From patchwork Thu Feb 8 03:05:02 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Xu X-Patchwork-Id: 1896433 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=e6OxU3Fr; 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 4TVhq24cmxz23h2 for ; Thu, 8 Feb 2024 14:11:58 +1100 (AEDT) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rXujp-0006E7-Bv; Wed, 07 Feb 2024 22:06:09 -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 1rXujk-0006Cs-1M for qemu-devel@nongnu.org; Wed, 07 Feb 2024 22:06:04 -0500 Received: from us-smtp-delivery-124.mimecast.com ([170.10.129.124]) by eggs.gnu.org with esmtps (TLS1.2:ECDHE_RSA_AES_256_GCM_SHA384:256) (Exim 4.90_1) (envelope-from ) id 1rXuji-0004Ls-Ez for qemu-devel@nongnu.org; Wed, 07 Feb 2024 22:06:03 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1707361561; 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=HOxM34KtLIN0cPcBEfxMbZlDBqVqASX9xZ8Un2UI8a4=; b=e6OxU3FrvygcTirN2NbBivQ08Q98fEEMiYk8uh+cQR7+QhM1vAxrpaHz8Xlpl/aMV5ryp6 7jnIP1ciok/dIeJjEosi0E0ZK04Vyqgh/HAB57zFQljdOsxR61iySDC3qookSqJakK8Z18 ij8lT8ahsR1ONqSSqqHIJOUtR9jceSQ= 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-193-zGiJ5iBoMm-RCvuNs_LyVA-1; Wed, 07 Feb 2024 22:05:58 -0500 X-MC-Unique: zGiJ5iBoMm-RCvuNs_LyVA-1 Received: from smtp.corp.redhat.com (int-mx09.intmail.prod.int.rdu2.redhat.com [10.11.54.9]) (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 34D52845E62; Thu, 8 Feb 2024 03:05:58 +0000 (UTC) Received: from x1n.redhat.com (unknown [10.72.116.45]) by smtp.corp.redhat.com (Postfix) with ESMTP id C0140492BC7; Thu, 8 Feb 2024 03:05:55 +0000 (UTC) From: peterx@redhat.com To: Peter Maydell , qemu-devel@nongnu.org Cc: peterx@redhat.com, Fabiano Rosas Subject: [PULL 08/34] migration/multifd: Simplify locking in sender thread Date: Thu, 8 Feb 2024 11:05:02 +0800 Message-ID: <20240208030528.368214-9-peterx@redhat.com> In-Reply-To: <20240208030528.368214-1-peterx@redhat.com> References: <20240208030528.368214-1-peterx@redhat.com> MIME-Version: 1.0 X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.9 Received-SPF: pass client-ip=170.10.129.124; envelope-from=peterx@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -21 X-Spam_score: -2.2 X-Spam_bar: -- X-Spam_report: (-2.2 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.106, 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_H2=-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 Reviewed-by: Fabiano Rosas Link: https://lore.kernel.org/r/20240202102857.110210-8-peterx@redhat.com 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 ea25bbe6bd..4d5a01ed93 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 (qatomic_read(&p->pending_job) == false) { - qatomic_set(&p->pending_job, true); next_channel = (i + 1) % migrate_multifd_channels(); break; } - qemu_mutex_unlock(&p->mutex); } + + qemu_mutex_lock(&p->mutex); assert(!p->pages->num); assert(!p->pages->block); - + /* + * 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(qatomic_read(&p->pending_job) == false); + qatomic_set(&p->pending_job, true); p->packet_num = multifd_send_state->packet_num++; multifd_send_state->pages = p->pages; p->pages = pages; @@ -709,8 +720,6 @@ 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); @@ -730,6 +739,7 @@ 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; } @@ -738,7 +748,6 @@ static void *multifd_send_thread(void *opaque) multifd_pages_reset(p->pages); p->next_packet_size = 0; - qemu_mutex_lock(&p->mutex); qatomic_set(&p->pending_job, false); qemu_mutex_unlock(&p->mutex); } else if (qatomic_read(&p->pending_sync)) {