Re: [Qemu-devel] [PATCH] io/channel-command: Delay the killing of the child after closing the pipe

[Daniel P . Berrangé]

On Tue, Feb 13, 2018 at 03:09:12PM +0000, Dr. David Alan Gilbert wrote:
> * Thomas Huth (address@hidden) wrote:
> > We are currently facing some migration failure on s390x when running
> > certain avocado tests, e.g. when running the test
> > type_specific.io-github-autotest-qemu.migrate.with_reboot.exec.gzip_exec.
> > This test is using 'migrate -d "exec:nc localhost 5200"' for the migration.
> > The problem is detected at the receiving side, where the migration stream
> > apparently ends too early. However, the cause for the problem is the
> > sending side: After writing the migration stream into the pipe to netcat,
> > the source QEMU calls qio_channel_command_close() which closes the pipe
> > and immediately (!) kills the child process afterwards. So if the
> > sending netcat did not read the final bytes from the pipe yet, or
> > if it did not manage to send out all its buffers yet, it is killed
> > before the whole migration stream is passed to the destination side.
> 
> Thanks for tracking that down!
> 
> > To ease the situation at least a little bit, we should give the child
> > process at least some few more time slices before we kill it with
> > SIGTERM and then with SIGKILL. With this change, the avocado test now
> > succeeds here in 10 out of 10 runs.
> > 
> > Signed-off-by: Thomas Huth <address@hidden>
> > ---
> >  io/channel-command.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/io/channel-command.c b/io/channel-command.c
> > index 319c5ed..f64db3e 100644
> > --- a/io/channel-command.c
> > +++ b/io/channel-command.c
> > @@ -177,11 +177,11 @@ static int 
> > qio_channel_command_abort(QIOChannelCommand *ioc,
> >              return -1;
> >          }
> >      } else if (ret == 0) {
> > -        if (step == 0) {
> > +        if (step == 4) {
> >              kill(ioc->pid, SIGTERM);
> > -        } else if (step == 1) {
> > +        } else if (step == 8) {
> >              kill(ioc->pid, SIGKILL);
> > -        } else {
> > +        } else if (step >= 9) {
> 
> Hmm.  This seems pretty arbitrary; if I understand correctly you're
> saying it'll get a SIGTERM after 4 (arbitrary) * 10ms (arbitrary).
> 
> Who is to say that's enough for a scp or gzip or the like?

We could conceivably implement the  qio_channel_shutdown() operation
for the QIOChannelCommand class. It would merely close the FD to the
child process, but leave it running. That would give it time to read
any data still in the pipe from QEMU IIUC.

Regards,
Daniel
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