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

[Paolo Bonzini <pbonzini@redhat.com>]

On 03/03/2017 14:26, Dr. David Alan Gilbert wrote:
> * 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.

Yeah, but I think we get there at the end of a failed migrate only.  So
perhaps we can introduce a new state RUN_STATE_FAILED_MIGRATE and forbid
"cont" from finish-migrate (only allow it from failed-migrate)?

Paolo

>> 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
>