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

[Stefan Hajnoczi]

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).

The next step after that would be to audit other monitor commands for
unnecessary vcpu synchronization.

Stefan