[PATCH 2/2] thread-pool: replace semaphore with condition variable

[Paolo Bonzini]

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: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 util/thread-pool.c | 31 ++++++++++++-------------------
 1 file changed, 12 insertions(+), 19 deletions(-)

diff --git a/util/thread-pool.c b/util/thread-pool.c
index 7e9e2c178b..5511706de8 100644
--- a/util/thread-pool.c
+++ b/util/thread-pool.c
@@ -57,7 +57,7 @@ struct ThreadPool {
     QEMUBH *completion_bh;
     QemuMutex lock;
     QemuCond worker_stopped;
-    QemuSemaphore sem;
+    QemuCond request_cond;
     int max_threads;
     QEMUBH *new_thread_bh;
 
@@ -85,15 +85,14 @@ static void *worker_thread(void *opaque)
         ThreadPoolElement *req;
         int ret;
 
-        do {
+        if (QTAILQ_EMPTY(&pool->request_list)) {
             pool->idle_threads++;
-            qemu_mutex_unlock(&pool->lock);
-            ret = qemu_sem_timedwait(&pool->sem, 10000);
-            qemu_mutex_lock(&pool->lock);
+            ret = qemu_cond_timedwait(&pool->request_cond, &pool->lock, 10000);
             pool->idle_threads--;
-        } while (ret == -1 && !QTAILQ_EMPTY(&pool->request_list));
-        if (ret == -1 || pool->stopping) {
-            break;
+            if (!ret && QTAILQ_EMPTY(&pool->request_list)) {
+                break;
+            }
+            continue;
         }
 
         req = QTAILQ_FIRST(&pool->request_list);
@@ -210,13 +209,7 @@ 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 &&
-        /* 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) {
+    if (elem->state == THREAD_QUEUED) {
         QTAILQ_REMOVE(&pool->request_list, elem, reqs);
         qemu_bh_schedule(pool->completion_bh);
 
@@ -261,7 +254,7 @@ 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;
 }
 
@@ -304,7 +297,7 @@ 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->max_threads = 64;
     pool->new_thread_bh = aio_bh_new(ctx, spawn_thread_bh_fn, pool);
 
@@ -336,15 +329,15 @@ 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);
-- 
2.31.1