11 a.m.
[adding libvirt]
On 03/24/2014 09:47 AM, Paolo Bonzini wrote:
Il 24/03/2014 14:04, arei.gonglei(a)huawei.com ha scritto:
> From: zengjunliang <zengjunliang(a)huawei.com>
>
> Return error for migrate cancel, when migration status is not
> MIG_STATE_SETUP or MIG_STATE_ACTIVE. Thus, libvirt can can
> perceive the operation fails.
>
> Signed-off-by: zengjunliang <zengjunliang(a)huawei.com>
> Signed-off-by: Gonglei <arei.gonglei(a)huawei.com>
I think this is done on purpose, because canceling migration is racy.
Instead, libvirt should do "query-migrate" and check if the migration
was completed or canceled.
Can you please give more details at how you are triggering the problem
with libvirt? I think Paolo is probably right - the bug is more likely
to be in libvirt not expecting the race and not recovering correctly
when the race occurs, than it is to be in changing qemu's state algorithm.
--
Eric Blake eblake redhat com +1-919-301-3266
Libvirt virtualization library http://libvirt.org
6:15 a.m.
New subject: [libvirt] [PATCH] migration: Fix possible bug for migrate cancel
-----Original Message-----
From: Eric Blake [mailto:eblake@redhat.com]
Sent: Tuesday, March 25, 2014 12:01 AM
To: Paolo Bonzini; Gonglei (Arei); qemu-devel(a)nongnu.org
Cc: quintela(a)redhat.com; owasserm(a)redhat.com; Yanqiangjun; Zhaoyanbin
(A); Zengjunliang; libvir-list(a)redhat.com
Subject: Re: [PATCH] migration: Fix possible bug for migrate cancel
[adding libvirt]
On 03/24/2014 09:47 AM, Paolo Bonzini wrote:
> Il 24/03/2014 14:04, arei.gonglei(a)huawei.com ha scritto:
>> From: zengjunliang <zengjunliang(a)huawei.com>
>>
>> Return error for migrate cancel, when migration status is not
>> MIG_STATE_SETUP or MIG_STATE_ACTIVE. Thus, libvirt can can
>> perceive the operation fails.
>>
>> Signed-off-by: zengjunliang <zengjunliang(a)huawei.com>
>> Signed-off-by: Gonglei <arei.gonglei(a)huawei.com>
>
> I think this is done on purpose, because canceling migration is racy.
> Instead, libvirt should do "query-migrate" and check if the migration
> was completed or canceled.
Can you please give more details at how you are triggering the problem
with libvirt? I think Paolo is probably right - the bug is more likely
to be in libvirt not expecting the race and not recovering correctly
when the race occurs, than it is to be in changing qemu's state algorithm.
When the migration progress reaches 100%, and the migration status becomes
MIG_STATE_COMPLETED in Qemu.
It will take some time which from MIG_STATE_COMPLETED to the migration thread resources
are recovered.
If we cancel the migration at this moment, the migrate_fd_cancel function will break
directly without reporting
error code. Then, libvirt considers the cancle operation a success, contrary facts.
Best regards,
-Gonglei
4:18 a.m.
New subject: [libvirt] [PATCH] migration: Fix possible bug for migrate cancel
> >> Return error for migrate cancel, when migration status
is not
> >> MIG_STATE_SETUP or MIG_STATE_ACTIVE. Thus, libvirt can can
> >> perceive the operation fails.
> >>
> >> Signed-off-by: zengjunliang <zengjunliang(a)huawei.com>
> >> Signed-off-by: Gonglei <arei.gonglei(a)huawei.com>
> >
> > I think this is done on purpose, because canceling migration is racy.
> > Instead, libvirt should do "query-migrate" and check if the
migration
> > was completed or canceled.
>
> Can you please give more details at how you are triggering the problem
> with libvirt? I think Paolo is probably right - the bug is more likely
> to be in libvirt not expecting the race and not recovering correctly
> when the race occurs, than it is to be in changing qemu's state algorithm.
>
When the migration progress reaches 100%, and the migration status becomes
MIG_STATE_COMPLETED in Qemu.
It will take some time which from MIG_STATE_COMPLETED to the migration
thread resources are recovered.
If we cancel the migration at this moment, the migrate_fd_cancel function will
break directly without reporting
error code. Then, libvirt considers the cancle operation a success, contrary
facts.
Ping...
Best regards,
-Gonglei
4:28 a.m.
New subject: [libvirt] [PATCH] migration: Fix possible bug for migrate cancel
Il 28/03/2014 10:18, Gonglei (Arei) ha scritto:
> > > Can you please give more details at how you are
triggering the problem
> > > with libvirt? I think Paolo is probably right - the bug is more likely
> > > to be in libvirt not expecting the race and not recovering correctly
> > > when the race occurs, than it is to be in changing qemu's state
algorithm.
> > >
> When the migration progress reaches 100%, and the migration status becomes
> MIG_STATE_COMPLETED in Qemu.
> It will take some time which from MIG_STATE_COMPLETED to the migration
> thread resources are recovered.
> If we cancel the migration at this moment, the migrate_fd_cancel function will
> break directly without reporting
> error code. Then, libvirt considers the cancle operation a success, contrary
> facts.
There is no error, once migration is completed you can still shutdown on
the destination and continue on the source. Libvirt should either:
1) poll with "query-migrate" after migrate_cancel, and report an error
there if it's the desired semantics;
2) toggle a "cancelled" flag before asking QEMU to cancel migration,
check it in the migration functions after "query-migrate" reported
completion; if it is true, do not resume on the destination.
Another reason for doing it in libvirt is that the serialization between
cancellation and completion of migration ultimately is controlled by
libvirt's lock. Doing this in QEMU makes it harder to reason about
concurrency.
Paolo
6:30 a.m.
New subject: [libvirt] [Qemu-devel] [PATCH] migration: Fix possible bug for migrate cancel
* Paolo Bonzini (pbonzini(a)redhat.com) wrote:
Il 28/03/2014 10:18, Gonglei (Arei) ha scritto:
>>> > Can you please give more details at how you are triggering the problem
>>> > with libvirt? I think Paolo is probably right - the bug is more
likely
>>> > to be in libvirt not expecting the race and not recovering correctly
>>> > when the race occurs, than it is to be in changing qemu's state
algorithm.
>>> >
>>When the migration progress reaches 100%, and the migration status becomes
>>MIG_STATE_COMPLETED in Qemu.
>>It will take some time which from MIG_STATE_COMPLETED to the migration
>>thread resources are recovered.
>>If we cancel the migration at this moment, the migrate_fd_cancel function will
>>break directly without reporting
>>error code. Then, libvirt considers the cancle operation a success, contrary
>>facts.
There is no error, once migration is completed you can still
shutdown on the destination and continue on the source. Libvirt
should either:
(I've rewritten my reply below about 4 times - swinging between
different answers, this stuff really isn't obvious, and certainly
not documented)
I think I agree that it's not an error; but I think migrate_fd_cancel
knows what the outcome will be.
If it was MIG_STATE_ERROR on entry to migrate_fd_cancel, then yes it
could tell you that the cancel failed because you were already in error.
If it was MIG_STATE_COMPLETED on entry to migrate_fd_cancel, then yes it
could tell you that the cancel failed because you already finished.
If it was MIG_STATE_ACTIVE on entry to migrate_fd_cancel - it will go to
MIG_STATE_CANCELLING and I believe eventually to MIG_STATE_CANCELLED;
I don't believe it can get to MIG_STATE_ERROR from that point, since
all of the places in the migrate_thread that transition to error
do explicit ACTIVE->ERROR transitions. I don't believe it can get to
MIG_STATE_COMPLETED for the same reason.
So migrate_fd_cancel knows that the eventual outcome will be Error
or Cancelled or completed, even if the state isn't there yet, and it
could reply to say that.
1) poll with "query-migrate" after migrate_cancel, and
report an
error there if it's the desired semantics;
2) toggle a "cancelled" flag before asking QEMU to cancel migration,
check it in the migration functions after "query-migrate" reported
completion; if it is true, do not resume on the destination.
I think you're right you have to poll with query-migrate until you
get one of cancelled/failed/completed.
However it's a bit odd; prior to the introduction of 'CANCELLING', the
state that you would get by a query-migrate after migrate_fd_cancel
returned would in principal be the state you ended up in - i.e.
cancelled/failed/completed. With cancelling added, query-migrate
might lie to you and say 'active' (when it's really hiding the
fact that cancelling is happening). So while 'cancelling' apparently
didn't alter the API it did, in that query-migrate after a cancel
can now return active where it couldn't before.
Another reason for doing it in libvirt is that the serialization
between cancellation and completion of migration ultimately is
controlled by libvirt's lock. Doing this in QEMU makes it harder to
reason about concurrency.
I think you have to be careful when you talk about 'cancellation and completion
of migration' - in that paragraph I don't think you mean the same thing
as MIG_STATE_CANCELLED and MIG_STATE_COMPLETED, I think you're talking
about the larger scale idea of completion after you take into account
that the VM might be paused after qemu has gone to MIG_STATE_COMPLETED and
libvirt might still decide it wants to give up and use the version on
the source that's still paused.
Dave
--
Dr. David Alan Gilbert / dgilbert(a)redhat.com / Manchester, UK
7:16 a.m.
New subject: [libvirt] [Qemu-devel] [PATCH] migration: Fix possible bug for migrate cancel
Il 28/03/2014 12:30, Dr. David Alan Gilbert ha scritto:
> > Another reason for doing it in libvirt is that the
serialization
> > between cancellation and completion of migration ultimately is
> > controlled by libvirt's lock. Doing this in QEMU makes it harder to
> > reason about concurrency.
I think you have to be careful when you talk about 'cancellation and completion
of migration' - in that paragraph I don't think you mean the same thing
as MIG_STATE_CANCELLED and MIG_STATE_COMPLETED, I think you're talking
about the larger scale idea of completion after you take into account
that the VM might be paused after qemu has gone to MIG_STATE_COMPLETED and
libvirt might still decide it wants to give up and use the version on
the source that's still paused.
Yes, exactly. This is why I considered the possibility of adding a
"cancelled" flag within libvirt.
Libvirt always uses -S on the destination, so it's always possible to
cancel migration even after MIG_STATE_COMPLETED.
Paolo
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5 comments
4 participants
participants (4)
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Dr. David Alan Gilbert -
Eric Blake -
Gonglei (Arei) -
Paolo Bonzini