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

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [Bug?] BQL about live migration
Date: Fri, 3 Mar 2017 14:14:00 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.7.0

On 03/03/2017 14:11, Dr. David Alan Gilbert wrote:
> * Paolo Bonzini (address@hidden) 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 
>>> 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'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. :(

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