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

[John Snow]

On Wed, Nov 15, 2023 at 12:23 PM Daniel P. Berrangé <berrange@redhat.com> 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.
>
> 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
>
>
> 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.

Tch. I see. Thank you for diagnosing this.

>From the machine.py perspective, you have to *opt in* to having a
console, so I hadn't considered that a caller would enable the console
and then ... not read from it. Surely that's a bug in the caller?

If you don't intend to read from the console, you shouldn't call set_console().

(The async rewrite I have been toying with on and off has a built-in
drainer that writes to a log file that would probably remedy this, but
the client tests should still be fixed, I think. Otherwise, do you
have any suggestions for how I might make this failure state more
obvious/friendly? I wonder if on close of the machine.py object I
could detect that the pipe is full and emit a warning about that.)

--js

>
> With regards,
> Daniel
> --
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