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 03:41:45PM +0000, Dr. David Alan Gilbert wrote:
> * Daniel P. Berrangé (address@hidden) wrote:
> > On Tue, Feb 13, 2018 at 03:25:30PM +0000, Dr. David Alan Gilbert wrote:
> > > * Daniel P. Berrangé (address@hidden) wrote:
> > > > 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.
> > > 
> > > Yeh that's better; although when would we call shutdown or close on it?
> > 
> > Doesn't QEMU alredy use  shutdown() during the right part of migration,
> > or is that only wrt post-copy ?
> 
> We only use it for cancel and errors, not during the normal behaviour.

So we could do with shutdown() for sake of post-copy anyway, but for
normal behaviour maybe the right answer is for close() to just wait a
real long time for the child app to exit ?  If we close the pipes, and
then wait 5 seconds or more before giving up ?

Regards,
Daniel
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