On Mon, Sep 18, 2017 at 06:09:29PM +0200, Marc-André Lureau wrote: > On Mon, Sep 18, 2017 at 1:26 PM, Dr. David Alan Gilbert > <dgilb...@redhat.com> wrote: > > * Marc-André Lureau (marcandre.lur...@gmail.com) wrote: > >> Hi > >> > >> On Mon, Sep 18, 2017 at 12:55 PM, Dr. David Alan Gilbert > >> <dgilb...@redhat.com> wrote: > >> > * Marc-André Lureau (marcandre.lur...@gmail.com) wrote: > >> >> Hi > >> >> > >> >> On Mon, Sep 18, 2017 at 10:37 AM, Peter Xu <pet...@redhat.com> wrote: > >> >> > On Fri, Sep 15, 2017 at 01:14:47PM +0200, Marc-André Lureau wrote: > >> >> >> Hi > >> >> >> > >> >> >> On Thu, Sep 14, 2017 at 9:46 PM, Peter Xu <pet...@redhat.com> wrote: > >> >> >> > On Thu, Sep 14, 2017 at 07:53:15PM +0100, Dr. David Alan Gilbert > >> >> >> > wrote: > >> >> >> >> * Marc-André Lureau (marcandre.lur...@gmail.com) wrote: > >> >> >> >> > Hi > >> >> >> >> > > >> >> >> >> > On Thu, Sep 14, 2017 at 9:50 AM, Peter Xu <pet...@redhat.com> > >> >> >> >> > wrote: > >> >> >> >> > > This series was born from this one: > >> >> >> >> > > > >> >> >> >> > > > >> >> >> >> > > https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg04310.html > >> >> >> >> > > > >> >> >> >> > > The design comes from Markus, and also the whole-bunch-of > >> >> >> >> > > discussions > >> >> >> >> > > in previous thread. My heartful thanks to Markus, Daniel, > >> >> >> >> > > Dave, > >> >> >> >> > > Stefan, etc. on discussing the topic (...again!), providing > >> >> >> >> > > shiny > >> >> >> >> > > ideas and suggestions. Finally we got such a solution that > >> >> >> >> > > seems to > >> >> >> >> > > satisfy everyone. > >> >> >> >> > > > >> >> >> >> > > I re-started the versioning since this series is totally > >> >> >> >> > > different > >> >> >> >> > > from previous one. Now it's version 1. > >> >> >> >> > > > >> >> >> >> > > In case new reviewers come along the way without reading > >> >> >> >> > > previous > >> >> >> >> > > discussions, I will try to do a summary on what this is all > >> >> >> >> > > about. > >> >> >> >> > > > >> >> >> >> > > What is OOB execution? > >> >> >> >> > > ====================== > >> >> >> >> > > > >> >> >> >> > > It's the shortcut of Out-Of-Band execution, its name is given > >> >> >> >> > > by > >> >> >> >> > > Markus. It's a way to quickly execute a QMP request. Say, > >> >> >> >> > > originally > >> >> >> >> > > QMP is going throw these steps: > >> >> >> >> > > > >> >> >> >> > > JSON Parser --> QMP Dispatcher --> Respond > >> >> >> >> > > /|\ (2) (3) | > >> >> >> >> > > (1) | \|/ (4) > >> >> >> >> > > +--------- main thread --------+ > >> >> >> >> > > > >> >> >> >> > > The requests are executed by the so-called QMP-dispatcher > >> >> >> >> > > after the > >> >> >> >> > > JSON is parsed. If OOB is on, we run the command directly in > >> >> >> >> > > the > >> >> >> >> > > parser and quickly returns. > >> >> >> >> > > >> >> >> >> > All commands should have the "id" field mandatory in this case, > >> >> >> >> > else > >> >> >> >> > the client will not distinguish the replies coming from the > >> >> >> >> > last/oob > >> >> >> >> > and the previous commands. > >> >> >> >> > > >> >> >> >> > This should probably be enforced upfront by client capability > >> >> >> >> > checks, > >> >> >> >> > more below. > >> >> >> > > >> >> >> > Hmm yes since the oob commands are actually running in async way, > >> >> >> > request ID should be needed here. However I'm not sure whether > >> >> >> > enabling the whole "request ID" thing is too big for this "try to > >> >> >> > be > >> >> >> > small" oob change... And IMHO it suites better to be part of the > >> >> >> > whole > >> >> >> > async work (no matter which implementation we'll use). > >> >> >> > > >> >> >> > How about this: we make "id" mandatory for "run-oob" requests only. > >> >> >> > For oob commands, they will always have ID then no ordering issue, > >> >> >> > and > >> >> >> > we can do it async; for the rest of non-oob commands, we still > >> >> >> > allow > >> >> >> > them to go without ID, and since they are not oob, they'll always > >> >> >> > be > >> >> >> > done in order as well. Would this work? > >> >> >> > >> >> >> This mixed-mode is imho more complicated to deal with than having the > >> >> >> protocol enforced one way or the other, but that should work. > >> >> >> > >> >> >> > > >> >> >> >> > > >> >> >> >> > > Yeah I know in current code the parser calls dispatcher > >> >> >> >> > > directly > >> >> >> >> > > (please see handle_qmp_command()). However it's not true > >> >> >> >> > > again after > >> >> >> >> > > this series (parser will has its own IO thread, and > >> >> >> >> > > dispatcher will > >> >> >> >> > > still be run in main thread). So this OOB does brings > >> >> >> >> > > something > >> >> >> >> > > different. > >> >> >> >> > > > >> >> >> >> > > There are more details on why OOB and the > >> >> >> >> > > difference/relationship > >> >> >> >> > > between OOB, async QMP, block/general jobs, etc.. but IMHO > >> >> >> >> > > that's > >> >> >> >> > > slightly out of topic (and believe me, it's not easy for me to > >> >> >> >> > > summarize that). For more information, please refers to [1]. > >> >> >> >> > > > >> >> >> >> > > Summary ends here. > >> >> >> >> > > > >> >> >> >> > > Some Implementation Details > >> >> >> >> > > =========================== > >> >> >> >> > > > >> >> >> >> > > Again, I mentioned that the old QMP workflow is this: > >> >> >> >> > > > >> >> >> >> > > JSON Parser --> QMP Dispatcher --> Respond > >> >> >> >> > > /|\ (2) (3) | > >> >> >> >> > > (1) | \|/ (4) > >> >> >> >> > > +--------- main thread --------+ > >> >> >> >> > > > >> >> >> >> > > What this series does is, firstly: > >> >> >> >> > > > >> >> >> >> > > JSON Parser QMP Dispatcher --> Respond > >> >> >> >> > > /|\ | /|\ (4) | > >> >> >> >> > > | | (2) | (3) | (5) > >> >> >> >> > > (1) | +-----> | \|/ > >> >> >> >> > > +--------- main thread <-------+ > >> >> >> >> > > > >> >> >> >> > > And further: > >> >> >> >> > > > >> >> >> >> > > queue/kick > >> >> >> >> > > JSON Parser ======> QMP Dispatcher --> Respond > >> >> >> >> > > /|\ | (3) /|\ (4) | > >> >> >> >> > > (1) | | (2) | | (5) > >> >> >> >> > > | \|/ | \|/ > >> >> >> >> > > IO thread main thread <-------+ > >> >> >> >> > > >> >> >> >> > Is the queue per monitor or per client? > >> >> >> > > >> >> >> > The queue is currently global. I think yes maybe at least we can > >> >> >> > do it > >> >> >> > per monitor, but I am not sure whether that is urgent or can be > >> >> >> > postponed. After all now QMPRequest (please refer to patch 11) is > >> >> >> > defined as (mon, id, req) tuple, so at least "id" namespace is > >> >> >> > per-monitor. > >> >> >> > > >> >> >> >> > And is the dispatching going > >> >> >> >> > to be processed even if the client is disconnected, and are new > >> >> >> >> > clients going to receive the replies from previous clients > >> >> >> >> > commands? > >> >> >> > > >> >> >> > [1] > >> >> >> > > >> >> >> > (will discuss together below) > >> >> >> > > >> >> >> >> > I > >> >> >> >> > believe there should be a per-client context, so there won't be > >> >> >> >> > "id" > >> >> >> >> > request conflicts. > >> >> >> > > >> >> >> > I'd say I am not familiar with this "client" idea, since after all > >> >> >> > IMHO one monitor is currently designed to mostly work with a single > >> >> >> > client. Say, unix sockets, telnet, all these backends are only > >> >> >> > single > >> >> >> > channeled, and one monitor instance can only work with one client > >> >> >> > at a > >> >> >> > time. Then do we really need to add this client layer upon it? > >> >> >> > IMHO > >> >> >> > the user can just provide more monitors if they wants more clients > >> >> >> > (and at least these clients should know the existance of the > >> >> >> > others or > >> >> >> > there might be problem, otherwise user2 will fail a migration, > >> >> >> > finally > >> >> >> > noticed that user1 has already triggered one), and the user should > >> >> >> > manage them well. > >> >> >> > >> >> >> qemu should support a management layer / libvirt restart/reconnect. > >> >> >> Afaik, it mostly work today. There might be a cases where libvirt can > >> >> >> be confused if it receives a reply from a previous connection > >> >> >> command, > >> >> >> but due to the sync processing of the chardev, I am not sure you can > >> >> >> get in this situation. By adding "oob" commands and queuing, the > >> >> >> client will have to remember which was the last "id" used, or it will > >> >> >> create more conflict after a reconnect. > >> >> >> > >> >> >> Imho we should introduce the client/connection concept to avoid this > >> >> >> confusion (unexpected reply & per client id space). > >> >> > > >> >> > Hmm I agree that the reconnect feature would be nice, but if so IMHO > >> >> > instead of throwing responses away when client disconnect, we should > >> >> > really keep them, and when the client reconnects, we queue the > >> >> > responses again. > >> >> > > >> >> > I think we have other quite simple ways to solve the "unexpected > >> >> > reply" and "per-client-id duplication" issues you have mentioned. > >> >> > > >> >> > Firstly, when client gets unexpected replies ("id" field not in its > >> >> > own request queue), the client should just ignore that reply, which > >> >> > seems natural to me. > >> >> > >> >> The trouble is that it may legitimately use the same "id" value for > >> >> new requests. And I don't see a simple way to handle that without > >> >> races. > >> > > >> > Under what circumstances can it reuse the same ID for new requests? > >> > Can't we simply tell it not to? > >> > >> I don't see any restriction today in the protocol in connecting with a > >> new client that may not know anything from a previous client. > > > > Well, it knows it's doing a reconnection. > > If you assume the "same client" reconnects to the monitor, I agree. > But this is a restriction of monitor usage.
In monitor_qmp_event(), we can empty the request queue when got CHR_EVENT_CLOSED. Would that be a solution? -- Peter Xu
