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

[Juan Quintela]

"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.

>
> 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.

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