Re: [Qemu-devel] [Bug?] BQL about live migration

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Paolo Bonzini (pbonzini@redhat.com) wrote:
> 
> 
> On 03/03/2017 14:11, Dr. David Alan Gilbert wrote:
> > * Paolo Bonzini (pbonzini@redhat.com) wrote:
> >>
> >>
> >> On 03/03/2017 13:00, Dr. David Alan Gilbert wrote:
> >>> Ouch that's pretty nasty; I remember Paolo explaining to me a while ago that
> >>> their were times when run_on_cpu would have to drop the BQL and I worried about it,
> >>> but this is the 1st time I've seen an error due to it.
> >>>
> >>> Do you know what the migration state was at that point? Was it MIGRATION_STATUS_CANCELLING?
> >>> I'm thinking perhaps we should stop 'cont' from continuing while migration is in
> >>> MIGRATION_STATUS_CANCELLING.  Do we send an event when we hit CANCELLED - so that
> >>> perhaps libvirt could avoid sending the 'cont' until then?
> >>
> >> No, there's no event, though I thought libvirt would poll until
> >> "query-migrate" returns the cancelled state.  Of course that is a small
> >> consolation, because a segfault is unacceptable.
> > 
> > I think you might get an event if you set the new migrate capability called
> > 'events' on!
> > 
> > void migrate_set_state(int *state, int old_state, int new_state)
> > {
> >     if (atomic_cmpxchg(state, old_state, new_state) == old_state) {
> >         trace_migrate_set_state(new_state);
> >         migrate_generate_event(new_state);
> >     }
> > }
> > 
> > static void migrate_generate_event(int new_state)
> > {
> >     if (migrate_use_events()) {
> >         qapi_event_send_migration(new_state, &error_abort); 
> >     }
> > }
> > 
> > That event feature went in sometime after 2.3.0.
> > 
> >> One possibility is to suspend the monitor in qmp_migrate_cancel and
> >> resume it (with add_migration_state_change_notifier) when we hit the
> >> CANCELLED state.  I'm not sure what the latency would be between the end
> >> of migrate_fd_cancel and finally reaching CANCELLED.
> > 
> > I don't like suspending monitors; it can potentially take quite a significant
> > time to do a cancel.
> > How about making 'cont' fail if we're in CANCELLING?
> 
> Actually I thought that would be the case already (in fact CANCELLING is
> internal only; the outside world sees it as "active" in query-migrate).
> 
> Lei, what is the runstate?  (That is, why did cont succeed at all)?

I suspect it's RUN_STATE_FINISH_MIGRATE - we set that before we do the device
save, and that's what we get at the end of a migrate and it's legal to restart
from there.

> Paolo
> 
> > I'd really love to see the 'run_on_cpu' being more careful about the BQL;
> > we really need all of the rest of the devices to stay quiesced at times.
> 
> That's not really possible, because of how condition variables work. :(

*Really* we need to find a solution to that - there's probably lots of 
other things that can spring up in that small window other than the
'cont'.

Dave

--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK