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Tue, 22 Sep 2026 23:34:47 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id 2hr5DJcQs2pueAAAD6G6ig (envelope-from ); Tue, 22 Sep 2026 23:34:47 +0000 From: Fabiano Rosas To: Peter Xu , qemu-devel@nongnu.org Cc: Peter Xu , Akash Kulhalli , Juraj Marcin Subject: Re: [PATCH 7/7] migration: Postpone the outgoing preempt channel for preempt_pre_7_2 In-Reply-To: <20260916213558.587304-8-peterx@redhat.com> References: <20260916213558.587304-1-peterx@redhat.com> <20260916213558.587304-8-peterx@redhat.com> Date: Tue, 22 Sep 2026 20:34:41 -0300 Message-ID: <87o6doami6.fsf@suse.de> MIME-Version: 1.0 Content-Type: text/plain X-Spamd-Result: default: False [-4.30 / 50.00]; BAYES_HAM(-3.00)[100.00%]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; MIME_GOOD(-0.10)[text/plain]; RCVD_VIA_SMTP_AUTH(0.00)[]; ARC_NA(0.00)[]; MIME_TRACE(0.00)[0:+]; MISSING_XM_UA(0.00)[]; TO_DN_SOME(0.00)[]; MID_RHS_MATCH_FROM(0.00)[]; RCVD_TLS_ALL(0.00)[]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; FROM_HAS_DN(0.00)[]; RCPT_COUNT_FIVE(0.00)[5]; FROM_EQ_ENVFROM(0.00)[]; TO_MATCH_ENVRCPT_ALL(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; DBL_BLOCKED_OPENRESOLVER(0.00)[imap1.dmz-prg2.suse.org:helo,suse.de:mid] Received-SPF: pass client-ip=2a07:de40:b251:101:10:150:64:2; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -20 X-Spam_score: -2.1 X-Spam_bar: -- X-Spam_report: (-2.1 / 5.0 requ) BAYES_00=-1.9, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, 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: qemu development 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 Peter Xu writes: > This is the corresponding outgoing fix for the same problem mentioned in > the previous patch. > > The idea is simple: either migration to an old QEMU or when TLS is enabled, > we rely on correct ordering of channels to make it work: first main, then > multifd, finally preempt. > > This patch re-orders multifd and postcopy preempt channel on outgoing side, > but only for the old machine types where preempt_pre_7_2 set, to make it > follow the right orders to create the channels. > > Keep the resume path alone because the resume path never involve multifd > channels, even if multifd is enabled. For that, fun fact: when multifd is > enabled with postcopy preempt, network interruption will break all multifd > channels, but they're never re-connected, until they got recycled at the > end of postcopy migration. Is this a separate bug? Do we need to add some cleanup? > OTOH, multifd_recv_all_channels_created() > reports true even after postcopy network down, as multifd_recv_state->count > is never decremented. > Seems normal to me. We're not even trying to reconnect multifd. > Fixes: e27418861288 ("migration: enable multifd and postcopy together") > Signed-off-by: Peter Xu > --- > migration/migration.c | 32 +++++++++++++++++++++++--------- > 1 file changed, 23 insertions(+), 9 deletions(-) > > diff --git a/migration/migration.c b/migration/migration.c > index 4cc4077a76..f8d664808f 100644 > --- a/migration/migration.c > +++ b/migration/migration.c > @@ -3733,6 +3733,20 @@ static void *migration_thread(void *opaque) > goto out; > } > > + /* > + * Only for old 7.1/7.2 machine types running on new binaries (that > + * supports multifd+postcopy+preempt). Makes sure to initiate the > + * connect() after multifd channels because the old (broken) binaries > + * assumes the order of channels, which we cannot fix anymore. > + * > + * multifd_send_setup() will make sure all multifd channels will be > + * connected first, then initiate preempt channel here guarantees the > + * ordering to those old binaries. > + */ It's a bit confusing what's old binary, what's new binary here. Old/new don't really apply I think as there are more than two cases when we look at the global picture. There's before 7.1, before 7.2, before the preempt_pre_7_2 flag, before the multifd+postcopy support, before this series, etc. Specially since this code is exactly the same as the one under if (resume), but there we're talking about a different new/old. Also, maybe we should avoid framing the assumption on the order of channels as "broken", that's how it always worked and is just a case of unimaginative assumptions, i.e. not knowing multifd would ever run along preempt. The real bug is that v10.1 added a "new" channel to the mix without working on compatibility. In this series you're doing both: fixing the bug that v10.1 introduced and improving the code to not assume the order of channels. > + if (migrate_postcopy_preempt() && s->preempt_pre_7_2) { > + postcopy_preempt_setup(s); > + } Don't you want to check for multifd as well and apply this fix only to the case that can actually trigger it? To avoid inadvertently changing the normal postcopy case. > + > bql_lock(); > qemu_savevm_state_header(s->to_dst_file); > bql_unlock(); > @@ -4013,16 +4027,16 @@ void migration_start_outgoing(MigrationState *s) > open_return_path_on_source(s); > } > > - /* > - * This needs to be done before resuming a postcopy. Note: for newer > - * QEMUs we will delay the channel creation until postcopy_start(), to > - * avoid disorder of channel creations. > - */ > - if (migrate_postcopy_preempt() && s->preempt_pre_7_2) { > - postcopy_preempt_setup(s); > - } > - Hm, this used to happen before qmp_migrate() returns. Now it's in the migration thread and after multifd channels connect, there's a big window there. We added a lot of code to ensure cleanup of multifd happens properly and without races with TLS thread creation, etc. Have you considered such issues? > if (resume) { > + /* > + * This needs to be done before resuming a postcopy. Note: for newer > + * QEMUs we will delay the channel creation until postcopy_start(), to > + * avoid disorder of channel creations. > + */ > + if (migrate_postcopy_preempt() && s->preempt_pre_7_2) { > + postcopy_preempt_setup(s); > + } > + > /* Wakeup the main migration thread to do the recovery */ > migrate_set_state(&s->state, MIGRATION_STATUS_POSTCOPY_RECOVER_SETUP, > MIGRATION_STATUS_POSTCOPY_RECOVER);