On Thu, Feb 22, 2024 at 10:49:12AM -0300, Fabiano Rosas wrote: > Peter Xu <pet...@redhat.com> writes: > > > 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. > > Well, on the other hand that's a major use-case of the BQL: protecting > state that's used by QMP. > > > > > 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? > > That more closely reflects the problem we're trying to solve, which is > just an ordering one. However, the QMP code already holds the BQL, we > could just take benefit of that instead of adding more complex > synchronization primitives. > > May I suggest we keep it simple and move that last migrate_set_state > into the BQL as well?
It's okay to me, but then let's also extend the comment a little bit on the two exact requirements we're persuing (atomicity of updating fields, ordering of state update v.s. mbps)? We can also rename migration_calculate_complete() to something like migration_completion_finalize()? Then move the state update into it. -- Peter Xu
