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Wed, 23 Sep 2026 22:30:43 +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 e5sPCBNTtGpNZwAAD6G6ig (envelope-from ); Wed, 23 Sep 2026 22:30:43 +0000 From: Fabiano Rosas To: Peter Xu Cc: qemu-devel@nongnu.org, Akash Kulhalli , Juraj Marcin Subject: Re: [PATCH 7/7] migration: Postpone the outgoing preempt channel for preempt_pre_7_2 In-Reply-To: References: <20260916213558.587304-1-peterx@redhat.com> <20260916213558.587304-8-peterx@redhat.com> <87o6doami6.fsf@suse.de> Date: Wed, 23 Sep 2026 19:30:36 -0300 Message-ID: <87a4p7a9df.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_THREE(0.00)[4]; 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=195.135.223.131; envelope-from=farosas@suse.de; helo=smtp-out2.suse.de X-Spam_score_int: -43 X-Spam_score: -4.4 X-Spam_bar: ---- X-Spam_report: (-4.4 / 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, RCVD_IN_DNSWL_MED=-2.3, 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: > On Tue, Sep 22, 2026 at 08:34:41PM -0300, Fabiano Rosas wrote: >> 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? > > Not a bug? The resume path only happens during postcopy phase, during > which multifd (even if enabled) is ignored. > Ah, I though you meant multifd+postcopy could leave multifd channels behind in case of network failure. >> >> > 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. > > The clean way to do this is, whenever we recycled all the channels we reset > it. Ideally they should have been recycled when switching to postcopy, but > that's non-issue, I agree. It's just not that intuitive, IMHO. But I can > definitely drop this paragraph if it caused confusions. > You're not wrong, but we can very well have something that counts up when channels are created and doesn't count down when they are removed. >> >> > 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 > > Yeah, I used "new binaries" to reference 10.1+ on the first line (while in > context of 7.1/7.2 machine types, they're "new"), but then used "old > (broken) binaries" on the 3rd line (while in context of after fixing > incoming side after previous patch, in which case not-fixed ones are > "old"). > > My bad, it's probably confusing whoever reads it. > > I wrote a new comment: > > /* > * Only for old 7.1/7.2 machine types (preempt mode supported since s/old// > * 7.1). Connect the preempt channel only after multifd channels, > * because some QEMU versions (v10.1 to v11.1) will assume the order of > * channels to be: (1) main, then (2) multifd channels, then (3) > * preempt channel. IOW, (3) must be after (2). The last part is not necessary. > * > * multifd_send_setup() makes sure all multifd channels be connected > * first, then initiate preempt channel here guarantees the ordering to > * those QEMU binaries. > */ > Thanks, that's better. >> 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 > > Most things before multifd+postcopy work do not matter much, because as you > also mentioned these binaries can't enable both features, so it is not a > concern in term of channel misorders. > >> 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. > > Just to mention, we need that separate connection for resuming case below > in migration_start_outgoing(), because the resuming case doesn't start a > new migration_thread but reuse the old one. It means that's the only place > we can reconnect the preempt channel in resume case. > >> >> 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. > > Yes, I feel like we're on the same page. I think it was used to how it > always worked before multifd+postcopy merged. After it merged, it's not > how it worked, it should never worked, IIUC, hence "broken" after 10.1+. > >> >> 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. > > Note that relying on order of channels are actually OK in 99.9% cases.. So > that's why I tend to make it a "fix", even if it smells like a feature. > > Or say, I'll backport this whole series, when ready, to our zstream, as a > fix too. This is the smallest change set I can come up with even if it > contains something looks like a feature.. > Yeah, I was trying to come up with a simple fix you could backport where we could just check s->preempt_pre_7_2 during discovery and force the order to be correct, but it didn't work. >> >> > + 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. > > It's my intention to move this for both cases when multifd is enabled or > not, because I think it's simpler and cleaner. > > If I check multifd here, I'll need to move the migration_start_outgoing() > code below to check !multifd instead, it'll look like: > > migration_start_outgoing() > ... > if (resume || !multifd) > connect_preempt(); > if (resume) > ... > > It looks uglier, so I chose this path forward. It seems all safe. > Ok, maybe add some words saying you intend to affect both cases. >> >> > + >> > 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 > > I think qmp_migrate() should have returned already when creating preempt > channel, either on master or after applying this patch? > > Only socket based URI can use preempt mode, so when reaching here (say, on > master branch), it should be part of socket_outgoing_migration(), async > operation of qio_channel_socket_connect_async(). > Yes, of course, I forgot it's async. >> 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? > > Hmm, I'm not aware of the races with TLS thread issue. > > /me asks for more hints.. > Oh, the work we did to ensure cleanup doesn't happen before all threads are up and running. The semaphore for multifd when creating channels I added and the changes to tls_thread_created you did. Nevermind, I got confused with the async part above. >> >> > 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); >>