From patchwork Wed Jan 31 10:31:03 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Peter Xu X-Patchwork-Id: 13539153 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 2908AC4828F for ; Wed, 31 Jan 2024 10:34:50 +0000 (UTC) Received: from localhost ([::1] helo=lists1p.gnu.org) by lists.gnu.org with esmtp (Exim 4.90_1) (envelope-from ) id 1rV7sq-0005F0-Pj; Wed, 31 Jan 2024 05:31:56 -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 1rV7sn-0005Af-OR for qemu-devel@nongnu.org; Wed, 31 Jan 2024 05:31:54 -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 1rV7sk-0005en-6n for qemu-devel@nongnu.org; Wed, 31 Jan 2024 05:31:52 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1706697109; 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=Xofmvl7UFAPSUm/4IrKBV85TkZhO+SISSyvdHG9E+DU=; b=CgiamVG6KJBWd0n4nIjWMgvoYTLnzyaqKNPWABm8+MNJzdUk2OzKbZ/GGvSb/q170s9LSy yyBLWTc9FW6kMSr1J3xeaO9yRY2BFw9N8F75A6RQLicbLJUKKhZGGu6kRTwscgJr+/7yMh htcxfAK3xHMMHMg9OSBBCswWEOEUOfw= Received: from mimecast-mx02.redhat.com (mx-ext.redhat.com [66.187.233.73]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-693-kHa6tFXyNxajaeM4W0-Bgg-1; Wed, 31 Jan 2024 05:31:46 -0500 X-MC-Unique: kHa6tFXyNxajaeM4W0-Bgg-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 0E1ED1C05154; Wed, 31 Jan 2024 10:31:46 +0000 (UTC) Received: from x1n.redhat.com (unknown [10.72.116.11]) by smtp.corp.redhat.com (Postfix) with ESMTP id 27B721BDB1; Wed, 31 Jan 2024 10:31:41 +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 06/14] migration/multifd: Separate SYNC request with normal jobs Date: Wed, 31 Jan 2024 18:31:03 +0800 Message-ID: <20240131103111.306523-7-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=170.10.129.124; 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_H4=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+qemu-devel=archiver.kernel.org@nongnu.org Sender: qemu-devel-bounces+qemu-devel=archiver.kernel.org@nongnu.org From: Peter Xu Multifd provide a threaded model for processing jobs. On sender side, there can be two kinds of job: (1) a list of pages to send, or (2) a sync request. The sync request is a very special kind of job. It never contains a page array, but only a multifd packet telling the dest side to synchronize with sent pages. Before this patch, both requests use the pending_job field, no matter what the request is, it will boost pending_job, while multifd sender thread will decrement it after it finishes one job. However this should be racy, because SYNC is special in that it needs to set p->flags with MULTIFD_FLAG_SYNC, showing that this is a sync request. Consider a sequence of operations where: - migration thread enqueue a job to send some pages, pending_job++ (0->1) - [...before the selected multifd sender thread wakes up...] - migration thread enqueue another job to sync, pending_job++ (1->2), setup p->flags=MULTIFD_FLAG_SYNC - multifd sender thread wakes up, found pending_job==2 - send the 1st packet with MULTIFD_FLAG_SYNC and list of pages - send the 2nd packet with flags==0 and no pages This is not expected, because MULTIFD_FLAG_SYNC should hopefully be done after all the pages are received. Meanwhile, the 2nd packet will be completely useless, which contains zero information. I didn't verify above, but I think this issue is still benign in that at least on the recv side we always receive pages before handling MULTIFD_FLAG_SYNC. However that's not always guaranteed and just tricky. One other reason I want to separate it is using p->flags to communicate between the two threads is also not clearly defined, it's very hard to read and understand why accessing p->flags is always safe; see the current impl of multifd_send_thread() where we tried to cache only p->flags. It doesn't need to be that complicated. This patch introduces pending_sync, a separate flag just to show that the requester needs a sync. Alongside, we remove the tricky caching of p->flags now because after this patch p->flags should only be used by multifd sender thread now, which will be crystal clear. So it is always thread safe to access p->flags. With that, we can also safely convert the pending_job into a boolean, because we don't support >1 pending jobs anyway. Signed-off-by: Peter Xu Reviewed-by: Fabiano Rosas --- migration/multifd.h | 13 +++++++++++-- migration/multifd.c | 29 ++++++++++++++++++----------- 2 files changed, 29 insertions(+), 13 deletions(-) diff --git a/migration/multifd.h b/migration/multifd.h index 3920bdbcf1..08f26ef3fe 100644 --- a/migration/multifd.h +++ b/migration/multifd.h @@ -99,8 +99,17 @@ typedef struct { uint32_t flags; /* global number of generated multifd packets */ uint64_t packet_num; - /* thread has work to do */ - int pending_job; + /* + * The sender thread has work to do if either of below boolean is set. + * + * @pending_job: a job is pending + * @pending_sync: a sync request is pending + * + * For both of these fields, they're only set by the requesters, and + * cleared by the multifd sender threads. + */ + bool pending_job; + bool pending_sync; /* array of pages to sent. * The owner of 'pages' depends of 'pending_job' value: * pending_job == 0 -> migration_thread can use it. diff --git a/migration/multifd.c b/migration/multifd.c index 8bb1fd95cf..6a4863edd2 100644 --- a/migration/multifd.c +++ b/migration/multifd.c @@ -443,7 +443,7 @@ static int multifd_send_pages(void) p = &multifd_send_state->params[i]; qemu_mutex_lock(&p->mutex); if (!p->pending_job) { - p->pending_job++; + p->pending_job = true; next_channel = (i + 1) % migrate_multifd_channels(); break; } @@ -631,8 +631,7 @@ int multifd_send_sync_main(void) qemu_mutex_lock(&p->mutex); p->packet_num = multifd_send_state->packet_num++; - p->flags |= MULTIFD_FLAG_SYNC; - p->pending_job++; + p->pending_sync = true; qemu_mutex_unlock(&p->mutex); qemu_sem_post(&p->sem); } @@ -688,7 +687,6 @@ static void *multifd_send_thread(void *opaque) if (p->pending_job) { uint64_t packet_num = p->packet_num; MultiFDPages_t *pages = p->pages; - uint32_t flags; if (use_zero_copy_send) { p->iovs_num = 0; @@ -704,13 +702,11 @@ static void *multifd_send_thread(void *opaque) } } multifd_send_fill_packet(p); - flags = p->flags; - p->flags = 0; p->num_packets++; p->total_normal_pages += pages->num; qemu_mutex_unlock(&p->mutex); - trace_multifd_send(p->id, packet_num, pages->num, flags, + trace_multifd_send(p->id, packet_num, pages->num, p->flags, p->next_packet_size); if (use_zero_copy_send) { @@ -738,12 +734,23 @@ 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--; + p->pending_job = false; qemu_mutex_unlock(&p->mutex); - - if (flags & MULTIFD_FLAG_SYNC) { - qemu_sem_post(&p->sem_sync); + } else if (p->pending_sync) { + p->flags = MULTIFD_FLAG_SYNC; + multifd_send_fill_packet(p); + ret = qio_channel_write_all(p->c, (void *)p->packet, + p->packet_len, &local_err); + if (ret != 0) { + qemu_mutex_unlock(&p->mutex); + break; } + /* p->next_packet_size will always be zero for a SYNC packet */ + stat64_add(&mig_stats.multifd_bytes, p->packet_len); + p->flags = 0; + p->pending_sync = false; + qemu_mutex_unlock(&p->mutex); + qemu_sem_post(&p->sem_sync); } else { qemu_mutex_unlock(&p->mutex); /* sometimes there are spurious wakeups */