Re: [Qemu-devel] [RFC v2 0/8] monitor: allow per-monitor thread

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Stefan Hajnoczi (stefanha@gmail.com) wrote:
> On Thu, Sep 7, 2017 at 1:02 PM, Peter Xu <peterx@redhat.com> wrote:
> > On Thu, Sep 07, 2017 at 11:09:29AM +0100, Stefan Hajnoczi wrote:
> >> On Thu, Sep 7, 2017 at 10:35 AM, Dr. David Alan Gilbert
> >> <dgilbert@redhat.com> wrote:
> >> > * Stefan Hajnoczi (stefanha@gmail.com) wrote:
> >> >> On Wed, Sep 6, 2017 at 4:14 PM, Dr. David Alan Gilbert
> >> >> <dgilbert@redhat.com> wrote:
> >> >> > * Stefan Hajnoczi (stefanha@gmail.com) wrote:
> >> >> >> On Wed, Aug 23, 2017 at 02:51:03PM +0800, Peter Xu wrote:
> >> >> >> > 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.
> >> >> >>
> >> >> >> I see a larger issue with postcopy: existing QEMU code assumes that
> >> >> >> guest memory access is instantaneous.
> >> >> >>
> >> >> >> Postcopy breaks this assumption and introduces blocking points that can
> >> >> >> now take unbounded time.
> >> >> >>
> >> >> >> This problem isn't specific to the monitor.  It can also happen to other
> >> >> >> components in QEMU like the gdbstub.
> >> >> >>
> >> >> >> Do we need an asynchronous memory API?  Synchronous memory access should
> >> >> >> only be allowed in vcpu threads.
> >> >> >
> >> >> > It would probably be useful for gdbstub where the overhead of async
> >> >> > doesn't matter;  but doing that for all IO emulation is hard.
> >> >>
> >> >> Why is it hard?
> >> >>
> >> >> Memory access can be synchronous in the vcpu thread.  That eliminates
> >> >> a lot of code straight away.
> >> >>
> >> >> Anything using dma-helpers.c is already async.  They just don't know
> >> >> that the memory access part is being made async too :).
> >> >
> >> > Can you point me to some info on that ?
> >>
> >> IDE and SCSI use dma-helpers.c to perform I/O:
> >> hw/ide/core.c:892:        s->bus->dma->aiocb =
> >> dma_blk_io(blk_get_aio_context(s->blk),
> >> hw/ide/macio.c:189:        s->bus->dma->aiocb =
> >> dma_blk_io(blk_get_aio_context(s->blk), &s->sg,
> >> hw/scsi/scsi-disk.c:348:        r->req.aiocb =
> >> dma_blk_io(blk_get_aio_context(s->qdev.conf.blk),
> >> hw/scsi/scsi-disk.c:551:        r->req.aiocb =
> >> dma_blk_io(blk_get_aio_context(s->qdev.conf.blk),
> >>
> >> They pass a scatter-gather list of guest RAM addresses to
> >> dma-helpers.c.  They receive a callback when I/O has finished.
> >>
> >> Try following the code path.  Request submission may be from a vcpu
> >> thread or IOThread.  Completion occurs in the main loop or an
> >> IOThread.
> >>
> >> The main point is that this API is already asynchronous.  If any
> >> changes are needed for async guest memory access (not sure, I haven't
> >> checked), then at least the dma-helpers.c users do not need to be
> >> modified.
> >>
> >> >> The remaining cases are virtio and some other devices.
> >> >>
> >> >> If you are worried about performance, the first rule is that async
> >> >> memory access is only needed on the destination side when post-copy is
> >> >> active.  Maybe use setjmp to return from the signal handler and queue
> >> >> a callback for when the page has been loaded.
> >> >
> >> > I'm not sure it's worth trying to be too clever at avoiding this;
> >> > I see the fact that we're doing IO with the bql held as a more
> >> > fundamental problem.
> >>
> >> QEMU should be doing I/O syscalls in async fashion or threadpool
> >> workers (no BQL) so the BQL is not an issue.  Anything else could
> >> cause unbounded waits even without postcopy.
> >
> > E.g. when vcpu got page faulted with BQL taken, while the main thread
> > needs the BQL to dispatch anything, including monitor commands.
> >
> > So I think it's a multiplex problem - we need to solve both (1) main
> > thread accessing guest memories which is still missing, and (2) BQL
> > deadlocks between vcpu threads and main thread.
> 
> I think we need a single solution and cannot treat these as separate.
> This is because the same virtio device emulation code may run in 3
> contexts:
> 1. vcpu thread (ioeventfd=off)
> 2. main loop thread (ioeventfd=on)
> 3. IOThread (ioeventfd=on, iothread=<id>)
> 
> If you try to solve them separately then the code won't work in all 3
> contexts anymore.

I think you can also get main loop thread hangs on things like
network packet reception.

Dave

> 
> Stefan
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK