Re: [PATCH 1/2] multifd: use qemu_sem_timedwait in multifd_recv_thread to avoid waiting forever

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Mon, Nov 29, 2021 at 11:20:08AM +0000, Dr. David Alan Gilbert wrote:
> > * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > > On Fri, Nov 26, 2021 at 04:31:53PM +0100, Li Zhang wrote:
> > > > When doing live migration with multifd channels 8, 16 or larger number,
> > > > the guest hangs in the presence of the network errors such as missing 
> > > > TCP ACKs.
> > > > 
> > > > At sender's side:
> > > > The main thread is blocked on qemu_thread_join, migration_fd_cleanup
> > > > is called because one thread fails on qio_channel_write_all when
> > > > the network problem happens and other send threads are blocked on 
> > > > sendmsg.
> > > > They could not be terminated. So the main thread is blocked on 
> > > > qemu_thread_join
> > > > to wait for the threads terminated.
> > > 
> > > Isn't the right answer here to ensure we've called 'shutdown' on
> > > all the FDs, so that the threads get kicked out of sendmsg, before
> > > trying to join the thread ?
> > 
> > I agree a timeout is wrong here; there is no way to get a good timeout
> > value.
> > However, I'm a bit confused - we should be able to try a shutdown on the
> > receive side using the 'yank' command. - that's what it's there for; Li
> > does this solve your problem?
> 
> Why do we even need to use 'yank' on the receive side ? Until migration
> has switched over from src to dst, the receive side is discardable and
> the whole process can just be teminated with kill(SIGTERM/SIGKILL).

True, although it's nice to be able to quit cleanly.

> On the source side 'yank' is needed, because the QEMU process is still
> running the live workload and thus is precious and mustn't be killed.

True.

Dave

> Regards,
> Daniel
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK