Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances

[Peter Xu]

On Wed, Sep 13, 2023 at 06:53:20PM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > On Mon, Sep 11, 2023 at 02:13:19PM -0300, Fabiano Rosas wrote:
> >> The core yank code is strict about balanced registering and
> >> unregistering of yank functions.
> >> 
> >> This creates a difficulty because the migration code registers one
> >> yank function per QIOChannel, but each QIOChannel can be referenced by
> >> more than one QEMUFile. The yank function should not be removed until
> >> all QEMUFiles have been closed.
> >> 
> >> Keep a reference count of how many QEMUFiles are using a QIOChannel
> >> that has a yank function. Only unregister the yank function when all
> >> QEMUFiles have been closed.
> >> 
> >> This improves the current code by removing the need for the programmer
> >> to know which QEMUFile is the last one to be cleaned up and fixes the
> >> theoretical issue of removing the yank function while another QEMUFile
> >> could still be using the ioc and require a yank.
> >> 
> >> Signed-off-by: Fabiano Rosas <farosas@suse.de>
> >> ---
> >>  migration/yank_functions.c | 81 ++++++++++++++++++++++++++++++++++----
> >>  migration/yank_functions.h |  8 ++++
> >>  2 files changed, 81 insertions(+), 8 deletions(-)
> >
> > I worry this over-complicate things.
> 
> It does. We ran out of simple options.
> 
> > If you prefer the cleaness that we operate always on qemufile level, can we
> > just register each yank function per-qemufile?
> 
> "just" hehe
> 
> we could, but:
> 
> i) the yank is a per-channel operation, so this is even more unintuitive;

I mean we can provide something like:

void migration_yank_qemufile(void *opaque)
{
    QEMUFile *file = opaque;
    QIOChannel *ioc = file->ioc;

    qio_channel_shutdown(ioc, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
}

void migration_qemufile_register_yank(QEMUFile *file)
{
    if (migration_ioc_yank_supported(file->ioc)) {
        yank_register_function(MIGRATION_YANK_INSTANCE,
                               migration_yank_qemufile,
                               file);
    }
}

> 
> ii) multifd doesn't have a QEMUFile, so it will have to continue using
>     the ioc;

We can keep using migration_ioc_[un]register_yank() for them if there's no
qemufile attached.  As long as the function will all be registered under
MIGRATION_YANK_INSTANCE we should be fine having different yank func.

> 
> iii) we'll have to add a yank to every new QEMUFile created during the
>      incoming migration (colo, rdma, etc), otherwise the incoming side
>      will be left using iocs while the src uses the QEMUFile;

For RDMA, IIUC it'll simply be a noop as migration_ioc_yank_supported()
will be a noop for it for either reg/unreg.

Currently it seems we will also unreg the ioc even for RDMA (even though we
don't reg for it).  But since unreg will be a noop it seems all fine even
if not paired.. maybe we should still try to pair it, e.g. register also in
rdma_start_outgoing_migration() for the rdma ioc so at least they're paired.

I don't see why COLO is special here, though.  Maybe I missed something.

> 
> iv) this is a functional change of the yank feature for which we have no
>     tests.

Having yank tested should be preferrable.  Lukas is in the loop, let's see
whether he has something. We can still smoke test it before a selftest
being there.

Taking one step back.. I doubt whether anyone is using yank for migration?
Knowing that migration already have migrate-cancel (for precopy) and
migrate-pause (for postcopy).  I never used it myself, and I don't think
it's supported for RHEL.  How's that in suse's case?

If no one is using it, maybe we can even avoid registering migration to
yank?

> 
> If that's all ok to you I'll go ahead and git it a try.
> 
> > I think qmp yank will simply fail the 2nd call on the qemufile if the
> > iochannel is shared with the other one, but that's totally fine, IMHO.
> >
> > What do you think?
> >
> > In all cases, we should probably at least merge patch 1-8 if that can
> > resolve the CI issue.  I think all of them are properly reviewed.
> 
> I agree. Someone needs to queue this though since Juan has been busy.

Yes, I'll see what I can do.

-- 
Peter Xu