On Wed, Feb 21, 2024 at 09:56:36AM -0300, Fabiano Rosas wrote: > Peter Xu <pet...@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 <faro...@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
