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[99.254.114.190]) by smtp.gmail.com with ESMTPSA id af79cd13be357-7b6b5a841b8sm86150485a.87.2024.12.05.10.53.05 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 05 Dec 2024 10:53:06 -0800 (PST) From: Peter Xu To: qemu-devel@nongnu.org Cc: peterx@redhat.com, Fabiano Rosas , "Maciej S . Szmigiero" , Alex Williamson , Avihai Horon , =?utf-8?q?C=C3=A9dric_Le_Goater?= Subject: [PATCH 1/2] migration/multifd: Further remove the SYNC on complete Date: Thu, 5 Dec 2024 13:53:02 -0500 Message-ID: <20241205185303.897010-2-peterx@redhat.com> X-Mailer: git-send-email 2.47.0 In-Reply-To: <20241205185303.897010-1-peterx@redhat.com> References: <20241205185303.897010-1-peterx@redhat.com> MIME-Version: 1.0 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: -50 X-Spam_score: -5.1 X-Spam_bar: ----- X-Spam_report: (-5.1 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-2.996, 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, RCVD_IN_VALIDITY_RPBL_BLOCKED=0.001, RCVD_IN_VALIDITY_SAFE_BLOCKED=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 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 Commit 637280aeb2 ("migration/multifd: Avoid the final FLUSH in complete()") removed the FLUSH operation on complete() which should avoid one global sync on destination side, because it's not needed. However that commit overlooked multifd_ram_flush_and_sync() part of things, as that's always used together with the FLUSH message on the main channel. For very old binaries (multifd_flush_after_each_section==true), that's still needed because each EOS received on destination will enforce all-channel sync once. For new binaries (multifd_flush_after_each_section==false), the flush and sync shouldn't be needed just like the FLUSH, with the same reasoning. Currently, on new binaries we'll still send SYNC messages on multifd channels, even if FLUSH is omitted at the end. It'll make all recv threads hang at SYNC message. Multifd is still all working fine because luckily recv side cleanup code (mostly multifd_recv_sync_main()) is smart enough to make sure even if recv threads are stuck at SYNC it'll get kicked out. And since this is the completion phase of migration, nothing else will be sent after the SYNCs. This may be needed for VFIO in the future because VFIO can have data to push after ram_save_complete(), hence we don't want the recv thread to be stuck in SYNC message. Remove this limitation will make src even slightly faster too to avoid some more code. Stable branches do not need this patch, as no real bug I can think of that will go wrong there.. so not attaching Fixes to be clear on the backport not needed. Signed-off-by: Peter Xu --- migration/ram.c | 13 ++++++++++--- 1 file changed, 10 insertions(+), 3 deletions(-) diff --git a/migration/ram.c b/migration/ram.c index 05ff9eb328..7284c34bd8 100644 --- a/migration/ram.c +++ b/migration/ram.c @@ -3283,9 +3283,16 @@ static int ram_save_complete(QEMUFile *f, void *opaque) } } - ret = multifd_ram_flush_and_sync(); - if (ret < 0) { - return ret; + if (migrate_multifd() && + migrate_multifd_flush_after_each_section()) { + /* + * Only the old dest QEMU will need this sync, because each EOS + * will require one SYNC message on each channel. + */ + ret = multifd_ram_flush_and_sync(); + if (ret < 0) { + return ret; + } } if (migrate_mapped_ram()) {