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Wed, 25 May 2022 12:29:06 -0700 (PDT) X-Google-Smtp-Source: ABdhPJzGEfjznm4OMOs1ScgNEBHkR6Lzy8B4PKewO9ZJ5XjcT5/0d3JMDLfFqB1grGpX7MX6SY0htg== X-Received: by 2002:a05:6402:896:b0:42b:a9e1:a60b with SMTP id e22-20020a056402089600b0042ba9e1a60bmr7776813edy.394.1653506945929; Wed, 25 May 2022 12:29:05 -0700 (PDT) Received: from goa-sendmail ([2001:b07:6468:f312:9af8:e5f5:7516:fa89]) by smtp.gmail.com with ESMTPSA id s23-20020a056402015700b0042ab1735552sm11005752edu.66.2022.05.25.12.29.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 25 May 2022 12:29:05 -0700 (PDT) From: Paolo Bonzini To: qemu-devel@nongnu.org Cc: =?UTF-8?q?Luk=C3=A1=C5=A1=20Doktor?= , Stefan Hajnoczi , Nicolas Saenz Julienne Subject: [PULL 04/17] thread-pool: replace semaphore with condition variable Date: Wed, 25 May 2022 21:28:39 +0200 Message-Id: <20220525192852.301633-5-pbonzini@redhat.com> X-Mailer: git-send-email 2.36.1 In-Reply-To: <20220525192852.301633-1-pbonzini@redhat.com> References: <20220525192852.301633-1-pbonzini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Type: text/plain Content-Transfer-Encoding: 8bit Received-SPF: pass client-ip=170.10.133.124; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -11 X-Spam_score: -1.2 X-Spam_bar: - X-Spam_report: (-1.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.082, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, PP_MIME_FAKE_ASCII_TEXT=0.999, RCVD_IN_DNSWL_NONE=-0.0001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=no autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org Sender: "Qemu-devel" 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 Suggested-by: Stefan Hajnoczi Reviewed-by: Nicolas Saenz Julienne Message-Id: <20220514065012.1149539-3-pbonzini@redhat.com> Signed-off-by: Paolo Bonzini --- util/thread-pool.c | 68 +++++++++++++++++++--------------------------- 1 file changed, 28 insertions(+), 40 deletions(-) diff --git a/util/thread-pool.c b/util/thread-pool.c index 4979f30ca3..6e3d4e4a2f 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); -- 2.36.1