Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread

[Markus Armbruster]

"Dr. David Alan Gilbert" <address@hidden> writes:

> * Daniel P. Berrange (address@hidden) wrote:
>> On Thu, Sep 07, 2017 at 04:13:41PM +0800, Peter Xu wrote:
>> > On Wed, Sep 06, 2017 at 12:54:28PM +0100, Daniel P. Berrange wrote:
>> > > On Wed, Sep 06, 2017 at 12:31:58PM +0100, Dr. David Alan Gilbert wrote:
>> > > > * Daniel P. Berrange (address@hidden) wrote:
>> > > > > This does imply that you need a separate monitor I/O processing, 
>> > > > > from the
>> > > > > command execution thread, but I see no need for all commands to 
>> > > > > suddenly
>> > > > > become async. Just allowing interleaved replies is sufficient from 
>> > > > > the
>> > > > > POV of the protocol definition. This interleaving is easy to handle 
>> > > > > from
>> > > > > the client POV - just requires a unique 'serial' in the request by 
>> > > > > the
>> > > > > client, that is copied into the reply by QEMU.
>> > > > 
>> > > > OK, so for that we can just take Marc-André's syntax and call it 'id':
>> > > >   https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg03634.html
>> > > > 
>> > > > then it's upto the caller to ensure those id's are unique.
>> > > 
>> > > Libvirt has in fact generated a unique 'id' for every monitor command
>> > > since day 1 of supporting QMP.
>> > > 
>> > > > I do worry about two things:
>> > > >   a) With this the caller doesn't really know which commands could be
>> > > >   in parallel - for example if we've got a recovery command that's
>> > > >   executed by this non-locking thread that's OK, we expect that
>> > > >   to be doable in parallel.  If in the future though we do
>> > > >   what you initially suggested and have a bunch of commands get
>> > > >   routed to the migration thread (say) then those would suddenly
>> > > >   operate in parallel with other commands that we're previously
>> > > >   synchronous.
>> > > 
>> > > We could still have an opt-in for async commands. eg default to executing
>> > > all commands in the main thread, unless the client issues an explicit
>> > > "make it async" command, to switch to allowing the migration thread to
>> > > process it async.
>> > > 
>> > >  { "execute": "qmp_allow_async",
>> > >    "data": { "commands": [
>> > >        "migrate_cancel",
>> > >    ] } }
>> > > 
>> > > 
>> > >  { "return": { "commands": [
>> > >        "migrate_cancel",
>> > >    ] } }
>> > > 
>> > > The server response contains the subset of commands from the request
>> > > for which async is supported.
>> > > 
>> > > That gives good negotiation ability going forward as we incrementally
>> > > support async on more commands.
>> > 
>> > I think this goes back to the discussion on which design we'd like to
>> > choose.  IMHO the whole async idea plus the per-command-id is indeed
>> > cleaner and nicer, and I believe that can benefit not only libvirt,
>> > but also other QMP users.  The problem is, I have no idea how long
>> > it'll take to let us have such a feature - I believe that will include
>> > QEMU and Libvirt to both support that.  And it'll be a pity if the
>> > postcopy recovery cannot work only because we cannot guarantee a
>> > stable monitor.
>> 
>> This is not a blocker for having postcopy recovery feature merged.
>> It merely means that in a situation where the mainloop is blocked,
>> then we can't recover, in other situations we'll be able to recover
>> fine. Sure it would be nice to fix that problem too, but I don't
>> see it as a block.
>
> It's probably OK to merge the recovery code before the monitor code;
> but I don't think it's something you'd want to tell users about -
> a 'postcopy recovery that only works rarely' isn't much use.

"Rarely"?  Are main loop hangs *that* common?

Can we quantify the problem to help gauge urgency?