Re: [PATCH v6 00/25] monitor: add asynchronous command type

["Marc-André Lureau" <marcandre.lureau@gmail.com>]

Hi

On Tue, Jan 7, 2020 at 9:17 AM Kevin Wolf <kwolf@redhat.com> wrote:
>
> Am 06.01.2020 um 19:21 hat Marc-André Lureau geschrieben:
> > > What my patch does is moving everything into a coroutine. This is wrong
> > > because not everything can be run in a coroutine, so it needs to be made
> > > optional (like you did with your async flag).
> >
> > "everything" is a bit too much ;) You proposal is to replace
> > qmp_dispatch_bh by a coroutine version (except for OOB commands). This
> > is nice because indeed, it allows to reenter the mainloop with a
> > simple yield in QMP commands. It is also simpler than my "async"
> > proposal, because some of the state is part of the coroutine, and
> > because it doesn't allow QMP commands concurrency (beside existing
> > OOB).
> >
> > Iow, coroutine (for async) + oob (for concurrency) make my proposal
> > kinda obsolete. I can only regret that a simple callback-based
> > solution looked simpler to me than one that mixes both threads &
> > coroutines, but I don't mind if everybody likes it better :) I can
> > definitely see the point for block commands, which rely on coroutines
> > anyway, and qemu is already that complex in general.
>
> Callbacks are indeed simple enough for implementing the infrastructure,
> but for the users they only look simple as long as they do trivial
> things. :-)
>
> Anyway, now that you have seen my POC hack, do you agree that this
> should help solving the screendump problem, too?

Yes, and I will work on it as soon as you have a working patch series
or branch :)

>
> > > The problem isn't with completely coroutine-unaware code, though: That
> > > one would just work, even if not taking advantage from the coroutine. A
> > > potential problem exists with code that behaves differently when run in
> > > a coroutine or outside of coroutine context (generally by checking
> > > qemu_in_coroutine())), or calls of coroutine-unaware code into such
> > > functions.
> > >
> > > Running some command handlers outside of coroutine context wouldn't be
> > > hard to add to my patch (basically just a BH), but I haven't looked into
> > > the QAPI side of making it an option.
> >
> > Yes, I think we should have a 'coroutine': true, for commands that
> > should be run with a coroutine.
> >
> > Or perhaps replace existing allow-oob with 'dispatch':
> > - 'bh' (default)
> > - 'coroutine'
> > - 'allow-oob' (oob + bh fallback, since oob don't have coroutine - at
> > this point)
>
> If it's "at this point", then making it two separate bools would make
> more sense. But I seem to remember that OOB handlers are fundamentally
> not supposed to block, so coroutine support would be pointless for them
> and an enum could work.

I think so too

>
> I'll defer to Markus on this one.

Yup, Markus should take a look at your proposal and give some
guidance. And hopefully, it won't take >2y.

>
> > Your patch looks quite good to me, but make check hangs. Have you
> > looked at it?
>
> I'm not sure if I was looking at some qemu-iotests cases or make check,
> but yes, I did see a hang. My case was a QMP command that just doesn't
> work correctly inside a coroutine without modifications, so requiring
> 'coroutine': true would fix it.

ok, then I suggest you do a "minimal" patch series that works.

thanks!

-- 
Marc-André Lureau