From patchwork Wed Jan 31 10:31:04 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Xu X-Patchwork-Id: 1893384 Return-Path: X-Original-To: incoming@patchwork.ozlabs.org Delivered-To: patchwork-incoming@legolas.ozlabs.org Authentication-Results: legolas.ozlabs.org; dkim=pass (1024-bit key; unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256 header.s=mimecast20190719 header.b=bZzNEKFj; dkim-atps=neutral Authentication-Results: legolas.ozlabs.org; spf=pass (sender SPF authorized) smtp.mailfrom=nongnu.org (client-ip=209.51.188.17; helo=lists.gnu.org; envelope-from=qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org; receiver=patchwork.ozlabs.org) Received: from lists.gnu.org (lists.gnu.org [209.51.188.17]) (using TLSv1.2 with cipher ECDHE-ECDSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by legolas.ozlabs.org (Postfix) with ESMTPS id 4TPz1k6Sj3z23h1 for ; Wed, 31 Jan 2024 21:34:50 +1100 (AEDT) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rV7sr-0005F7-CP; Wed, 31 Jan 2024 05:31:57 -0500 Received: from eggs.gnu.org ([2001:470:142:3::10]) by lists.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rV7sq-0005Ep-EJ for qemu-devel@nongnu.org; Wed, 31 Jan 2024 05:31:56 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.133.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rV7so-0005f4-Jk for qemu-devel@nongnu.org; Wed, 31 Jan 2024 05:31:56 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1706697113; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=14HBcwus+qwKczG43PdMeM13AKTcXtuWl/N5uwY+Yu8=; b=bZzNEKFjU3nX2BkimZFODoC4LTCeeamzusK+stuYSUSmHllAAjeQzxyo1n61DFy7XUhxUZ RPEfUUBfimgsebJAmPn7i9Dm1XLrbj8Qxh3ueg9ENzRv7BWAQQdBU4Ez/qO9/yhMW5qPFz DK3R+1iRJobOVP7vnSgESO9Xjw3gGuI= Received: from mimecast-mx02.redhat.com (mimecast-mx02.redhat.com [66.187.233.88]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-300-502OBX22PaK-tgN-E-saYA-1; Wed, 31 Jan 2024 05:31:51 -0500 X-MC-Unique: 502OBX22PaK-tgN-E-saYA-1 Received: from smtp.corp.redhat.com (int-mx05.intmail.prod.int.rdu2.redhat.com [10.11.54.5]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mimecast-mx02.redhat.com (Postfix) with ESMTPS id F1519185A782; Wed, 31 Jan 2024 10:31:50 +0000 (UTC) Received: from x1n.redhat.com (unknown [10.72.116.11]) by smtp.corp.redhat.com (Postfix) with ESMTP id EE5EE107BD; Wed, 31 Jan 2024 10:31:46 +0000 (UTC) From: peterx@redhat.com To: qemu-devel@nongnu.org Cc: Bryan Zhang , Prasad Pandit , Fabiano Rosas , peterx@redhat.com, Yuan Liu , Avihai Horon , Hao Xiang Subject: [PATCH 07/14] migration/multifd: Simplify locking in sender thread Date: Wed, 31 Jan 2024 18:31:04 +0800 Message-ID: <20240131103111.306523-8-peterx@redhat.com> In-Reply-To: <20240131103111.306523-1-peterx@redhat.com> References: <20240131103111.306523-1-peterx@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.5 Received-SPF: pass client-ip=170.10.133.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -33 X-Spam_score: -3.4 X-Spam_bar: --- X-Spam_report: (-3.4 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-1.292, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H3=0.001, RCVD_IN_MSPIKE_WL=0.001, 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+incoming=patchwork.ozlabs.org@nongnu.org Sender: qemu-devel-bounces+incoming=patchwork.ozlabs.org@nongnu.org From: Peter Xu The sender thread will yield the p->mutex before IO starts, trying to not block the requester thread. This may be unnecessary lock optimizations, because the requester can already read pending_job safely even without the lock, because the requester is currently the only one who can assign a task. Drop that lock complication on both sides: (1) in the sender thread, always take the mutex until job done (2) in the requester thread, check pending_job clear lockless Signed-off-by: Peter Xu --- migration/multifd.c | 23 ++++++++++++++++------- 1 file changed, 16 insertions(+), 7 deletions(-) diff --git a/migration/multifd.c b/migration/multifd.c index 6a4863edd2..4dc5af0a15 100644 --- a/migration/multifd.c +++ b/migration/multifd.c @@ -429,7 +429,9 @@ static int multifd_send_pages(void) return -1; } + /* We wait here, until at least one channel is ready */ qemu_sem_wait(&multifd_send_state->channels_ready); + /* * next_channel can remain from a previous migration that was * using more channels, so ensure it doesn't overflow if the @@ -441,17 +443,26 @@ static int multifd_send_pages(void) return -1; } p = &multifd_send_state->params[i]; - qemu_mutex_lock(&p->mutex); + /* + * Lockless read to p->pending_job is safe, because only multifd + * sender thread can clear it. + */ if (!p->pending_job) { - p->pending_job = true; next_channel = (i + 1) % migrate_multifd_channels(); break; } - qemu_mutex_unlock(&p->mutex); } + + qemu_mutex_lock(&p->mutex); + /* + * Double check on pending_job==false with the lock. In the future if + * we can have >1 requester thread, we can replace this with a "goto + * retry", but that is for later. + */ + assert(p->pending_job == false); + p->pending_job = true; assert(!p->pages->num); assert(!p->pages->block); - p->packet_num = multifd_send_state->packet_num++; multifd_send_state->pages = p->pages; p->pages = pages; @@ -704,8 +715,6 @@ static void *multifd_send_thread(void *opaque) multifd_send_fill_packet(p); p->num_packets++; p->total_normal_pages += pages->num; - qemu_mutex_unlock(&p->mutex); - trace_multifd_send(p->id, packet_num, pages->num, p->flags, p->next_packet_size); @@ -725,6 +734,7 @@ static void *multifd_send_thread(void *opaque) ret = qio_channel_writev_full_all(p->c, p->iov, p->iovs_num, NULL, 0, p->write_flags, &local_err); if (ret != 0) { + qemu_mutex_unlock(&p->mutex); break; } @@ -733,7 +743,6 @@ static void *multifd_send_thread(void *opaque) multifd_pages_reset(p->pages); p->next_packet_size = 0; - qemu_mutex_lock(&p->mutex); p->pending_job = false; qemu_mutex_unlock(&p->mutex); } else if (p->pending_sync) {