Re: [Qemu-devel] [PATCH v4 33/47] Postcopy: Postcopy startup in migration thread

[Dr. David Alan Gilbert]

* David Gibson (address@hidden) wrote:
> On Fri, Oct 03, 2014 at 06:47:39PM +0100, Dr. David Alan Gilbert (git) wrote:
> > From: "Dr. David Alan Gilbert" <address@hidden>
> > 
> > Rework the migration thread to setup and start postcopy.
> > 
> > Signed-off-by: Dr. David Alan Gilbert <address@hidden>
> > ---
> >  include/migration/migration.h |   3 +
> >  migration.c                   | 201 
> > ++++++++++++++++++++++++++++++++++++++----
> >  2 files changed, 185 insertions(+), 19 deletions(-)
> > 
> > diff --git a/include/migration/migration.h b/include/migration/migration.h
> > index b01cc17..f401775 100644
> > --- a/include/migration/migration.h
> > +++ b/include/migration/migration.h

> > +    DPRINTF("postcopy_start: sending req 2\n");
> > +    qemu_savevm_send_reqack(ms->file, 2);
> 
> Are these reqacks just for debugging, or do they affect the protocol?

Just for debugging; comment added.  They just make it easy to line
traces up between the source and destination, and also make it easy to figure
out how far stuff has got if it jams up.

> > +    if (migrate_postcopy_ram()) {
> > +        /* Now tell the dest that it should open it's end so it can reply 
> > */
> 
> s/it's/its/

Fixed.

> > +        qemu_savevm_send_openrp(s->file);
> > +
> > +        /* And ask it to send an ack that will make stuff easier to debug 
> > */
> > +        qemu_savevm_send_reqack(s->file, 1);
> > +
> > +        /* Tell the destination that we *might* want to do postcopy later;
> > +         * if the other end can't do postcopy it should fail now, nice and
> > +         * early.
> > +         */
> > +        qemu_savevm_send_postcopy_ram_advise(s->file);
> > +    }
> > +
> >      s->setup_time = qemu_clock_get_ms(QEMU_CLOCK_HOST) - setup_start;
> > +    current_active_type = MIG_STATE_ACTIVE;
> >      migrate_set_state(s, MIG_STATE_SETUP, MIG_STATE_ACTIVE);
> >  
> >      DPRINTF("setup complete\n");
> > @@ -945,37 +1057,74 @@ static void *migration_thread(void *opaque)
> >                      " nonpost=%" PRIu64 ")\n",
> >                      pending_size, max_size, pend_post, pend_nonpost);
> >              if (pending_size && pending_size >= max_size) {
> > +                /* Still a significant amount to transfer */
> > +
> > +                current_time = qemu_clock_get_ms(QEMU_CLOCK_REALTIME);
> > +                if (migrate_postcopy_ram() &&
> > +                    s->state != MIG_STATE_POSTCOPY_ACTIVE &&
> > +                    pend_nonpost == 0 && s->start_postcopy) {
> 
> Hrm.  This is checking for pend_nonpost == 0, rather than just close
> to zero.  IIUC this will only work if all "live sendable" state is
> also postcopyable.  But if we have live sendable data that's not
> postcopyable - like the power hash page table - we'll need some
> threshold here, like we currently have for entering the stopped vm
> phase of a precopy migration.
> 
> Or am I missing something?

Hmm, I think you're right; I've changed this to:
        pend_nonpost <= max_size 

so that it's the same cut-off logic as the normal end-of-migrate;
I think that will work; i.e. it gets small enough to be expected
to complete quickly in the _complete phase that's at the start
of postcopy.

Dave

--
Dr. David Alan Gilbert / address@hidden / Manchester, UK