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bh=ZXF88j6L+HU+EILPLvbQTVGt5UFQGI3yIKbM1yEgjSo=; b=Z6Q4GRFQKgNyfBr0zwQ+nguFfrU6ledwdsoM8mXFwAssGgPhe3BObcULovH1/SU/RPk7ME igSfgzx1Z/Z9+35mwKQEq+htoTYxxA+5vkTQ/BbZK+8D6mRLBu0M7e3aI5GCFMp9s3AgRI P46nmdfQNrkJizmopMtRiZfRMIz51jU= X-MC-Unique: 8-uARfx-P22rhz_M9YV0Kg-1 From: peterx@redhat.com To: Peter Maydell , qemu-devel@nongnu.org Cc: peterx@redhat.com, Fabiano Rosas Subject: [PULL 07/34] migration/multifd: Separate SYNC request with normal jobs Date: Thu, 8 Feb 2024 11:05:01 +0800 Message-ID: <20240208030528.368214-8-peterx@redhat.com> In-Reply-To: <20240208030528.368214-1-peterx@redhat.com> References: <20240208030528.368214-1-peterx@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.9 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=170.10.133.124; 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_H5=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+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1707361902690100003 Content-Type: text/plain; charset="utf-8" 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=3DMULTIFD_FLAG_SYNC - multifd sender thread wakes up, found pending_job=3D=3D2 - send the 1st packet with MULTIFD_FLAG_SYNC and list of pages - send the 2nd packet with flags=3D=3D0 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. Always use atomic ops to access both flags to make sure no cache effect. When at it, drop the initial setting of "pending_job =3D 0" because it's always allocated using g_new0(). Reviewed-by: Fabiano Rosas Link: https://lore.kernel.org/r/20240202102857.110210-7-peterx@redhat.com Signed-off-by: Peter Xu --- migration/multifd.h | 13 +++++++++++-- migration/multifd.c | 39 +++++++++++++++++++++++++-------------- 2 files changed, 36 insertions(+), 16 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 =3D=3D 0 -> migration_thread can use it. diff --git a/migration/multifd.c b/migration/multifd.c index 8bb1fd95cf..ea25bbe6bd 100644 --- a/migration/multifd.c +++ b/migration/multifd.c @@ -442,8 +442,8 @@ static int multifd_send_pages(void) } p =3D &multifd_send_state->params[i]; qemu_mutex_lock(&p->mutex); - if (!p->pending_job) { - p->pending_job++; + if (qatomic_read(&p->pending_job) =3D=3D false) { + qatomic_set(&p->pending_job, true); next_channel =3D (i + 1) % migrate_multifd_channels(); break; } @@ -631,8 +631,12 @@ int multifd_send_sync_main(void) =20 qemu_mutex_lock(&p->mutex); p->packet_num =3D multifd_send_state->packet_num++; - p->flags |=3D MULTIFD_FLAG_SYNC; - p->pending_job++; + /* + * We should be the only user so far, so not possible to be set by + * others concurrently. + */ + assert(qatomic_read(&p->pending_sync) =3D=3D false); + qatomic_set(&p->pending_sync, true); qemu_mutex_unlock(&p->mutex); qemu_sem_post(&p->sem); } @@ -685,10 +689,9 @@ static void *multifd_send_thread(void *opaque) } qemu_mutex_lock(&p->mutex); =20 - if (p->pending_job) { + if (qatomic_read(&p->pending_job)) { uint64_t packet_num =3D p->packet_num; MultiFDPages_t *pages =3D p->pages; - uint32_t flags; =20 if (use_zero_copy_send) { p->iovs_num =3D 0; @@ -704,13 +707,11 @@ static void *multifd_send_thread(void *opaque) } } multifd_send_fill_packet(p); - flags =3D p->flags; - p->flags =3D 0; p->num_packets++; p->total_normal_pages +=3D pages->num; qemu_mutex_unlock(&p->mutex); =20 - 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); =20 if (use_zero_copy_send) { @@ -738,12 +739,23 @@ static void *multifd_send_thread(void *opaque) multifd_pages_reset(p->pages); p->next_packet_size =3D 0; qemu_mutex_lock(&p->mutex); - p->pending_job--; + qatomic_set(&p->pending_job, false); qemu_mutex_unlock(&p->mutex); - - if (flags & MULTIFD_FLAG_SYNC) { - qemu_sem_post(&p->sem_sync); + } else if (qatomic_read(&p->pending_sync)) { + p->flags =3D MULTIFD_FLAG_SYNC; + multifd_send_fill_packet(p); + ret =3D qio_channel_write_all(p->c, (void *)p->packet, + p->packet_len, &local_err); + if (ret !=3D 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 =3D 0; + qatomic_set(&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 */ @@ -907,7 +919,6 @@ int multifd_save_setup(Error **errp) qemu_mutex_init(&p->mutex); qemu_sem_init(&p->sem, 0); qemu_sem_init(&p->sem_sync, 0); - p->pending_job =3D 0; p->id =3D i; p->pages =3D multifd_pages_init(page_count); p->packet_len =3D sizeof(MultiFDPacket_t) --=20 2.43.0