Re: [Qemu-devel] [PATCH v7 23/42] MIGRATION_STATUS_POSTCOPY_ACTIVE: Add new migration state

[Dr. David Alan Gilbert]

* Juan Quintela (address@hidden) wrote:
> "Dr. David Alan Gilbert" <address@hidden> wrote:
> > * Juan Quintela (address@hidden) wrote:
> >> "Dr. David Alan Gilbert (git)" <address@hidden> wrote:
> >> > From: "Dr. David Alan Gilbert" <address@hidden>
> >> >
> >> > 'MIGRATION_STATUS_POSTCOPY_ACTIVE' is entered after 
> >> > migrate_start_postcopy
> >> >
> >> > 'migration_postcopy_phase' is provided for other sections to know if
> >> > they're in postcopy.
> >> >
> >> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> >> > Reviewed-by: David Gibson <address@hidden>
> >> > Reviewed-by: Eric Blake <address@hidden>
> >> 
> >> Reviewed-by: Juan Quintela <address@hidden>
> >> 
> >> 
> >> But (there is always a but....)
> >> 
> >> 
> >> > @@ -358,6 +359,39 @@ MigrationInfo *qmp_query_migrate(Error **errp)
> >> >  
> >> >          get_xbzrle_cache_stats(info);
> >> >          break;
> >> > +    case MIGRATION_STATUS_POSTCOPY_ACTIVE:
> >> > +        /* Mostly the same as active; TODO add some postcopy stats */
> >> > +        info->has_status = true;
> >> > +        info->has_total_time = true;
> >> > +        info->total_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME)
> >> > +            - s->total_time;
> >> > +        info->has_expected_downtime = true;
> >> > +        info->expected_downtime = s->expected_downtime;
> >> > +        info->has_setup_time = true;
> >> > +        info->setup_time = s->setup_time;
> >> > +
> >> > +        info->has_ram = true;
> >> > +        info->ram = g_malloc0(sizeof(*info->ram));
> >> > +        info->ram->transferred = ram_bytes_transferred();
> >> > +        info->ram->remaining = ram_bytes_remaining();
> >> > +        info->ram->total = ram_bytes_total();
> >> > +        info->ram->duplicate = dup_mig_pages_transferred();
> >> > +        info->ram->skipped = skipped_mig_pages_transferred();
> >> > +        info->ram->normal = norm_mig_pages_transferred();
> >> > +        info->ram->normal_bytes = norm_mig_bytes_transferred();
> >> > +        info->ram->dirty_pages_rate = s->dirty_pages_rate;
> >> > +        info->ram->mbps = s->mbps;
> >> > +
> >> > +        if (blk_mig_active()) {
> >> > +            info->has_disk = true;
> >> > +            info->disk = g_malloc0(sizeof(*info->disk));
> >> > +            info->disk->transferred = blk_mig_bytes_transferred();
> >> > +            info->disk->remaining = blk_mig_bytes_remaining();
> >> > +            info->disk->total = blk_mig_bytes_total();
> >> > +        }
> >> 
> >> Can we have block migration active with postcopy?  I would assume that
> >> this would get disk corruption, no?  Or if you preffer the other
> >> question, what protects us from disk corruption?
> >
> > I think you can, I've not tried it; however I also think it should
> > be safe.
> >
> >  migration/block.c's block_save_pending always puts a value in the
> > non_postcopiable_pending return value (and 0 in the postcopiable_pending);
> > the migrate thread checks the non_postcopiable_pending size to
> > decide when it can switch to postcopy, and performs a call to the complete
> > method on each device before it does.  Thus the block migration should
> > be finished before we start doing the actual postcopy stage, and thus
> > before the destination CPU starts running.
> 
> I mean that as it is right now, the info under blk_mig_active() check
> would be zero/the same than before entering postcopy.

Ah, yes;  would blk_mig_bytes_total/transferred still have valid values you
would want to display, even at the end of the block migration phase?

> >
> > A possibly harder question is what happens if block.c did implement
> > postcopy and you had both block postcopy and ram postcopy active at
> > the same time; again I think it should work but I'm not sure if one
> > would starve the other.
> >
> >> Once here, I guess we can get the migrate_already_active() bit without
> >> problem?
> >
> > I'm not sure of the question here; but the idea of 
> > migration_already_active()
> > is just to avoid all of the open-coded checks for each possible state;
> > now we've added anothe state they were getting messy.
> 
> Sorry.  I mean that the migrate_already_active() bits can get in without
> further ado.  Don't need to wait for postcopy to be integrated.

Yes; do you want it split out?

Dave

> 
> >
> > Dave
> >
> >> 
> >> Later, Juan.
> > --
> > Dr. David Alan Gilbert / address@hidden / Manchester, UK
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK