Re: [Qemu-devel] [RFC 29/29] migration: reset migrate thread vars when resumed

[Peter Xu <peterx@redhat.com>]

On Fri, Aug 04, 2017 at 10:52:27AM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > On Thu, Aug 03, 2017 at 02:54:35PM +0100, Dr. David Alan Gilbert wrote:

[...]

> > > > @@ -2319,6 +2327,7 @@ static void *migration_thread(void *opaque)
> > > >      /* The active state we expect to be in; ACTIVE or POSTCOPY_ACTIVE */
> > > >      enum MigrationStatus current_active_state = MIGRATION_STATUS_ACTIVE;
> > > >      bool enable_colo = migrate_colo_enabled();
> > > > +    MigThrError thr_error;
> > > >  
> > > >      rcu_register_thread();
> > > >  
> > > > @@ -2395,8 +2404,17 @@ static void *migration_thread(void *opaque)
> > > >           * Try to detect any kind of failures, and see whether we
> > > >           * should stop the migration now.
> > > >           */
> > > > -        if (migration_detect_error(s)) {
> > > > +        thr_error = migration_detect_error(s);
> > > > +        if (thr_error == MIG_THR_ERR_FATAL) {
> > > > +            /* Stop migration */
> > > >              break;
> > > > +        } else if (thr_error == MIG_THR_ERR_RECOVERED) {
> > > > +            /*
> > > > +             * Just recovered from a e.g. network failure, reset all
> > > > +             * the local variables.
> > > > +             */
> > > > +            initial_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> > > > +            initial_bytes = 0;
> > > 
> > > They don't seem that important to reset?
> > 
> > The problem is that we have this in migration_thread():
> > 
> >         if (current_time >= initial_time + BUFFER_DELAY) {
> >             uint64_t transferred_bytes = qemu_ftell(s->to_dst_file) -
> >                                          initial_bytes;
> >             uint64_t time_spent = current_time - initial_time;
> >             double bandwidth = (double)transferred_bytes / time_spent;
> >             threshold_size = bandwidth * s->parameters.downtime_limit;
> >             ...
> >         }
> > 
> > Here qemu_ftell() would possibly be very small since we have just
> > resumed... and then transferred_bytes will be extremely huge since
> > "qemu_ftell(s->to_dst_file) - initial_bytes" is actually negative...
> > Then, with luck, we'll got extremely huge "bandwidth" as well.
> 
> Ah yes that's a good reason to reset it then; add a comment like
> 'important to avoid breaking transferred_bytes and bandwidth
> calculation'

Will do.

-- 
Peter Xu