads = ctx->thread_pool_min;
 298     pool->max_threads = ctx->thread_pool_max;
 299
 300     /*
 301      * We either have to:
 302      *  - Increase the number available of threads until over the min_threads
 303      *    threshold.
 304      *  - Bump the worker threads so that they exit, until under the max_threads
 305      *    threshold.
 306      *  - Do nothing. The current number of threads fall in between the min and
 307      *    max thresholds. We'll let the pool manage itself.
 308      */
 309     for (int i = pool->cur_threads; i < pool->min_threads; i++) {
 310         spawn_thread(pool);
 311     }
 312
 313     for (int i = pool->cur_threads; i > pool->max_threads; i--) {
 314         qemu_cond_signal(&pool->request_cond);
 315     }
 316
 317     qemu_mutex_unlock(&pool->lock);
 318 }
 319
 320 static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
 321 {
 322     if (!ctx) {
 323         ctx = qemu_get_aio_context();
 324     }
 325
 326     memset(pool, 0, sizeof(*pool));
 327     pool->ctx = ctx;
 328     pool->completion_bh = aio_bh_new(ctx, thread_pool_completion_bh, pool);
 329     qemu_mutex_init(&pool->lock);
 330     qemu_cond_init(&pool->worker_stopped);
 331     qemu_cond_init(&pool->request_cond);
 332     pool->new_thread_bh = aio_bh_new(ctx, spawn_thread_bh_fn, pool);
 333
 334     QLIST_INIT(&pool->head);
 335     QTAILQ_INIT(&pool->request_list);
 336
 337     thread_pool_update_params(pool, ctx);
 338 }
 339
 340 ThreadPool *thread_pool_new(AioContext *ctx)
 341 {
 342     ThreadPool *pool = g_new(ThreadPool, 1);
 343     thread_pool_init_one(pool, ctx);
 344     return pool;
 345 }
 346
 347 void thread_pool_free(ThreadPool *pool)
 348 {
 349     if (!pool) {
 350         return;
 351     }
 352
 353     assert(QLIST_EMPTY(&pool->head));
 354
 355     qemu_mutex_lock(&pool->lock);
 356
 357     /* Stop new threads from spawning */
 358     qemu_bh_delete(pool->new_thread_bh);
 359     pool->cur_threads -= pool->new_threads;
 360     pool->new_threads = 0;
 361
 362     /* Wait for worker threads to terminate */
 363     pool->max_threads = 0;
 364     qemu_cond_broadcast(&pool->request_cond);
 365     while (pool->cur_threads > 0) {
 366         qemu_cond_wait(&pool->worker_stopped, &pool->lock);
 367     }
 368
 369     qemu_mutex_unlock(&pool->lock);
 370
 371     qemu_bh_delete(pool->completion_bh);
 372     qemu_cond_destroy(&pool->request_cond);
 373     qemu_cond_destroy(&pool->worker_stopped);
 374     qemu_mutex_destroy(&pool->lock);
 375     g_free(pool);
 376 }