Peter Xu <pet...@redhat.com> writes: > 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)?
Ok, I'll respin with these changes. > > We can also rename migration_calculate_complete() to something like > migration_completion_finalize()? Then move the state update into it. I've been planning to merge migration_completion() and migration_iteration_finish(). It's too unintuitive to do the completion routine deep inside migration_iteration_run(). AFAICS those are all tail calls, so we could bring migration_completion() up into the migration_thread top level. So if you'll allow me I think I'll refrain from moving the state into migration_calculate_complete() for now.
