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

[Dr. David Alan Gilbert]

* Stefan Hajnoczi (address@hidden) wrote:
> On Thu, Sep 7, 2017 at 9:13 AM, Peter Xu <address@hidden> 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.
> 
> Please don't rush in a hack, they often introduce new bugs that we
> have to support long-term when they are part of the QMP API.
> 
> In your original email you mentioned "info cpus".  Have you considered
> modifying this command so it does not sync the CPU?  I'm not sure
> callers really need to sync the CPU, typically they just want to know
> the vcpu numbers, thread IDs, and current state (halted, running,
> etc).

But it has the pc as well, so that's actual state.

Dave

> The next step after that would be to audit other monitor commands for
> unnecessary vcpu synchronization.
> 
> Stefan
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK