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Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank insta
From: |
Fabiano Rosas |
Subject: |
Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances |
Date: |
Thu, 14 Sep 2023 10:23:38 -0300 |
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().
>>
>> 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.
>>
>> 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.
- [PATCH v6 06/10] migration: Replace the return path retry logic, (continued)
- [PATCH v6 06/10] migration: Replace the return path retry logic, Fabiano Rosas, 2023/09/11
- [PATCH v6 07/10] migration: Move return path cleanup to main migration thread, Fabiano Rosas, 2023/09/11
- [PATCH v6 08/10] migration/yank: Use channel features, Fabiano Rosas, 2023/09/11
- [PATCH v6 04/10] migration: Remove redundant cleanup of postcopy_qemufile_src, Fabiano Rosas, 2023/09/11
- [PATCH v6 09/10] migration/yank: Keep track of registered yank instances, Fabiano Rosas, 2023/09/11
- Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances, Peter Xu, 2023/09/13
- Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances, Fabiano Rosas, 2023/09/13
- Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances, Peter Xu, 2023/09/13
- Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances,
Fabiano Rosas <=
- Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances, Peter Xu, 2023/09/14
- Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances, Lukas Straub, 2023/09/25
- Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances, Fabiano Rosas, 2023/09/25
- Re: [PATCH v6 09/10] migration/yank: Keep track of registered yank instances, Lukas Straub, 2023/09/25
[PATCH v6 10/10] migration: Add a wrapper to cleanup migration files, Fabiano Rosas, 2023/09/11