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Re: [PATCH 1/2] multifd: use qemu_sem_timedwait in multifd_recv_thread t

From: Dr. David Alan Gilbert
Subject: Re: [PATCH 1/2] multifd: use qemu_sem_timedwait in multifd_recv_thread to avoid waiting forever
Date: Mon, 29 Nov 2021 11:20:08 +0000
User-agent: Mutt/2.1.3 (2021-09-10)

* 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
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?

multifd_load_cleanup already kicks sem_sync before trying to do a
thread_join - so have we managed to trigger that on the receive side?


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

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