aboutsummaryrefslogtreecommitdiff
path: root/util
diff options
context:
space:
mode:
authorPaolo Bonzini <pbonzini@redhat.com>2022-05-14 08:50:11 +0200
committerPaolo Bonzini <pbonzini@redhat.com>2022-05-25 21:26:13 +0200
commit900fa208f50623672a6f879374222a7fd4717791 (patch)
treec69c05bc7c2031c08cc2c21bae67843e226c2614 /util
parent3c7b72ddca9ce85a9d1e8a98fd0996b74597b5ae (diff)
downloadqemu-900fa208f50623672a6f879374222a7fd4717791.zip
qemu-900fa208f50623672a6f879374222a7fd4717791.tar.gz
qemu-900fa208f50623672a6f879374222a7fd4717791.tar.bz2
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>
Diffstat (limited to 'util')
-rw-r--r--util/thread-pool.c68
1 files changed, 28 insertions, 40 deletions
diff --git a/util/thread-pool.c b/util/thread-pool.c
index 4979f30..6e3d4e4 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;
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_sem_timedwait(&pool->sem, 10000);
- qemu_mutex_lock(&pool->lock);
+ ret = qemu_cond_timedwait(&pool->request_cond, &pool->lock, 10000);
pool->idle_threads--;
- } while (back_to_sleep(pool, ret));
- if (ret == -1 || pool->stopping ||
- pool->cur_threads > pool->max_threads) {
- break;
+ if (ret == 0 &&
+ QTAILQ_EMPTY(&pool->request_list) &&
+ pool->cur_threads > pool->min_threads) {
+ /* 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 &&
- /* 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);
@@ -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);