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

[Stefan Hajnoczi <stefanha@gmail.com>]

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.

Stefan