On Thu, Feb 22, 2024 at 05:40:41PM +0800, Peter Xu wrote: > 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.
Hmm perhaps I wrote too soon.. it should only guarantee the ordering of the update on the lock variable itself v.s. any previous R&Ws, nothing else. Only if the other side uses bql_lock() will it guarantee proper ordering. Put them in bql should work, but I hesitate such use to start using bql to protect state updates. How about we drop the lock, but use an explicit smp_mb_release()? We may also want to use smb_load_acquire() in fill_source_migration_info() to use on reading &s->state (all will need some comment). To me, making sure the total mbps is valid seems more important; while the other races are less harmful, and may not be a major concern? PS: logically I think smp_mb_release() is not needed either, because state is updated using qatomic_cmpxchg(), which implies a full __ATOMIC_SEQ_CST. > > > > > 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 -- Peter Xu
