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>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
util/thread-pool.c | 64 +++++++++++++++++-----------------------------
1 file changed, 24 insertions(+), 40 deletions(-)
diff --git a/util/thread-pool.c b/util/thread-pool.c
index 4979f30ca3..da189d9338 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 +82,27 @@ 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) {
+ if (QTAILQ_EMPTY(&pool->request_list) &&
+ pool->cur_threads > pool->min_threads) {
+ /* Timed out + no work to do + no need for warm threads = exit. */
+ break;
+ } else {
+ /*
+ * Even if there is some work to do, check if there aren't
+ * too many worker threads before picking it up.
+ */
+ continue;
+ }
+ }
}
req = QTAILQ_FIRST(&pool->request_list);
@@ -229,13 +219,7 @@ 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 +264,7 @@ 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 +307,7 @@ 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 +317,7 @@ 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 +334,7 @@ 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 +367,15 @@ 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);
--
2.36.0
Hi Paolo,
On Thu, 2022-05-12 at 12:43 +0200, Paolo Bonzini wrote:
[...]
> diff --git a/util/thread-pool.c b/util/thread-pool.c
> index 4979f30ca3..da189d9338 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 +82,27 @@ 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 ||
I think, you need to check for 'pool->stopping' upon exiting wait_cond().
Otherwise it'll blindly try to dequeue a request from a list that is otherwise
empty.
BTW, I see there is no thread_pool_free() unit test.
> - pool->cur_threads > pool->max_threads) {
> - break;
> + if (ret == 0) {
> + if (QTAILQ_EMPTY(&pool->request_list) &&
> + pool->cur_threads > pool->min_threads) {
> + /* Timed out + no work to do + no need for warm
> threads = exit. */
> + break;
> + } else {
> + /*
> + * Even if there is some work to do, check if
> there aren't
> + * too many worker threads before picking it up.
> + */
> + continue;
> + }
> + }
> }
>
> req = QTAILQ_FIRST(&pool->request_list);
> @@ -229,13 +219,7 @@ 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);
The 'thread-pool cancel' unit test fails.
I think it's because there is an assumption in worker_thread() that if you get
woken up, you'll have a pending request. And you're now 'stealing' work
requests, without 'stealing' a wakeup (what qemu_sem_timedwait(sem, 0) achieved
in the past).
Regards,
--
Nicolás Sáenz
On 5/13/22 13:56, Nicolas Saenz Julienne wrote:
>> 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 ||
>
> I think, you need to check for 'pool->stopping' upon exiting wait_cond().
> Otherwise it'll blindly try to dequeue a request from a list that is otherwise
> empty.
Good point, thanks.
>> - 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);
>
> The 'thread-pool cancel' unit test fails.
>
> I think it's because there is an assumption in worker_thread() that if you get
> woken up, you'll have a pending request. And you're now 'stealing' work
> requests, without 'stealing' a wakeup (what qemu_sem_timedwait(sem, 0) achieved
> in the past).
You don't need to steal a wakeup because cond_wait does not "count", but
yeah it's essentially the same issue that you mentioned above.
Paolo
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