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

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

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.


> 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.

With regards,
Daniel
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