Re: [PATCH 0/5] Introduce 'yank' oob qmp command to recover from hanging qemu

[Kevin Wolf]

Am 13.05.2020 um 12:53 hat Dr. David Alan Gilbert geschrieben:
> * Kevin Wolf (address@hidden) wrote:
> > Am 12.05.2020 um 11:43 hat Daniel P. Berrangé geschrieben:
> > > On Tue, May 12, 2020 at 11:32:06AM +0200, Lukas Straub wrote:
> > > > On Mon, 11 May 2020 16:46:45 +0100
> > > > "Dr. David Alan Gilbert" <address@hidden> wrote:
> > > > 
> > > > > * Daniel P. Berrangé (address@hidden) wrote: 
> > > > > > ...
> > > > > > That way if QEMU does get stuck, you can start by tearing down the
> > > > > > least distruptive channel. eg try tearing down the migration 
> > > > > > connection
> > > > > > first (which shouldn't negatively impact the guest), and only if 
> > > > > > that
> > > > > > doesn't work then, move on to tear down the NBD connection (which 
> > > > > > risks
> > > > > > data loss)  
> > > > > 
> > > > > I wonder if a different way would be to make all network connections
> > > > > register with yank, but then make yank take a list of connections to
> > > > > shutdown(2).
> > > > 
> > > > Good Idea. We could name the connections (/yank callbacks) in the
> > > > form "nbd:<node-name>", "chardev:<chardev-name>" and "migration"
> > > > (and add "netdev:...", etc. in the future). Then make yank take a
> > > > list of connection names as you suggest and silently ignore connections
> > > > that don't exist. And maybe even add a 'query-yank' oob command 
> > > > returning
> > > > a list of registered connections so the management application can do
> > > > pattern matching if it wants.
> > 
> > I'm generally not a big fan of silently ignoring things. Is there a
> > specific requirement to do it in this case, or can management
> > applications be expected to know which connections exist?
> > 
> > > Yes, that would make the yank command much more flexible in how it can
> > > be used.
> > > 
> > > As an alternative to using formatted strings like this, it could be
> > > modelled more explicitly in QAPI
> > > 
> > >   { 'struct':  'YankChannels',
> > >     'data': { 'chardev': [ 'string' ],
> > >               'nbd': ['string'],
> > >         'migration': bool } }
> > > 
> > > In this example, 'chardev' would accept a list of chardev IDs which
> > > have it enabled, 'nbd' would accept a list of block node IDs which
> > > have it enabled, and migration is a singleton on/off.
> > 
> > Of course, it also means that the yank code needs to know about every
> > single object that supports the operation, whereas if you only have
> > strings, the objects could keep registering their connection with a
> > generic function like yank_register_function() in this version.
> > 
> > I'm not sure if the additional complexity is worth the benefits.
> 
> I tend to agree; although we do have to ensure we either use an existing
> naming scheme (e.g. QOM object names?) or make sure we've got a well
> defined list of prefixes.

Not everything that has a network connection is a QOM object (in fact,
neither migration nor chardev nor nbd are QOM objects).

I guess it would be nice to have a single namespace for everything in
QEMU, but the reality is that we have a few separate ones. As long as we
consistently add a prefix that identifies the namespace in question, I
think that would work.

This means that if we're using node-name to identify the NBD connection,
the namespace should be 'block' rather than 'nbd'.

One more thing to consider is, what if a single object has multiple
connections? In the case of node-names, we have a limited set of allowed
characters, so we can use one of the remaining characters as a separator
and then suffix a counter. In other places, the identifier isn't
restricted, so suffixing doesn't work. Maybe prefixing does, but it
would have to be there from the beginning then.

And another thing: Do we really want to document this as limited to
network connections? Another common cause of hangs is when you have
image files on an NFS mount and the connection goes away. Of course, in
the end this is still networking, but inside of QEMU it looks like
accessing any other file. I'm not sure that we'll allow yanking access
to image files anytime soon, but it might not hurt to keep it at the
back of our mind as a potential option we might want the design to
allow.

Kevin