Re: [PATCH] migration: Fix qmp_query_migrate mbps value

[Peter Xu]

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.

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