Re: [PATCH 13/20] migration: Move channel setup out of postcopy_try_recover()

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Peter Xu (peterx@redhat.com) wrote:
> On Tue, Feb 22, 2022 at 10:57:34AM +0000, Dr. David Alan Gilbert wrote:
> > * Peter Xu (peterx@redhat.com) wrote:
> > > We used to use postcopy_try_recover() to replace migration_incoming_setup() to
> > > setup incoming channels.  That's fine for the old world, but in the new world
> > > there can be more than one channels that need setup.  Better move the channel
> > > setup out of it so that postcopy_try_recover() only handles the last phase of
> > > switching to the recovery phase.
> > > 
> > > To do that in migration_fd_process_incoming(), move the postcopy_try_recover()
> > > call to be after migration_incoming_setup(), which will setup the channels.
> > > While in migration_ioc_process_incoming(), postpone the recover() routine right
> > > before we'll jump into migration_incoming_process().
> > > 
> > > A side benefit is we don't need to pass in QEMUFile* to postcopy_try_recover()
> > > anymore.  Remove it.
> > > 
> > > Signed-off-by: Peter Xu <peterx@redhat.com>
> > 
> > OK, but note one question below:
> > 
> > Reviewed-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> 
> Thanks.
> 
> > 
> > > ---
> > >  migration/migration.c | 23 +++++++++++------------
> > >  1 file changed, 11 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/migration/migration.c b/migration/migration.c
> > > index 67520d3105..b2e6446457 100644
> > > --- a/migration/migration.c
> > > +++ b/migration/migration.c
> > > @@ -665,19 +665,20 @@ void migration_incoming_process(void)
> > >  }
> > >  
> > >  /* Returns true if recovered from a paused migration, otherwise false */
> > > -static bool postcopy_try_recover(QEMUFile *f)
> > > +static bool postcopy_try_recover(void)
> > >  {
> > >      MigrationIncomingState *mis = migration_incoming_get_current();
> > >  
> > >      if (mis->state == MIGRATION_STATUS_POSTCOPY_PAUSED) {
> > >          /* Resumed from a paused postcopy migration */
> > >  
> > > -        mis->from_src_file = f;
> > > +        /* This should be set already in migration_incoming_setup() */
> > > +        assert(mis->from_src_file);
> > >          /* Postcopy has standalone thread to do vm load */
> > > -        qemu_file_set_blocking(f, true);
> > > +        qemu_file_set_blocking(mis->from_src_file, true);
> > 
> > Does that set_blocking happen on the 2nd channel somewhere?
> 
> Nop.  I think the rational is that by default all channels are blocking.
> 
> Then what happened is: migration code only sets the main channel to
> non-blocking on incoming, that's in migration_incoming_setup().  Hence for
> postcopy recovery we need to tweak it to blocking here.

OK, yes, so the rule seems to be if it's done in it's own thread, we
make it blocking.

> The 2nd new channel is not operated by migration_incoming_setup(), but by
> postcopy_preempt_new_channel(), so it keeps the original blocking state,
> which should be blocking.
> 
> If we want to make that clear, we can proactively set non-blocking too in
> postcopy_preempt_new_channel() on the 2nd channel.  It's just that it
> should be optional as long as blocking is the default for any new fd of a
> socket.

OK, I notice that in 9e4d2b9 made it explicit on the outgoing side.

Dave

> Thanks,
> 
> -- 
> Peter Xu
> 
-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK