Re: [Qemu-devel] [RFC v2 2/8] monitor: allow monitor to create thread to poll

[Peter Xu <peterx@redhat.com>]

On Mon, Aug 28, 2017 at 12:11:38PM +0200, Marc-André Lureau wrote:
> Hi
> 
> On Mon, Aug 28, 2017 at 5:05 AM, Peter Xu <peterx@redhat.com> wrote:
> > On Fri, Aug 25, 2017 at 04:07:34PM +0000, Marc-André Lureau wrote:
> >> On Fri, Aug 25, 2017 at 5:33 PM Dr. David Alan Gilbert <dgilbert@redhat.com>
> >> wrote:
> >>
> >> > * Marc-André Lureau (marcandre.lureau@gmail.com) wrote:
> >> > > Hi
> >> > >
> >> > > On Wed, Aug 23, 2017 at 8:52 AM Peter Xu <peterx@redhat.com> wrote:
> >> > >
> >> > > > Firstly, introduce Monitor.use_thread, and set it for monitors that are
> >> > > > using non-mux typed backend chardev.  We only do this for monitors, so
> >> > > > mux-typed chardevs are not suitable (when it connects to, e.g., serials
> >> > > > and the monitor together).
> >> > > >
> >> > > > When use_thread is set, we create standalone thread to poll the monitor
> >> > > > events, isolated from the main loop thread.  Here we still need to take
> >> > > > the BQL before dispatching the tasks since some of the monitor commands
> >> > > > are not allowed to execute without the protection of BQL.  Then this
> >> > > > gives us the chance to avoid taking the BQL for some monitor commands
> >> > in
> >> > > > the future.
> >> > > >
> >> > > > * Why this change?
> >> > > >
> >> > > > We need these per-monitor threads to make sure we can have at least one
> >> > > > monitor that will never stuck (that can receive further monitor
> >> > > > commands).
> >> > > >
> >> > > > * So when will monitors stuck?  And, how do they stuck?
> >> > > >
> >> > > > After we have postcopy and remote page faults, it's simple to achieve a
> >> > > > stuck in the monitor (which is also a stuck in main loop thread):
> >> > > >
> >> > > > (1) Monitor deadlock on BQL
> >> > > >
> >> > > > As we may know, when postcopy is running on destination VM, the vcpu
> >> > > > threads can stuck merely any time as long as it tries to access an
> >> > > > uncopied guest page.  Meanwhile, when the stuck happens, it is possible
> >> > > > that the vcpu thread is holding the BQL.  If the page fault is not
> >> > > > handled quickly, you'll find that monitors stop working, which is
> >> > trying
> >> > > > to take the BQL.
> >> > > >
> >> > > > If the page fault cannot be handled correctly (one case is a paused
> >> > > > postcopy, when network is temporarily down), monitors will hang
> >> > > > forever.  Without current patch, that means the main loop hanged.
> >> > We'll
> >> > > > never find a way to talk to VM again.
> >> > > >
> >> > >
> >> > > Could the BQL be pushed down to the monitor commands level instead? That
> >> > > way we wouldn't need a seperate thread to solve the hang on commands that
> >> > > do not need BQL.
> >> >
> >> > If the main thread is stuck though I don't see how that helps you; you
> >> > have to be able to run these commands on another thread.
> >> >
> >>
> >> Why would the main thread be stuck? In (1) If the vcpu thread takes the BQL
> >> and the command doesn't need it, it would work.  In (2),  info cpus
> >> shouldn't keep the BQL (my qapi-async series would probably help here)
> >
> > (Thanks for joining the discussion)
> >
> > AFAIK the main thread can be stuck for many reasons.  I have seen one
> > stack when the VGA code (IIUC) was trying to writting to guest graphic
> > memory in main loop thread but luckily that guest page is still not
> > copied yet from source.  As long as the main thread is stuck for any
> > reason, no chance for monitor commands, even if the commands support
> > async operations.
> 
> If that command becomes async (it probably should, any command doing
> IO probaly should), then the main loop can keep running.

The problem is that, it's not blocked at "a command", but a task
running on the main thread.  The task can access guest memory, and
when the guest page is not there, the main thread hangs.  Then it
hangs every monitors, and all other tasks that are bounded to main
thread.

> 
> >
> > So IMHO the only solution is doing these things in separate threads,
> > rather than all in a single one.
> 
> I wouldn't say it's the only solution. I think the monitor can touch
> many areas that haven't been written with multi-threading in mind. My
> proposal is probably safer, although I don't know how hard it would be
> to push the BQL down to QMP commands, and make async existing IO
> commands. The benefits of this work are quite interesting imho,
> because a stuck mainloop is basically a stuck qemu, and an additional
> thread will not solve it...
> 
> -- 
> Marc-André Lureau

-- 
Peter Xu