Re: [PATCH] tests/avocado/reverse_debugging: Disable the ppc64 tests by default

[Daniel P . Berrangé]

On Thu, Nov 16, 2023 at 05:14:43PM +1000, Nicholas Piggin wrote:
> On Thu Nov 16, 2023 at 1:55 PM AEST, Ani Sinha wrote:
> >
> >
> > > On 16-Nov-2023, at 6:45 AM, Nicholas Piggin <npiggin@gmail.com> wrote:
> > > 
> > > On Thu Nov 16, 2023 at 3:22 AM AEST, Daniel P. Berrangé wrote:
> > >> On Wed, Nov 15, 2023 at 01:14:53PM +0000, Daniel P. Berrangé wrote:
> > >>> On Wed, Nov 15, 2023 at 07:23:01AM +0100, Thomas Huth wrote:
> > >>>> On 15/11/2023 02.15, Nicholas Piggin wrote:
> > >>>>> On Wed Nov 15, 2023 at 4:29 AM AEST, Thomas Huth wrote:
> > >>>>>> On 14/11/2023 17.37, Philippe Mathieu-Daudé wrote:
> > >>>>>>> On 14/11/23 17:31, Thomas Huth wrote:
> > >>>>>>>> The tests seem currently to be broken. Disable them by default
> > >>>>>>>> until someone fixes them.
> > >>>>>>>> 
> > >>>>>>>> Signed-off-by: Thomas Huth <thuth@redhat.com>
> > >>>>>>>> ---
> > >>>>>>>>   tests/avocado/reverse_debugging.py | 7 ++++---
> > >>>>>>>>   1 file changed, 4 insertions(+), 3 deletions(-)
> > >>>>>>> 
> > >>>>>>> Similarly, I suspect 
> > >>>>>>> https://gitlab.com/qemu-project/qemu/-/issues/1961
> > >>>>>>> which has a fix ready:
> > >>>>>>> https://lore.kernel.org/qemu-devel/20231110170831.185001-1-richard.henderson@linaro.org/
> > >>>>>>> 
> > >>>>>>> Maybe wait the fix gets in first?
> > >>>>>> 
> > >>>>>> No, I applied Richard's patch, but the problem persists. Does this 
> > >>>>>> test
> > >>>>>> still work for you?
> > >>>>> 
> > >>>>> I bisected it to 1d4796cd008373 ("python/machine: use socketpair() for
> > >>>>> console connections"),
> > >>>> 
> > >>>> Maybe John (who wrote that commit) can help?
> > >>> 
> > >>> I find it hard to believe this commit is a direct root cause of the
> > >>> problem since all it does is change the QEMU startup sequence so that
> > >>> instead of QEMU listening for a monitor connection, it is given a
> > >>> pre-opened monitor connection.
> > >>> 
> > >>> At the very most that should affect the startup timing a little.
> > >>> 
> > >>> I notice all the reverse debugging tests have a skip on gitlab
> > >>> with a comment:
> > >>> 
> > >>>    # unidentified gitlab timeout problem
> > >>> 
> > >>> this makes be suspicious that John's patch has merely made this
> > >>> (henceforth undiagnosed) timeout more likely to ocurr.
> > >> 
> > >> After an absolutely horrendous hours long debugging session I think
> > >> I figured out the problem. The QEMU process is blocking in
> > >> 
> > >>    qemu_chr_write_buffer
> > >> 
> > >> spinning in the loop on EAGAIN.
> > > 
> > > Great work.
> > > 
> > > Why does this make the gdb socket give an empty response? Something
> > > just times out?
> > > 
> > >> 
> > >> The Python  Machine() class has passed one of a pre-created socketpair
> > >> FDs for the serial port chardev. The guest is trying to write to this
> > >> and blocking.  Nothing in the Machine() class is reading from the
> > >> other end of the serial port console.
> > >> 
> > >> 
> > >> Before John's change, the serial port uses a chardev in server mode
> > >> and crucially  'wait=off', and the Machine() class never opened the
> > >> console socket unless the test case wanted to read from it.
> > >> 
> > >> IOW, QEMU had a background job setting there waiting for a connection
> > >> that would never come.
> > >> 
> > >> As a result when QEMU started executing the guest, all the serial port
> > >> writes get sent into to the void.
> > >> 
> > >> 
> > >> So John's patch has had a semantic change in behaviour, because the
> > >> console socket is permanently open, and thus socket buffers are liable
> > >> to fill up.
> > >> 
> > >> As a demo I increased the socket buffers to 1MB and everything then
> > >> succeeded.
> > >> 
> > >> @@ -357,6 +360,10 @@ def _pre_launch(self) -> None:
> > >> 
> > >>         if self._console_set:
> > >>             self._cons_sock_pair = socket.socketpair()
> > >> +            self._cons_sock_pair[0].setsockopt(socket.SOL_SOCKET, 
> > >> socket.SO_SNDBUF, 1024*1024);
> > >> +            self._cons_sock_pair[0].setsockopt(socket.SOL_SOCKET, 
> > >> socket.SO_RCVBUF, 1024*1024);
> > >> +            self._cons_sock_pair[1].setsockopt(socket.SOL_SOCKET, 
> > >> socket.SO_SNDBUF, 1024*1024);
> > >> +            self._cons_sock_pair[1].setsockopt(socket.SOL_SOCKET, 
> > >> socket.SO_RCVBUF, 1024*1024);
> > >>             os.set_inheritable(self._cons_sock_pair[0].fileno(), True)
> > >> 
> > >>         # NOTE: Make sure any opened resources are *definitely* freed in
> > > 
> > > So perhaps ppc64 fails just because it prints more to the console in early
> > > boot than other targets?
> > > 
> > >> The Machine class doesn't know if anything will ever use the console,
> > >> so as is the change is unsafe.
> > >> 
> > >> The original goal of John's change was to guarantee we capture early
> > >> boot messages as some test need that.  
> > >> 
> > >> I think we need to be able to have a flag to say whether the caller needs
> > >> an "early console" facility, and only use the pre-opened FD passing for
> > >> that case. Tests we need early console will have to ask for that 
> > >> guarantee
> > >> explicitly.
> > > 
> > > The below patch makes this test work. Maybe as a quick fix it is
> > > better than disabling the test.
> > > 
> > > I guess we still have a problem if a test invokes vm.launch()
> > > directly without subsequently waiting for a console pattern or
> > > doing something with the console as you say. Your suggesstion is
> > > add something like vm.launch(console=True) ? 
> >
> > I think what he is saying is to add a new property for QEMUMachine() with 
> > which the test can explicitly tell the machine init code that it is going 
> > to drain the console logs. By default it can be false. When tests use 
> > console_drainer, they can set the property to true and inspect the early 
> > console logs after draining it. 
> 
> Hmm... well we do have QEMUMachine.set_console already. Is this enough?
> If the test case is not going to drain or interact with the console
> then it could set it to false. Or am I missing something?

Yeah, set_console is enough - i missed that that exists.

Thus problem is more specific. It hits when a test calls
set_console(True), but then fails to read from the console.


With regards,
Daniel
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