Re: [Qemu-devel] test-filter-mirror hangs

[Peter Xu <peterx@redhat.com>]

On Thu, Jan 24, 2019 at 09:11:15AM +0000, Dr. David Alan Gilbert wrote:
> * Jason Wang (jasowang@redhat.com) wrote:
> > 
> > On 2019/1/24 上午3:53, Dr. David Alan Gilbert wrote:
> > > * Jason Wang (jasowang@redhat.com) wrote:
> > > > On 2019/1/22 上午2:56, Peter Maydell wrote:
> > > > > On Thu, 17 Jan 2019 at 09:46, Jason Wang<jasowang@redhat.com>  wrote:
> > > > > > On 2019/1/15 上午12:33, Zhang Chen wrote:
> > > > > > > On Sat, Jan 12, 2019 at 12:15 AM Dr. David Alan Gilbert
> > > > > > > <dgilbert@redhat.com  <mailto:dgilbert@redhat.com>> wrote:
> > > > > > > 
> > > > > > >       * Peter Maydell (peter.maydell@linaro.org
> > > > > > >       <mailto:peter.maydell@linaro.org>) wrote:
> > > > > > >       > Recently I've noticed that test-filter-mirror has been hanging
> > > > > > >       > intermittently, typically when run on some other TCG architecture.
> > > > > > >       > In the instance I've just looked at, this was with s390x guest on
> > > > > > >       > x86-64 host, though I've also seen it on other host archs and
> > > > > > >       > perhaps with other guests.
> > > > > > > 
> > > > > > >       Watch out to see if you really do see it for other guests;
> > > > > > >       it carefully avoids using virtio-net to avoid vhost; but on s390x it
> > > > > > >       uses virtio-net-ccw - could that hit the vhost it was trying to avoid?
> > > > > > > 
> > > > > > >       > Below is a backtrace, though it seems to be pretty unhelpful.
> > > > > > >       > Anybody got any theories ? Does the mirror test rely on dirty
> > > > > > >       > memory bitmaps like the migration test (which also hangs
> > > > > > >       > occasionally with TCG due to some bug I'm sure we've investigated
> > > > > > >       > in the past) ?
> > > > > > > 
> > > > > > >       I don't think it relies on the CPU at all.
> > > > > > >    I have no idea about this currently, but Jason and I designed the
> > > > > > > test case.
> > > > > > > Add Jason: Have any comments about this ?
> > > > > > I can't reproduce this locally with s390x-softmmu. It looks to me the
> > > > > > test should be independent to any kinds of emulation. It should pass
> > > > > > when mainloop work.
> > > > > I've just seen a hang with ppc64 guest on s390x host, so it is
> > > > > indeed not specific to s390x guest (and so not specific to
> > > > > virtio-net either, since the ppc64 guest setup uses e1000).
> > > > > 
> > > > > thanks
> > > > > -- PMM
> > > > Finally reproduced locally after hundreds (sometimes thousands) times of
> > > > running.
> > > > 
> > > > Bisection points to OOB monitor[1].
> > > > 
> > > > It looks to me after OOB is used unconditionally we lose a barrier to make
> > > > sure socket is connected before sending packets in test-filter-mirror.c. Is
> > > > there any other similar and simple thing that we could do to kick the
> > > > mainloop?
> > > Do you mean the:
> > > 
> > >      /* send a qmp command to guarantee that 'connected' is setting to true. */
> > >      qmp_discard_response(qts, "{ 'execute' : 'query-status'}");
> > 
> > 
> > Yes.
> > 
> > 
> > > 
> > > why was that ever sufficient to know the socket was ready?
> > 
> > 
> > It was suggested by Fam, I don't remember the details. Can we make sure all
> > pending events has been processed (UNIX socket was set to connected) after
> > query-status is returned with an non OOB monitor?
> 
> I'm not sure - it doesn't sound like a 'query-status' should ensure
> anything else.
> How about something like a 'query-chardev' - can that tell you what you
> need and loop until it's ready?

Yeah it sounds hacky to use "query status" to make sure a specific
chardev is connected even before the OOB...

I saw that currently the chardev requires "nowait":

    qts = qtest_initf(
        "-netdev socket,id=qtest-bn0,fd=%d "
        "-device %s,netdev=qtest-bn0,id=qtest-e0 "
        "-chardev socket,id=mirror0,path=%s,server,nowait "
        "-object filter-mirror,id=qtest-f0,netdev=qtest-bn0,queue=tx,outdev=mirror0 "
        , send_sock[1], devstr, sock_path);

Could it work without "nowait"?  Would that make sure QEMU will wait
until connection established before going on?

Regards,

-- 
Peter Xu