* Daniel P. Berrange (berra...@redhat.com) wrote: > On Wed, Sep 06, 2017 at 11:57:05AM +0100, Dr. David Alan Gilbert wrote: > > * Daniel P. Berrange (berra...@redhat.com) wrote: > > > On Wed, Sep 06, 2017 at 11:48:51AM +0100, Dr. David Alan Gilbert wrote: > > > > * Daniel P. Berrange (berra...@redhat.com) wrote: > > > > > On Wed, Sep 06, 2017 at 10:48:46AM +0100, Dr. David Alan Gilbert > > > > > wrote: > > > > > > * Daniel P. Berrange (berra...@redhat.com) wrote: > > > > > > > On Wed, Aug 23, 2017 at 02:51:03PM +0800, Peter Xu wrote: > > > > > > > > v2: > > > > > > > > - fixed "make check" error that patchew reported > > > > > > > > - moved the thread_join upper in monitor_data_destroy(), before > > > > > > > > resources are released > > > > > > > > - added one new patch (current patch 3) that fixes a nasty risk > > > > > > > > condition with IOWatchPoll. Please see commit message for > > > > > > > > more > > > > > > > > information. > > > > > > > > - added a g_main_context_wakeup() to make sure the separate loop > > > > > > > > thread can be kicked always when we want to destroy the > > > > > > > > per-monitor > > > > > > > > threads. > > > > > > > > - added one new patch (current patch 8) to introduce migration > > > > > > > > mgmt > > > > > > > > lock for migrate_incoming. > > > > > > > > > > > > > > > > This is an extended work for migration postcopy recovery. This > > > > > > > > series > > > > > > > > is tested with the following series to make sure it solves the > > > > > > > > monitor > > > > > > > > hang problem that we have encountered for postcopy recovery: > > > > > > > > > > > > > > > > [RFC 00/29] Migration: postcopy failure recovery > > > > > > > > [RFC 0/6] migration: re-use migrate_incoming for postcopy > > > > > > > > recovery > > > > > > > > > > > > > > > > The root problem is that, monitor commands are all handled in > > > > > > > > main > > > > > > > > loop thread now, no matter how many monitors we specify. And, > > > > > > > > if main > > > > > > > > loop thread hangs due to some reason, all monitors will be > > > > > > > > stuck. > > > > > > > > This can be done in reversed order as well: if any of the > > > > > > > > monitor > > > > > > > > hangs, it will hang the main loop, and the rest of the monitors > > > > > > > > (if > > > > > > > > there is any). > > > > > > > > > > > > > > > > That affects postcopy recovery, since the recovery requires > > > > > > > > user input > > > > > > > > on destination side. If monitors hang, the destination VM dies > > > > > > > > and > > > > > > > > lose hope for even a final recovery. > > > > > > > > > > > > > > > > So, sometimes we need to make sure the monitor be alive, at > > > > > > > > least one > > > > > > > > of them. > > > > > > > > > > > > > > > > The whole idea of this series is that instead if handling > > > > > > > > monitor > > > > > > > > commands all in main loop thread, we do it separately in > > > > > > > > per-monitor > > > > > > > > threads. Then, even if main loop thread hangs at any point by > > > > > > > > any > > > > > > > > reason, per-monitor thread can still survive. Further, we add > > > > > > > > hint in > > > > > > > > QMP/HMP to show whether a command can be executed without QMP, > > > > > > > > if so, > > > > > > > > we avoid taking BQL when running that command. It greatly > > > > > > > > reduced > > > > > > > > contention of BQL. Now the only user of that new parameter > > > > > > > > (currently > > > > > > > > I call it "without-bql") is "migrate-incoming" command, which > > > > > > > > is the > > > > > > > > only command to rescue a paused postcopy migration. > > > > > > > > > > > > > > > > However, even with the series, it does not mean that per-monitor > > > > > > > > threads will never hang. One example is that we can still run > > > > > > > > "info > > > > > > > > vcpus" in per-monitor threads during a paused postcopy (in that > > > > > > > > state, > > > > > > > > page faults are never handled, and "info cpus" will never > > > > > > > > return since > > > > > > > > it tries to sync every vcpus). So to make sure it does not > > > > > > > > hang, we > > > > > > > > not only need the per-monitor thread, the user should be > > > > > > > > careful as > > > > > > > > well on how to use it. > > > > > > > > > > > > > > > > For postcopy recovery, we may need dedicated monitor channel for > > > > > > > > recovery. In other words, a destination VM that supports > > > > > > > > postcopy > > > > > > > > recovery would possibly need: > > > > > > > > > > > > > > > > -qmp MAIN_CHANNEL -qmp RECOVERY_CHANNEL > > > > > > > > > > > > > > I think this is a really horrible thing to expose to management > > > > > > > applications. > > > > > > > They should not need to be aware of fact that QEMU is buggy and > > > > > > > thus requires > > > > > > > that certain commands be run on different monitors to work around > > > > > > > the bug. > > > > > > > > > > > > It's unfortunately baked in way too deep to fix in the near term; > > > > > > the > > > > > > BQL is just too cantagious and we have a fundamental design of > > > > > > running > > > > > > all the main IO emulation in one thread. > > > > > > > > > > > > > I'd much prefer to see the problem described handled > > > > > > > transparently inside > > > > > > > QEMU. One approach is have a dedicated thread in QEMU responsible > > > > > > > for all > > > > > > > monitor I/O. This thread should never actually execute monitor > > > > > > > commands > > > > > > > though, it would simply parse the command request and put data > > > > > > > onto a queue > > > > > > > of pending commands, thus it could never hang. The command queue > > > > > > > could be > > > > > > > processed by the main thread, or by another thread that is > > > > > > > interested. > > > > > > > eg the migration thread could process any queued commands related > > > > > > > to > > > > > > > migration directly. > > > > > > > > > > > > That requires a change in the current API to allow async command > > > > > > completion (OK that is something Marc-Andre's world has) so that > > > > > > from the one connection you can have multiple outstanding commands. > > > > > > Hmm unless.... > > > > > > > > > > > > We've also got problems that some commands don't like being run > > > > > > outside > > > > > > of the main thread (see Fam's reply on the 21st pointing out that a > > > > > > lot > > > > > > of block commands would assert). > > > > > > > > > > > > I think the way to move to what you describe would be: > > > > > > a) A separate thread for monitor IO > > > > > > This seems a separate problem > > > > > > How hard is that? Will all the current IO mechanisms used > > > > > > for monitors just work if we run them in a separate thread? > > > > > > What about mux? > > > > > > > > > > > > b) Initially all commands get dispatched to the main thread > > > > > > so nothing changes about the API. > > > > > > > > > > > > c) We create a new thread for the lock-free commands, and route > > > > > > lock-free commands down it. > > > > > > > > > > > > d) We start with a rule that on any one monitor connection we > > > > > > don't allow you to start a command until the previous one has > > > > > > finished > > > > > > > > > > > > (d) allows us to avoid any API changes, but allows us to do > > > > > > lock-free > > > > > > stuff on a separate connection like Peter's world. > > > > > > We can drop (d) once we have a way of doing async commands. > > > > > > We can add dispatching to more threads once someone describes > > > > > > what they want from those threads. > > > > > > > > > > > > Does that work for you Dan? > > > > > > > > > > It would *provided* that we do (c) for the commands Peter wants for > > > > > this migration series. IOW, I don't want to have to have logic in > > > > > libvirt that either needs to add a 2nd monitor server, or open a 2nd > > > > > monitor connection, to deal with migration post-copy recovery in some > > > > > versions of QEMU. So whatever is needed to make post-copy recovery > > > > > work has to be done for (c). > > > > > > > > But then doesn't that mean you're requiring us to break (d) and change > > > > the QMP interface to libvirt so it can do async stuff? > > > > > > Depends on your definition of break - I'm assuming there's either a way > > > to opt-in to use of a async mode for existing commands in (c), or that > > > async commands would be added in parallel with existing sync commands. > > > IOW, its not a API breakage - its an opt-in extension of existing > > > functionality. > > > > But you'd need to do async commands for all commands you issued to avoid > > blocking the io thread so that you could then issue the recovery > > commands. > > I don't see why that has to be the case. In order to issue an async command > all that needs to be the case is that command replies should be allowed to > be sent out of order. > > IOW if command A is blocking and command B is async, then we shoudl be > allowed to have the following > > req A > req B > res A > res B > > Or > > req A > req B > res B > res A > > Or > > req B > req A > res B > res A > > etc. > > This does imply that you need a separate monitor I/O processing, from the > command execution thread, but I see no need for all commands to suddenly > become async. Just allowing interleaved replies is sufficient from the > POV of the protocol definition. This interleaving is easy to handle from > the client POV - just requires a unique 'serial' in the request by the > client, that is copied into the reply by QEMU.
OK, so for that we can just take Marc-André's syntax and call it 'id': https://lists.gnu.org/archive/html/qemu-devel/2017-01/msg03634.html then it's upto the caller to ensure those id's are unique. I do worry about two things: a) With this the caller doesn't really know which commands could be in parallel - for example if we've got a recovery command that's executed by this non-locking thread that's OK, we expect that to be doable in parallel. If in the future though we do what you initially suggested and have a bunch of commands get routed to the migration thread (say) then those would suddenly operate in parallel with other commands that we're previously synchronous. b) I still worry how the various IO channels will behave on another thread. But that's more a general feeling rather than anything specific. Dave > Regards, > Daniel > -- > |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| > |: https://libvirt.org -o- https://fstop138.berrange.com :| > |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :| -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
