Re: [PATCH 0/3] monitor: only run coroutine commands in qemu_aio_context

[YangHang Liu]

It's easily for me to encounter " ../block/qcow2.c:5263:
ImageInfoSpecific *qcow2_get_specific_info(BlockDriverState *, Error
**): Assertion `false' failed" issue during 1Q vhost-user interface +
RT VM + post-copy migration

After applying this patch, the issue is still not reproduced even if I
repeat the same migration test for 60 times.

Tested-by: Yanghang Liu <yanghliu@redhat.com>


Best Regards,
YangHang Liu

On Mon, Jan 29, 2024 at 7:39 PM Peter Maydell <peter.maydell@linaro.org> wrote:
>
> On Tue, 16 Jan 2024 at 19:01, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >
> > Several bugs have been reported related to how QMP commands are rescheduled 
> > in
> > qemu_aio_context:
> > - https://gitlab.com/qemu-project/qemu/-/issues/1933
> > - https://issues.redhat.com/browse/RHEL-17369
> > - https://bugzilla.redhat.com/show_bug.cgi?id=2215192
> > - https://bugzilla.redhat.com/show_bug.cgi?id=2214985
> >
> > The first instance of the bug interacted with drain_call_rcu() temporarily
> > dropping the BQL and resulted in vCPU threads entering device emulation code
> > simultaneously (something that should never happen). I set out to make
> > drain_call_rcu() safe to use in this environment, but Paolo and Kevin 
> > discussed
> > the possibility of avoiding rescheduling the monitor_qmp_dispatcher_co()
> > coroutine for non-coroutine commands. This would prevent monitor commands 
> > from
> > running during vCPU thread aio_poll() entirely and addresses the root cause.
> >
> > This patch series implements this idea. qemu-iotests is sensitive to the 
> > exact
> > order in which QMP events and responses are emitted. Running QMP handlers in
> > the iohandler AioContext causes some QMP events to be ordered differently 
> > than
> > before. It is therefore necessary to adjust the reference output in many 
> > test
> > cases. The actual QMP code change is small and everything else is just to 
> > make
> > qemu-iotests happy.
>
> Hi; we have a suspicion that this change has resulted in a flaky-CI
> test: iotest-144 sometimes fails, apparently because a "return"
> result from QMP isn't always returned at the same place in relation
> to other QMP events. Could you have a look at it?
>
> https://gitlab.com/qemu-project/qemu/-/issues/2126
>
> thanks
> -- PMM
>