From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 6AB14C433EF for ; Thu, 12 May 2022 11:11:25 +0000 (UTC) Received: from localhost ([::1]:43174 helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1np6j6-0004ZZ-KH for qemu-devel@archiver.kernel.org; Thu, 12 May 2022 07:11:24 -0400 Received: from eggs.gnu.org ([2001:470:142:3::10]:57362) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1np6IZ-0004G5-Gs for qemu-devel@nongnu.org; Thu, 12 May 2022 06:43:59 -0400 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]:49673) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1np6IX-0003Jb-LI for qemu-devel@nongnu.org; Thu, 12 May 2022 06:43:59 -0400 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1652352237; 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-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=quFxEO95jnlD94ibyjfWuzrFAbqey91/3t0dPTHAVwA=; b=J4347WT11ly8FBK99GpEC4CvU7teQYORfwgVYtm+U15QJZB0TIxJFMGwYxbXXGohnuXhlu 42UMrUhYJK/neQXHZlfogVtoOHe+xstWugM0kQOqys2w+y7gJ7fCosF/s3Yna2QIa9HhAi mvzYoAuzFIHzcvQPeyzIprMh4nCx4N4= Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_GCM_SHA384) id us-mta-142-uo_lZZuwM3OLByjIlJAY8A-1; Thu, 12 May 2022 06:43:56 -0400 X-MC-Unique: uo_lZZuwM3OLByjIlJAY8A-1 Received: by mail-ed1-f72.google.com with SMTP id k15-20020a508acf000000b0042a448a43cdso19864edk.13 for ; Thu, 12 May 2022 03:43:55 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=quFxEO95jnlD94ibyjfWuzrFAbqey91/3t0dPTHAVwA=; b=1HbLbPzZ7ktwDRXfh9+ju6+RlMFau9xFPwKuViy2R4iVGi6Hwu9qbsVM1ntuylxleS /rL6wzs4CCXkYyFH7olO5omUutyDFFavqMqcgNRZyNbQ6kTxZN6qtLrf+jxfp7FUsyfS ydB21OzVfSSTyYNf2fTlrJl8WURPhHyR5SIB/9nTxuLkA7JeUxzm4EQh0B3Wzaf3JyY1 /b4q/FLGajG0V6GYirKMjLS3nthImRX/+PWoHnpKolBMPsQ6uYjzlmDh8jH1ViA0TgyP 92luBdiSzNazKSsHo1l+W5GodVQFpfK5BXK0CYJNNkze04+Cmu0+YIApYFXNp6g9Yus2 xD6w== X-Gm-Message-State: AOAM5336q9EmnWMnICf2XaoUijN0gvH1tLdgWQtIyb51r6ikjvaybu/X FmA6+Klj+N/mssR42LHkEmqqZvofJiFkcoM3HNTAoyiuKodbktqbzy8Lfo/mz8aokuoHEmegJwa QfyGwmliLXBDrdUuxhXhGY6iwcU51/tGUqDZIT7F8h+UrJg8BTZtuofAF5j5qyyl8wCA= X-Received: by 2002:a17:907:c02:b0:6df:fb64:2770 with SMTP id ga2-20020a1709070c0200b006dffb642770mr30062054ejc.221.1652352234335; Thu, 12 May 2022 03:43:54 -0700 (PDT) X-Google-Smtp-Source: ABdhPJz4X2CqxBYTT9bqG3JAurN6kuNHKSFw+crInL4ZKXDuj5grLyTDzW3ZfyEiIAQUwI3jVpsp/w== X-Received: by 2002:a17:907:c02:b0:6df:fb64:2770 with SMTP id ga2-20020a1709070c0200b006dffb642770mr30062018ejc.221.1652352233934; Thu, 12 May 2022 03:43:53 -0700 (PDT) Received: from [192.168.10.118] ([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id b7-20020a0564021f0700b0042617ba63d0sm2222036edb.90.2022.05.12.03.43.52 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 12 May 2022 03:43:53 -0700 (PDT) From: Paolo Bonzini To: qemu-devel@nongnu.org Cc: nsaenzju@redhat.com, stefanha@redhat.com, =?UTF-8?q?Luk=C3=A1=C5=A1=20Doktor?= Subject: [PATCH v2 2/3] thread-pool: replace semaphore with condition variable Date: Thu, 12 May 2022 12:43:45 +0200 Message-Id: <20220512104346.865536-3-pbonzini@redhat.com> X-Mailer: git-send-email 2.36.0 In-Reply-To: <20220512104346.865536-1-pbonzini@redhat.com> References: <20220512104346.865536-1-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: -18 X-Spam_score: -1.9 X-Spam_bar: - X-Spam_report: (-1.9 / 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, PP_MIME_FAKE_ASCII_TEXT=0.999, 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+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 Signed-off-by: Paolo Bonzini --- 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