Re: [PATCH v2 01/27] migration: Network Failover can't work with a paused guest

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

On Thu, Dec 03, 2020 at 06:21:47AM -0500, Michael S. Tsirkin wrote:
> On Wed, Dec 02, 2020 at 12:01:21PM +0000, Daniel P. Berrangé wrote:
> > On Wed, Dec 02, 2020 at 06:37:46AM -0500, Michael S. Tsirkin wrote:
> > > On Wed, Dec 02, 2020 at 11:26:39AM +0000, Daniel P. Berrangé wrote:
> > > > On Wed, Dec 02, 2020 at 06:19:29AM -0500, Michael S. Tsirkin wrote:
> > > > > On Wed, Dec 02, 2020 at 10:55:15AM +0000, Daniel P. Berrangé wrote:
> > > > > > On Wed, Dec 02, 2020 at 11:51:05AM +0100, Juan Quintela wrote:
> > > > > > > "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > > > > > > > On Wed, Dec 02, 2020 at 05:31:53AM -0500, Michael S. Tsirkin wrote:
> > > > > > > >> On Wed, Dec 02, 2020 at 10:27:18AM +0000, Daniel P. Berrangé wrote:
> > > > > > > >> > On Wed, Dec 02, 2020 at 05:13:18AM -0500, Michael S. Tsirkin wrote:
> > > > > > > >> > > On Wed, Nov 18, 2020 at 09:37:22AM +0100, Juan Quintela wrote:
> > > > > > > >> > > > If we have a paused guest, it can't unplug the network VF device, so
> > > > > > > >> > > > we wait there forever.  Just change the code to give one error on that
> > > > > > > >> > > > case.
> > > > > > > >> > > > 
> > > > > > > >> > > > Signed-off-by: Juan Quintela <quintela@redhat.com>
> > > > > > > >> > > 
> > > > > > > >> > > It's certainly possible but it's management that created
> > > > > > > >> > > this situation after all - why do we bother to enforce
> > > > > > > >> > > a policy? It is possible that management will unpause immediately
> > > > > > > >> > > afterwards and everything will proceed smoothly.
> > > > > > > >> > > 
> > > > > > > >> > > Yes migration will not happen until guest is
> > > > > > > >> > > unpaused but the same it true of e.g. a guest that is stuck
> > > > > > > >> > > because of a bug.
> > > > > > > >> > 
> > > > > > > >> > That's pretty different behaviour from how migration normally handles
> > > > > > > >> > a paused guest, which is that it is guaranteed to complete the migration
> > > > > > > >> > in as short a time as network bandwidth allows.
> > > > > > > >> > 
> > > > > > > >> > Just ignoring the situation I think will lead to surprise apps / admins,
> > > > > > > >> > because the person/entity invoking the migration is not likely to have
> > > > > > > >> > checked wether this particular guest uses net failover or not before
> > > > > > > >> > invoking - they'll just be expecting a paused migration to run fast and
> > > > > > > >> > be guaranteed to complete.
> > > > > > > >> > 
> > > > > > > >> > Regards,
> > > > > > > >> > Daniel
> > > > > > > >> 
> > > > > > > >> Okay I guess. But then shouldn't we handle the reverse situation too:
> > > > > > > >> pausing guest after migration started but before device was
> > > > > > > >> unplugged?
> > > > > > > >> 
> > > > > > > >
> > > > > > > > Thinking of which, I have no idea how we'd handle it - fail
> > > > > > > > pausing guest until migration is cancelled?
> > > > > > > >
> > > > > > > > All this seems heavy handed to me ...
> > > > > > > 
> > > > > > > This is the minimal fix that I can think of.
> > > > > > > 
> > > > > > > Further solution would be:
> > > > > > > - Add a new migration parameter: migrate-paused
> > > > > > > - change libvirt to use the new parameter if it exist
> > > > > > > - in qemu, when we do start migration (but after we wait for the unplug
> > > > > > >   device) paused the guest before starting migration and resume it after
> > > > > > >   migration finish.
> > > > > > 
> > > > > > It would also have to handle issuing of paused after migration has
> > > > > > been started - delay the pause request until the nuplug is complete
> > > > > > is one answer.
> > > > > 
> > > > > Hmm my worry would be that pausing is one way to give cpu
> > > > > resources back to host. It's problematic if guest can delay
> > > > > that indefinitely.
> > > > 
> > > > hmm, yes, that is awkward.  Perhaps we should just report an explicit
> > > > error then.
> > > 
> > > Report an error in response to which command? Do you mean
> > > fail migration?
> > 
> > If mgt attempt to pause an existing migration that hasn't finished
> > the PCI unplug stage, then fail the pause request.
> 
> Pause guest not migration ...
> Might be tricky ...
> 
> Let me ask this, why not just produce a warning
> that migration wan't finish until guest actually runs?
> User will then know and unpause the guest when he wants
> migration to succeed ...

A warning is going to be essentally invisible if the pause command
succeeeds. 

Regards,
Daniel
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