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bh=14HBcwus+qwKczG43PdMeM13AKTcXtuWl/N5uwY+Yu8=; b=bZzNEKFjU3nX2BkimZFODoC4LTCeeamzusK+stuYSUSmHllAAjeQzxyo1n61DFy7XUhxUZ RPEfUUBfimgsebJAmPn7i9Dm1XLrbj8Qxh3ueg9ENzRv7BWAQQdBU4Ez/qO9/yhMW5qPFz DK3R+1iRJobOVP7vnSgESO9Xjw3gGuI= X-MC-Unique: 502OBX22PaK-tgN-E-saYA-1 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 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.11.54.5 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: -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+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1706697289143100003 Content-Type: text/plain; charset="utf-8" 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; } =20 + /* 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 =3D &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 =3D true; next_channel =3D (i + 1) % migrate_multifd_channels(); break; } - qemu_mutex_unlock(&p->mutex); } + + qemu_mutex_lock(&p->mutex); + /* + * Double check on pending_job=3D=3Dfalse with the lock. In the futur= e if + * we can have >1 requester thread, we can replace this with a "goto + * retry", but that is for later. + */ + assert(p->pending_job =3D=3D false); + p->pending_job =3D true; assert(!p->pages->num); assert(!p->pages->block); - p->packet_num =3D multifd_send_state->packet_num++; multifd_send_state->pages =3D p->pages; p->pages =3D pages; @@ -704,8 +715,6 @@ static void *multifd_send_thread(void *opaque) multifd_send_fill_packet(p); p->num_packets++; p->total_normal_pages +=3D pages->num; - qemu_mutex_unlock(&p->mutex); - trace_multifd_send(p->id, packet_num, pages->num, p->flags, p->next_packet_size); =20 @@ -725,6 +734,7 @@ static void *multifd_send_thread(void *opaque) ret =3D qio_channel_writev_full_all(p->c, p->iov, p->iovs_num,= NULL, 0, p->write_flags, &local_er= r); if (ret !=3D 0) { + qemu_mutex_unlock(&p->mutex); break; } =20 @@ -733,7 +743,6 @@ static void *multifd_send_thread(void *opaque) =20 multifd_pages_reset(p->pages); p->next_packet_size =3D 0; - qemu_mutex_lock(&p->mutex); p->pending_job =3D false; qemu_mutex_unlock(&p->mutex); } else if (p->pending_sync) { --=20 2.43.0