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

[Peter Xu]

On Thu, Sep 14, 2023 at 10:23:38AM -0300, Fabiano Rosas wrote:
> Peter Xu <peterx@redhat.com> writes:
> 
> > 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);
> >     }
> > }
> 
> Sure, this is what I was thinking as well. IMO it will be yet another
> operation that happens on the channel, but it performed via the
> file. Just like qio_channel_close() at qemu_fclose(). Not the end of the
> world, of course, I just find it error-prone.
> 
> >> 
> >> 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.
> >
> 
> ok
> 
> >> 
> >> 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.
> 
> For colo I was thinking we'd have to register the yank just to be sure
> that all paths unregistering it have something to unregister.
> 
> Maybe I should move the register into qemu_file_new_impl() with a
> matching unregister at qemu_fclose().

Sounds good.  Or...

> 
> >> 
> >> 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).
> 
> Right, both already call qio_channel_shutdown().
> 
> > I never used it myself, and I don't think
> > it's supported for RHEL.  How's that in suse's case?
> 
> Never heard mention of it and I don't see it in our virtualization
> documentation.
> 
> >
> > If no one is using it, maybe we can even avoid registering migration to
> > yank?
> >
> 
> Seems reasonable to me.

... let's wait for a few days from Lukas to see whether he as any more
input, or I'd vote for dropping yank for migration as a whole. It caused
mostly more crashes that I knew than benefits, so far..

I also checked libvirt is not using 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.
> 
> Thanks. I could even send a pull request myself if it would make things
> easier. Let me know.

That's definitely an option.  But I want to make sure it's the same thing;
I replied in Stefan's report.  We can continue the discussion there for that.

-- 
Peter Xu