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 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?

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.

Dave

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