thread-pool: replace semaphore with condition variable

Since commit f9fc8932b1 ("thread-posix: remove the posix semaphore
support", 2022-04-06) QemuSemaphore has its own mutex and condition
variable; this adds unnecessary overhead on I/O with small block sizes.

Check the QTAILQ directly instead of adding the indirection of a
semaphore's count.  Using a semaphore has not been necessary since
qemu_cond_timedwait was introduced; the new code has to be careful about
spurious wakeups but it is simpler, for example thread_pool_cancel does
not have to worry about synchronizing the semaphore count with the number
of elements of pool->request_list.

Note that the return value of qemu_cond_timedwait (0 for timeout, 1 for
signal or spurious wakeup) is different from that of qemu_sem_timedwait
(-1 for timeout, 0 for success).

Reported-by: Lukáš Doktor <ldoktor@redhat.com>
Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Message-Id: <20220514065012.1149539-3-pbonzini@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

util/thread-pool.c

@@ -57,7 +57,7 @@ struct ThreadPool {
     QEMUBH *completion_bh;
     QemuMutex lock;
     QemuCond worker_stopped;
-    QemuSemaphore sem;
+    QemuCond request_cond;
     QEMUBH *new_thread_bh;
 
     /* The following variables are only accessed from one AioContext. */
@@ -74,23 +74,6 @@ struct ThreadPool {
     int max_threads;
 };
 
-static inline bool back_to_sleep(ThreadPool *pool, int ret)
-{
-    /*
-     * The semaphore timed out, we should exit the loop except when:
-     *  - There is work to do, we raced with the signal.
-     *  - The max threads threshold just changed, we raced with the signal.
-     *  - The thread pool forces a minimum number of readily available threads.
-     */
-    if (ret == -1 && (!QTAILQ_EMPTY(&pool->request_list) ||
-            pool->cur_threads > pool->max_threads ||
-            pool->cur_threads <= pool->min_threads)) {
-            return true;
-    }
-
-    return false;
-}
-
 static void *worker_thread(void *opaque)
 {
     ThreadPool *pool = opaque;
@@ -99,20 +82,25 @@ static void *worker_thread(void *opaque)
     pool->pending_threads--;
     do_spawn_thread(pool);
 
-    while (!pool->stopping) {
+    while (!pool->stopping && pool->cur_threads <= pool->max_threads) {
         ThreadPoolElement *req;
         int ret;
 
-        do {
+        if (QTAILQ_EMPTY(&pool->request_list)) {
             pool->idle_threads++;
-            qemu_mutex_unlock(&pool->lock);
+            ret = qemu_cond_timedwait(&pool->request_cond, &pool->lock, 10000);
-            ret = qemu_sem_timedwait(&pool->sem, 10000);
-            qemu_mutex_lock(&pool->lock);
             pool->idle_threads--;
-        } while (back_to_sleep(pool, ret));
+            if (ret == 0 &&
-        if (ret == -1 || pool->stopping ||
+                QTAILQ_EMPTY(&pool->request_list) &&
-            pool->cur_threads > pool->max_threads) {
+                pool->cur_threads > pool->min_threads) {
-            break;
+                /* Timed out + no work to do + no need for warm threads = exit.  */
+                break;
+            }
+            /*
+             * Even if there was some work to do, check if there aren't
+             * too many worker threads before picking it up.
+             */
+            continue;
         }
 
         req = QTAILQ_FIRST(&pool->request_list);
@@ -134,6 +122,12 @@ static void *worker_thread(void *opaque)
     pool->cur_threads--;
     qemu_cond_signal(&pool->worker_stopped);
     qemu_mutex_unlock(&pool->lock);
+
+    /*
+     * Wake up another thread, in case we got a wakeup but decided
+     * to exit due to pool->cur_threads > pool->max_threads.
+     */
+    qemu_cond_signal(&pool->request_cond);
     return NULL;
 }
 
@@ -229,13 +223,7 @@ static void thread_pool_cancel(BlockAIOCB *acb)
     trace_thread_pool_cancel(elem, elem->common.opaque);
 
     QEMU_LOCK_GUARD(&pool->lock);
-    if (elem->state == THREAD_QUEUED &&
+    if (elem->state == THREAD_QUEUED) {
-        /* No thread has yet started working on elem. we can try to "steal"
-         * the item from the worker if we can get a signal from the
-         * semaphore.  Because this is non-blocking, we can do it with
-         * the lock taken and ensure that elem will remain THREAD_QUEUED.
-         */
-        qemu_sem_timedwait(&pool->sem, 0) == 0) {
         QTAILQ_REMOVE(&pool->request_list, elem, reqs);
         qemu_bh_schedule(pool->completion_bh);
 
@@ -280,7 +268,7 @@ BlockAIOCB *thread_pool_submit_aio(ThreadPool *pool,
     }
     QTAILQ_INSERT_TAIL(&pool->request_list, req, reqs);
     qemu_mutex_unlock(&pool->lock);
-    qemu_sem_post(&pool->sem);
+    qemu_cond_signal(&pool->request_cond);
     return &req->common;
 }
 
@@ -323,7 +311,7 @@ void thread_pool_update_params(ThreadPool *pool, AioContext *ctx)
      * We either have to:
      *  - Increase the number available of threads until over the min_threads
      *    threshold.
-     *  - Decrease the number of available threads until under the max_threads
+     *  - Bump the worker threads so that they exit, until under the max_threads
      *    threshold.
      *  - Do nothing. The current number of threads fall in between the min and
      *    max thresholds. We'll let the pool manage itself.
@@ -333,7 +321,7 @@ void thread_pool_update_params(ThreadPool *pool, AioContext *ctx)
     }
 
     for (int i = pool->cur_threads; i > pool->max_threads; i--) {
-        qemu_sem_post(&pool->sem);
+        qemu_cond_signal(&pool->request_cond);
     }
 
     qemu_mutex_unlock(&pool->lock);
@@ -350,7 +338,7 @@ static void thread_pool_init_one(ThreadPool *pool, AioContext *ctx)
     pool->completion_bh = aio_bh_new(ctx, thread_pool_completion_bh, pool);
     qemu_mutex_init(&pool->lock);
     qemu_cond_init(&pool->worker_stopped);
-    qemu_sem_init(&pool->sem, 0);
+    qemu_cond_init(&pool->request_cond);
     pool->new_thread_bh = aio_bh_new(ctx, spawn_thread_bh_fn, pool);
 
     QLIST_INIT(&pool->head);
@@ -383,15 +371,15 @@ void thread_pool_free(ThreadPool *pool)
 
     /* Wait for worker threads to terminate */
     pool->stopping = true;
+    qemu_cond_broadcast(&pool->request_cond);
     while (pool->cur_threads > 0) {
-        qemu_sem_post(&pool->sem);
         qemu_cond_wait(&pool->worker_stopped, &pool->lock);
     }
 
     qemu_mutex_unlock(&pool->lock);
 
     qemu_bh_delete(pool->completion_bh);
-    qemu_sem_destroy(&pool->sem);
+    qemu_cond_destroy(&pool->request_cond);
     qemu_cond_destroy(&pool->worker_stopped);
     qemu_mutex_destroy(&pool->lock);
     g_free(pool);