Re: [Qemu-devel] [RFC v5 18/26] monitor: send event when request queue full

[Peter Xu]

On Thu, Dec 14, 2017 at 11:41:36AM +0000, Stefan Hajnoczi wrote:
> On Tue, Dec 05, 2017 at 01:51:52PM +0800, Peter Xu wrote:
> > Set maximum QMP request queue length to 8.  If queue full, instead of
> > queue the command, we directly return a "request-dropped" event, telling
> > client that specific command is dropped.
> > 
> > Note that this flow control mechanism is only valid if OOB is enabled.
> > If it's not, the effective queue length will always be 1, which strictly
> > follows original behavior of QMP command handling (which never drop
> > messages).
> > 
> > Signed-off-by: Peter Xu <address@hidden>
> > ---
> >  monitor.c | 15 +++++++++++++++
> >  1 file changed, 15 insertions(+)
> > 
> > diff --git a/monitor.c b/monitor.c
> > index c20e659740..f7923c4590 100644
> > --- a/monitor.c
> > +++ b/monitor.c
> > @@ -4029,6 +4029,8 @@ static void monitor_qmp_bh_dispatcher(void *data)
> >      }
> >  }
> >  
> > +#define  QMP_REQ_QUEUE_LEN_MAX  (8)
> > +
> >  static void handle_qmp_command(JSONMessageParser *parser, GQueue *tokens,
> >                                 void *opaque)
> >  {
> > @@ -4061,6 +4063,19 @@ static void handle_qmp_command(JSONMessageParser 
> > *parser, GQueue *tokens,
> >      req_obj->id = id;
> >      req_obj->req = req;
> >  
> > +    if (qmp_oob_enabled(mon)) {
> > +        /* Drop the request if queue is full. */
> > +        if (mon->qmp.qmp_requests->length >= QMP_REQ_QUEUE_LEN_MAX) {
> 
> qmp_queue_lock must be held.  Perhaps it's simplest to move this check
> into the qmp_queue_lock critical section below and unlock before calling
> qapi_event_send_request_dropped()...
> 
> > +            qapi_event_send_request_dropped(id,
> > +                                            REQUEST_DROP_REASON_QUEUE_FULL,
> > +                                            NULL);
> > +            qobject_decref(id);
> > +            qobject_decref(req);
> > +            g_free(req_obj);
> > +            return;
> > +        }
> > +    }
> > +
> >      /*
> >       * Put the request to the end of queue so that requests will be
> >       * handled in time order.  Ownership for req_obj, req, id,
> 
> ... down here.

Yeah can do.  I think it's not really a must (IMHO the worst case
won't be that worse if current queue length is 8)? but it's obviously
nicer.

Thanks,

-- 
Peter Xu