Re: [PATCH] migration: Fix qmp_query_migrate mbps value

[Peter Xu <peterx@redhat.com>]

On Wed, Feb 21, 2024 at 09:56:36AM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Mon, Feb 19, 2024 at 04:44:57PM -0300, Fabiano Rosas wrote:
> >> The QMP command query_migrate might see incorrect throughput numbers
> >> if it runs after we've set the migration completion status but before
> >> migration_calculate_complete() has updated s->total_time and s->mbps.
> >> 
> >> The migration status would show COMPLETED, but the throughput value
> >> would be the one from the last iteration and not the one from the
> >> whole migration. This will usually be a larger value due to the time
> >> period being smaller (one iteration).
> >> 
> >> Move migration_calculate_complete() earlier so that the status
> >> MIGRATION_STATUS_COMPLETED is only emitted after the final counters
> >> update.
> >> 
> >> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> >> ---
> >> CI run: https://gitlab.com/farosas/qemu/-/pipelines/1182405776
> >> ---
> >>  migration/migration.c | 10 ++++++----
> >>  1 file changed, 6 insertions(+), 4 deletions(-)
> >> 
> >> diff --git a/migration/migration.c b/migration/migration.c
> >> index ab21de2cad..7486d59da0 100644
> >> --- a/migration/migration.c
> >> +++ b/migration/migration.c
> >> @@ -102,6 +102,7 @@ static int migration_maybe_pause(MigrationState *s,
> >>                                   int new_state);
> >>  static void migrate_fd_cancel(MigrationState *s);
> >>  static bool close_return_path_on_source(MigrationState *s);
> >> +static void migration_calculate_complete(MigrationState *s);
> >>  
> >>  static void migration_downtime_start(MigrationState *s)
> >>  {
> >> @@ -2746,6 +2747,7 @@ static void migration_completion(MigrationState *s)
> >>          migrate_set_state(&s->state, MIGRATION_STATUS_ACTIVE,
> >>                            MIGRATION_STATUS_COLO);
> >>      } else {
> >> +        migration_calculate_complete(s);
> >>          migrate_set_state(&s->state, current_active_state,
> >>                            MIGRATION_STATUS_COMPLETED);
> >>      }
> >> @@ -2784,6 +2786,7 @@ static void bg_migration_completion(MigrationState *s)
> >>          goto fail;
> >>      }
> >>  
> >> +    migration_calculate_complete(s);
> >>      migrate_set_state(&s->state, current_active_state,
> >>                        MIGRATION_STATUS_COMPLETED);
> >>      return;
> >> @@ -2993,12 +2996,15 @@ static void migration_calculate_complete(MigrationState *s)
> >>      int64_t end_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> >>      int64_t transfer_time;
> >>  
> >> +    /* QMP could read from these concurrently */
> >> +    bql_lock();
> >>      migration_downtime_end(s);
> >>      s->total_time = end_time - s->start_time;
> >>      transfer_time = s->total_time - s->setup_time;
> >>      if (transfer_time) {
> >>          s->mbps = ((double) bytes * 8.0) / transfer_time / 1000;
> >>      }
> >> +    bql_unlock();
> >
> > The lock is not needed?
> >
> > AFAIU that was needed because of things like runstate_set() rather than
> > setting of these fields.
> >
> 
> Don't we need to keep the total_time and mbps update atomic? Otherwise
> query-migrate might see (say) total_time=0 and mbps=<correct value> or
> total_time=<correct value> and mbps=<previous value>.

I thought it wasn't a major concern, but what you said makes sense; taking
it one more time doesn't really hurt after all to provide such benefit.

> 
> Also, what orders s->mbps update before the s->state update? I'd say we
> should probably hold the lock around the whole total_time,mbps,state
> update.

IMHO that's fine; mutex unlock implies a RELEASE.  See atomic.rst:

- ``pthread_mutex_lock`` has acquire semantics, ``pthread_mutex_unlock`` has
  release semantics and synchronizes with a ``pthread_mutex_lock`` for the
  same mutex.

> 
> I'm not entirely sure, what do you think?
> 
> > See migration_update_counters() where it also updates mbps without holding
> > a lock.
> 
> Here it might be less important since it's the middle of the migration,
> there will proabably be more than one query-migrate which would see the
> correct values.

Yep.  I queued this.

Thanks,

-- 
Peter Xu