Re: [PATCH v3 07/10] migration: add new migration state wait-unplug

[Jens Freimann]

On Tue, Oct 15, 2019 at 11:50:08AM +0100, Dr. David Alan Gilbert wrote:
> * Jens Freimann (address@hidden) wrote:
> > On Fri, Oct 11, 2019 at 06:11:33PM +0100, Dr. David Alan Gilbert wrote:
> > > * Jens Freimann (address@hidden) wrote:
> > > > This patch adds a new migration state called wait-unplug.  It is entered
> > > > after the SETUP state and will transition into ACTIVE once all devices
> > > > were succesfully unplugged from the guest.
> > > >
> > > > So if a guest doesn't respond or takes long to honor the unplug request
> > > > the user will see the migration state 'wait-unplug'.
> > > >
> > > > In the migration thread we query failover devices if they're are still
> > > > pending the guest unplug. When all are unplugged the migration
> > > > continues. We give it a defined number of iterations including small
> > > > waiting periods before we proceed.
> > > >
> > > > Signed-off-by: Jens Freimann <address@hidden>
> > [..]
> > > > +    while (i < FAILOVER_UNPLUG_RETRIES &&
> > > > +           s->state == MIGRATION_STATUS_WAIT_UNPLUG) {
> > > > +        i++;
> > > > +        qemu_sem_timedwait(&s->wait_unplug_sem, 
> > FAILOVER_GUEST_UNPLUG_WAIT);
> > > > +        all_unplugged = qemu_savevm_state_guest_unplug_pending();
> > > > +        if (all_unplugged) {
> > > > +            break;
> > > > +        }
> > > > +    }
> > > > +
> > > > +    if (all_unplugged) {
> > > > +        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG,
> > > > +                MIGRATION_STATUS_ACTIVE);
> > > > +    } else {
> > > > +        migrate_set_state(&s->state, MIGRATION_STATUS_WAIT_UNPLUG,
> > > > +                          MIGRATION_STATUS_CANCELLING);
> > > > +    }
> > >
> > > I think you can get rid of both the timeout and the count and just make
> > > sure that migrate_cancel works at this point.
> >
> > I see, I need to add the new state to migration_is_setup_or_active() or
> > a cancel won't work.
>
> You probably need to do that anyway given all the other places
> is_setup_or_active is called.

Yes, done.

> > > This pushes the problem up a layer, which I think is fine.
> >
> > Seems good to me. To be clear, you're saying I should just poll on
> > the device unplugged state? Like
> >
> >         while (s->state == MIGRATION_STATUS_WAIT_UNPLUG &&
> >                !qemu_savevm_state_guest_unplug_pending()) {
> > _            /* This block intentionally left blank */
> >         }
>
> I'd keep the qemu_sem_timedwait in there, but with a short time out
> (e.g. 250ms say); that way it doesn't eat cpu, but also the cancel still
> happens quickly.

Yes, that's what I do now and it works fine.

Thanks!

regards,
Jens