RE: [PATCH 1/2] migration/colo: Optimize COLO start code path

[Zhang, Chen]

> -----Original Message-----
> From: Juan Quintela <quintela@redhat.com>
> Sent: Wednesday, November 17, 2021 12:28 AM
> To: Zhang, Chen <chen.zhang@intel.com>
> Cc: Hailiang Zhang <zhang.zhanghailiang@huawei.com>; Dr . David Alan
> Gilbert <dgilbert@redhat.com>; qemu-dev <qemu-devel@nongnu.org>
> Subject: Re: [PATCH 1/2] migration/colo: Optimize COLO start code path
> 
> Zhang Chen <chen.zhang@intel.com> wrote:
> > There is no need to start COLO through MIGRATION_STATUS_ACTIVE.
> 
> Hi
> 
> I don't understand what you are trying to do.  In my reading, at least the
> commit message is wrong:
> 
> void migrate_start_colo_process(MigrationState *s) {
>     ...
>     migrate_set_state(&s->state, MIGRATION_STATUS_ACTIVE,
>                       MIGRATION_STATUS_COLO);
>     ...
> }
> 
> and
> 
> void *colo_process_incoming_thread(void *opaque) {
>     ...
>     migrate_set_state(&mis->state, MIGRATION_STATUS_ACTIVE,
>                       MIGRATION_STATUS_COLO);
> 
> So colo starts with MIGRATION_STATUS_ACTIVE.

Yes, this patch just optimized COLO primary code 
path(migrate_start_colo_process()).
We can see this patch removed the 
 migrate_set_state(&s->state, MIGRATION_STATUS_ACTIVE,
                      MIGRATION_STATUS_COLO);
In the migrate_start_colo_process().

Current COLO status path:
 MIGRATION_STATUS_XXX   --->   MIGRATION_STATUS_ACTIVE ---> 
MIGRATION_STATUS_COLO ---> MIGRATION_STATUS_COMPLETED

This patch try to remove redundant " MIGRATION_STATUS_ACTIVE " in COLO start. 
MIGRATION_STATUS_XXX   ---> MIGRATION_STATUS_COLO ---> 
MIGRATION_STATUS_COMPLETED

Actually COLO primary code did nothing when running on 
"MIGRATION_STATUS_ACTIVE".
But for COLO secondary (void *colo_process_incoming_thread()), it shared some 
code with normal migration. No need to do this.

So, I will fix commit message to:
Optimize COLO primary start path to:
MIGRATION_STATUS_XXX   ---> MIGRATION_STATUS_COLO ---> 
MIGRATION_STATUS_COMPLETED
No need to start primary COLO through "MIGRATION_STATUS_ACTIVE".

How about it?

> 
> 
> > Signed-off-by: Zhang Chen <chen.zhang@intel.com>
> > ---
> >  migration/colo.c      |  2 --
> >  migration/migration.c | 18 +++++++++++-------
> >  2 files changed, 11 insertions(+), 9 deletions(-)
> >
> > diff --git a/migration/colo.c b/migration/colo.c index
> > 2415325262..ad1a4426b3 100644
> > --- a/migration/colo.c
> > +++ b/migration/colo.c
> > @@ -667,8 +667,6 @@ void migrate_start_colo_process(MigrationState *s)
> >                                  colo_checkpoint_notify, s);
> >
> >      qemu_sem_init(&s->colo_exit_sem, 0);
> > -    migrate_set_state(&s->state, MIGRATION_STATUS_ACTIVE,
> > -                      MIGRATION_STATUS_COLO);
> >      colo_process_checkpoint(s);
> >      qemu_mutex_lock_iothread();
> >  }
> > diff --git a/migration/migration.c b/migration/migration.c index
> > abaf6f9e3d..4c8662a839 100644
> > --- a/migration/migration.c
> > +++ b/migration/migration.c
> > @@ -3222,7 +3222,10 @@ static void migration_completion(MigrationState
> *s)
> >          goto fail_invalidate;
> >      }
> >
> > -    if (!migrate_colo_enabled()) {
> > +    if (migrate_colo_enabled()) {
> > +        migrate_set_state(&s->state, current_active_state,
> > +                          MIGRATION_STATUS_COLO);
> > +    } else {
> >          migrate_set_state(&s->state, current_active_state,
> >                            MIGRATION_STATUS_COMPLETED);
> >      }
> 
> This moves the setup to MIGRATION_STATUS_COLO to completion time
> instead of the beggining of the process.  I have no clue why.  I guess you can
> put a comment/commit message to say what you ar.e trynig to do.

You are right, no need to setup here.
I will remove this in next version.

> 
> > @@ -3607,12 +3610,7 @@ static void
> migration_iteration_finish(MigrationState *s)
> >          migration_calculate_complete(s);
> >          runstate_set(RUN_STATE_POSTMIGRATE);
> >          break;
> > -
> > -    case MIGRATION_STATUS_ACTIVE:
> > -        /*
> > -         * We should really assert here, but since it's during
> > -         * migration, let's try to reduce the usage of assertions.
> > -         */
> > +    case MIGRATION_STATUS_COLO:
> >          if (!migrate_colo_enabled()) {
> >              error_report("%s: critical error: calling COLO code without "
> >                           "COLO enabled", __func__); @@ -3622,6
> > +3620,12 @@ static void migration_iteration_finish(MigrationState *s)
> >           * Fixme: we will run VM in COLO no matter its old running state.
> >           * After exited COLO, we will keep running.
> >           */
> > +         /* Fallthrough */
> > +    case MIGRATION_STATUS_ACTIVE:
> > +        /*
> > +         * We should really assert here, but since it's during
> > +         * migration, let's try to reduce the usage of assertions.
> > +         */
> >          s->vm_was_running = true;
> >          /* Fallthrough */
> >      case MIGRATION_STATUS_FAILED:
> 
> I guess this change is related to the previous one, but I don't understand 
> colo
> enough to review it.

I think this patch is the general code, little background needed.
You can simple understand COLO is two VMs(primary node and secondary node) 
entered a state of cyclic migration.
Thanks your comments.

Thanks
Chen
 

> 
> Later, Juan.