Re: [PATCH] migration: Fix qmp_query_migrate mbps value

[Peter Xu <peterx@redhat.com>]

On Thu, Feb 22, 2024 at 10:49:12AM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@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 <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.
> >
> > 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