Re: [PATCH] migration: Apply migration specific keep-alive defaults to inet socket

["Daniel P. Berrangé" <berrange@redhat.com>]

On Fri, Sep 12, 2025 at 11:02:12AM -0400, Peter Xu wrote:
> On Fri, Sep 12, 2025 at 11:20:01AM +0100, Daniel P. Berrangé wrote:
> > On Wed, Sep 10, 2025 at 12:36:44PM -0400, Peter Xu wrote:
> > > On Wed, Sep 10, 2025 at 08:10:57AM +0100, Daniel P. Berrangé wrote:
> > > > On Tue, Sep 09, 2025 at 05:58:49PM -0400, Peter Xu wrote:
> > > > > On Tue, Sep 09, 2025 at 04:09:23PM +0100, Daniel P. Berrangé wrote:
> > > > > > On Tue, Sep 09, 2025 at 05:01:24PM +0200, Juraj Marcin wrote:
> > > > > > > From: Juraj Marcin <jmarcin@redhat.com>
> > > > > > > 
> > > > > > > Usual system defaults for TCP keep-alive options are too long for
> > > > > > > migration workload. On Linux, a TCP connection waits idle for 2 hours
> > > > > > > before it starts checking if the connection is not broken.
> > > > > > > 
> > > > > > > Now when InetSocketAddress supports keep-alive options [1], this patch
> > > > > > > applies migration specific defaults if they are not supplied by the user
> > > > > > > or the management software. With these defaults, a migration TCP stream
> > > > > > > waits idle for 1 minute and then sends 5 TCP keep-alive packets in 30
> > > > > > > second interval before considering the connection as broken.
> > > > > > > 
> > > > > > > System defaults can be still used by explicitly setting these parameters
> > > > > > > to 0.
> > > > > > 
> > > > > > IMHO this is not a good idea. This is a very short default, which
> > > > > > may be fine for the scenario where your network conn is permanently
> > > > > > dead, but it is going to cause undesirable failures when the network
> > > > > > conn is only temporarily dead.
> > > > > > 
> > > > > > Optimizing defaults for temporary outages is much more preferrable
> > > > > > as that maximises reliability of migration. In the case of permanent
> > > > > > outages, it is already possible to tear down the connection without
> > > > > > waiting for a keep-alive timeout, and liveliness checks can also be
> > > > > > perform by the mgmt app at a higher level too. The TCP keepalives
> > > > > > are just an eventual failsafe, and having those work on a long
> > > > > > timeframe is OK.
> > > > > 
> > > > > For precopy it looks fine indeed, because migrate_cancel should always work
> > > > > on src if src socket hanged, and even if dest QEMU socket hanged, it can
> > > > > simply be killed if src QEMU can be gracefully cancelled and rolled back to
> > > > > RUNNING, disregarding the socket status on dest QEMU.
> > > > > 
> > > > > For postcopy, we could still use migrate_pause to enforce src shutdown().
> > > > > Initially I thought we have no way of doing that for dest QEMU, but I just
> > > > > noticed two years ago I added that to dest QEMU for migrate_paused when
> > > > > working on commit f8c543e808f20b..  So looks like that part is covered too,
> > > > > so that if dest QEMU socket hanged we can also kick it out.
> > > > > 
> > > > > I'm not 100% sure though, on whether shutdown() would always be able to
> > > > > successfully kick out the hanged socket while the keepalive is ticking.  Is
> > > > > it guaranteed?
> > > > 
> > > > I don't know about shutdown(), but close() certainly works. If shutdown()
> > > > is not sufficient, then IMHO the migration code would need the ability to
> > > > use close() to deal with this situation.
> > > > 
> > > > 
> > > > > I also am not sure if that happens, whether libvirt would automatically do
> > > > > that, or provide some way so the user can trigger that.  The goal IIUC here
> > > > > is we shouldn't put user into a situation where the migration hanged but
> > > > > without any way to either cancel or recover.  With the default values Juraj
> > > > > provided here, it makes sure the hang won't happen more than a few minutes,
> > > > > which sounds like a sane timeout value.
> > > > 
> > > > Sufficient migration QMP commands should exist to ensure migration can
> > > > always be cancelled. Short keepalive timeouts should not be considered
> > > > a solution to any gaps in that respect.
> > > > 
> > > > Also there is yank, but IMHO apps shouldn't have to rely on yank - I see
> > > > yank as a safety net for apps to workaround limitations in QEMU.
> > > 
> > > The QMP facility looks to be all present, which is migrate-cancel and
> > > migrate-pause mentioned above.
> > > 
> > > For migrate_cancel (of precopy), is that Ctrl-C of "virsh migrate"?
> > > 
> > > Does libvirt exposes migrate_pause via any virsh command?  IIUC that's the
> > > only official way of pausing a postcopy VM on either side.  I also agree we
> > > shouldn't make yank the official tool to use.
> > 
> > virsh will call virDomainAbortJob when Ctrl-C is done to a 'migrate'
> > command.
> > 
> > virDomainAbortJob will call migrate-cancel for pre-copy, or
> > 'migrate-pause' for post-copy.
> 
> Would it call "migrate-pause" on both sides?

Not 100% sure, but with virDomainAbortJob I think libvirt only calls
migrate-pause on the source host.

> I believe the problem we hit was, when during postcopy and the NIC was
> misfunctioning, src fell into postcopy-paused successfully but dest didn't,
> stuck in postcopy-active.

If something has interrupted src<->dst host comms for QEMU it may well
impact libvirt <-> libvirt comms too, unless migration was being done
over a separate NIC than the mgmt LAN.  IOW, it may be impossible for
libvirt to call migrate-pause on both sides, at least not until the
NIC problem has been resolved.

> We'll want to make sure both sides to be kicked into paused stage to
> recover.  Otherwise dest can hang in the stage for hours until the watchdog
> timeout triggers.

Once the network problem has been resolved, then it ought to be possible
to get libvirt to issue 'migrate-pause' on both hosts, and thus be able
to recover.

Possibly the act of starting migration recovery in libvirt should attempt
to issue 'migrate-pause' to cleanup the previously running migration if
it is still in the stuck state.

> 
> > 
> > 
> > > OTOH, the default timeouts work without changing libvirt, making sure the
> > > customers will not be stuck in a likely-failing network for hours without
> > > providing a way to properly detach and recover when it's wanted.
> > 
> > "timeouts work" has the implicit assumpton that the only reason a
> > timeout will fire is due to a unrecoverable situation. IMHO that
> > assumption is not valid.
> 
> I agree adjusting timeout is not the best.
> 
> If we can have solid way to kick two sides out, I think indeed we don't
> need to change the timeout.
> 
> If not, we may still need to provide a way to allow user to try continue
> when the user found that the network is behaving abnormal.
> 
> Here adjusting timeout is slightly better than any adhoc socket timeout
> that we'll adjust: it's the migration timeout, and we only have two cases:
> (1) precopy, which is ok to fail and retried, (2) postcopy, which is also
> ok to fail and recovered.

Fail & retry/recover is not without cost / risk though. Users can have
successful migrations that are many hours long when dealing with big
VMs. IOW, returning to the start of pre-copy could be a non-trivial
time delay.

Consider if the reason for the migration is to evacuate workloads off
a host that is suffering technical problems. It could well be that
periodic unexpected network outages are what is triggering the need
to evacuate workloads. If we timeout a migration with keepalives they
may never be able to get through a migration op quickly enough, or
they can be delayed such that the host has a fatal error loosing the
workload before the retried migration is complete.

IMHO, once a migration has been started we should not proactively
interrupt that with things like keepalives, unless the admin made a
concious decision they wanted that behaviour enabled.

With regards,
Daniel
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