Re: [Qemu-devel] [RFC v5 21/26] qmp: isolate responses into io thread

[Stefan Hajnoczi]

On Mon, Dec 18, 2017 at 03:32:29PM +0800, Peter Xu wrote:
> On Mon, Dec 18, 2017 at 01:52:17PM +0800, Peter Xu wrote:
> > On Thu, Dec 14, 2017 at 01:43:59PM +0000, Stefan Hajnoczi wrote:
> > > On Tue, Dec 05, 2017 at 01:51:55PM +0800, Peter Xu wrote:
> > > > @@ -4429,6 +4515,13 @@ void monitor_cleanup(void)
> > > >       */
> > > >      iothread_stop(mon_global.mon_iothread);
> > > >  
> > > > +    /*
> > > > +     * After we have IOThread to send responses, it's possible that
> > > > +     * when we stop the IOThread there are still replies queued in the
> > > > +     * responder queue.  Flush all of them.
> > > > +     */
> > > > +    monitor_qmp_bh_responder(NULL);
> > > 
> > > This doesn't work because monitor_qmp_bh_responder() does not guarantee
> > > that the full response has been written when it returns.
> > > 
> > > When qemu_chr_fe_write() returns EAGAIN then qemu_chr_fe_add_watch() is
> > > used to register an event loop callback when the chardev becomes
> > > writable again.  But you stopped the event loop using iothread_stop() so
> > > we will never complete the write.
> > 
> > Good catch...
> > 
> > Actually I just noticed that for char frontend I missed a place to use
> > the chardev context for polling.  So before the flushing I possibly
> > need this:
> > 
> > diff --git a/chardev/char-fe.c b/chardev/char-fe.c                          
> >    
> > index ee6d596100..462c529f19 100644    
> > --- a/chardev/char-fe.c                
> > +++ b/chardev/char-fe.c                
> > @@ -356,7 +356,7 @@ guint qemu_chr_fe_add_watch(CharBackend *be, 
> > GIOCondition cond,                                                          
> >                  
> >      }                                 
> >                                        
> >      g_source_set_callback(src, (GSourceFunc)func, user_data, NULL);        
> >    
> > -    tag = g_source_attach(src, NULL); 
> > +    tag = g_source_attach(src, be->chr->gcontext);                         
> >    
> >      g_source_unref(src);              
> >                                        
> >      return tag;                       
> > 
> > Otherwise it'll be still be run in main thread always.
> > 
> > (I guess I haven't yet encountered an EAGAIN for it so far)
> > 
> > > 
> > > I suggest draining the monitor while the IOThread is still running
> > > (that way the AioContext and GMainContext are still operational).  You
> > > can:
> > > 1. Suspend the monitor so new commands will not be read.
> > > 2. Wait until all responses and outbuf are empty.
> > > 
> > > Another option is moving the chardev back to the main loop but I'm not
> > > sure if the chardev subsystem supports that.
> > 
> > Your suggestion is good to me.  I'll do that in IOThread before it
> > stops.  Thanks!
> 
> Hmm, after a second thought, I think maybe it would be nicer to just
> call monitor_flush() after the monitor iothread is stopped.
> 
> Firstly, that is perfectly legal - since AFAICT commit 6cff3e8594
> ("monitor: protect outbuf and mux_out with mutex", 2014-06-23) is
> tailored for doing that, though that was for block iothread, not
> monitor.
> 
> Then, it's still possible that the write buffer is full and we got
> EAGAIN during flushing.  Then we'll plug another task into the monitor
> gcontext which will never run since the iothread is stopped already.
> IMHO that's totally fine - I think it's already our best effort to
> flush the out buffer once at this point.  If the buffer is still full
> after one attemp of monitor_flush(), we should just give up the 2nd
> flushing instead of waiting here.  After all, we are quitting QEMU.
> 
> As a summary, my planned change of this issue would be quite
> straightforward: call monitor_flush() when looping over monitors,
> before destroying them.  And when destroying, if there is dangling
> Monitor.out_watch, detach them.  Hope that works.  Thanks,

I think QEMU currently doesn't drain the monitor output buffer either.
Please make sure to write the code and commit description so it is clear
this is just a best-effort attempt.

Stefan

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