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Fri, 06 May 2022 07:47:21 -0400 Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-642-eG9koSJIMWWzw3v4Vfunhg-1; Fri, 06 May 2022 07:47:15 -0400 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id 6FE5318A6582 for ; Fri, 6 May 2022 11:47:15 +0000 (UTC) Received: from virtlab701.virt.lab.eng.bos.redhat.com (virtlab701.virt.lab.eng.bos.redhat.com [10.19.152.228]) by smtp.corp.redhat.com (Postfix) with ESMTP id 4FCB29E75; Fri, 6 May 2022 11:47:15 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1651837637; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=oYPejR+xm7AfJPjZe039oam3tIUkevBzq5/pi33NtEc=; b=DpOLcAas61ha68DYs1NUYTz0PDQuWQCRAlnzfbSzVAi23+9AAWHUi6DfxLgOp8frTWU2YB R402HPGA/YMBSMSKUtqLpimQE54Bcib12RpWwZhVlKak3Kzz3DhCJ1ojHiQPUXnAHHxnPp 9QKWwJOFsPWDCwFe4qZy2rU3XNqB/J8= X-MC-Unique: eG9koSJIMWWzw3v4Vfunhg-1 From: Paolo Bonzini To: qemu-devel@nongnu.org Cc: stefanha@redhat.com, ldoktor@redhat.com Subject: [PATCH 2/2] thread-pool: replace semaphore with condition variable Date: Fri, 6 May 2022 07:47:11 -0400 Message-Id: <20220506114711.1398662-3-pbonzini@redhat.com> In-Reply-To: <20220506114711.1398662-1-pbonzini@redhat.com> References: <20220506114711.1398662-1-pbonzini@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 2.79 on 10.11.54.5 Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.133.74; envelope-from=pbonzini@redhat.com; helo=us-smtp-delivery-74.mimecast.com X-Spam_score_int: -28 X-Spam_score: -2.9 X-Spam_bar: -- X-Spam_report: (-2.9 / 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, RCVD_IN_DNSWL_LOW=-0.7, SPF_HELO_NONE=0.001, SPF_PASS=-0.001, T_SCC_BODY_TEXT_LINE=-0.01 autolearn=ham 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+importer=patchew.org@nongnu.org Sender: "Qemu-devel" X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1651837800509100001 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: Luk=C3=A1=C5=A1 Doktor Suggested-by: Stefan Hajnoczi Reviewed-by: Stefan Hajnoczi Signed-off-by: Paolo Bonzini --- 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; =20 @@ -85,15 +85,14 @@ static void *worker_thread(void *opaque) ThreadPoolElement *req; int ret; =20 - do { + if (QTAILQ_EMPTY(&pool->request_list)) { pool->idle_threads++; - qemu_mutex_unlock(&pool->lock); - ret =3D qemu_sem_timedwait(&pool->sem, 10000); - qemu_mutex_lock(&pool->lock); + ret =3D qemu_cond_timedwait(&pool->request_cond, &pool->lock, = 10000); pool->idle_threads--; - } while (ret =3D=3D -1 && !QTAILQ_EMPTY(&pool->request_list)); - if (ret =3D=3D -1 || pool->stopping) { - break; + if (!ret && QTAILQ_EMPTY(&pool->request_list)) { + break; + } + continue; } =20 req =3D QTAILQ_FIRST(&pool->request_list); @@ -210,13 +209,7 @@ static void thread_pool_cancel(BlockAIOCB *acb) trace_thread_pool_cancel(elem, elem->common.opaque); =20 QEMU_LOCK_GUARD(&pool->lock); - if (elem->state =3D=3D 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) =3D=3D 0) { + if (elem->state =3D=3D THREAD_QUEUED) { QTAILQ_REMOVE(&pool->request_list, elem, reqs); qemu_bh_schedule(pool->completion_bh); =20 @@ -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; } =20 @@ -304,7 +297,7 @@ static void thread_pool_init_one(ThreadPool *pool, AioC= ontext *ctx) pool->completion_bh =3D aio_bh_new(ctx, thread_pool_completion_bh, poo= l); 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 =3D 64; pool->new_thread_bh =3D aio_bh_new(ctx, spawn_thread_bh_fn, pool); =20 @@ -336,15 +329,15 @@ void thread_pool_free(ThreadPool *pool) =20 /* Wait for worker threads to terminate */ pool->stopping =3D 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); } =20 qemu_mutex_unlock(&pool->lock); =20 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); --=20 2.31.1