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

[Li Zhang]

On 11/29/21 4:49 PM, Dr. David Alan Gilbert wrote:
> * 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.

I found that the 'yank' function has been registered on receive side 
actually.

It's different from the send side.

It's in the function:

void migration_channel_process_incoming(QIOChannel *ioc)
{
    MigrationState *s = migrate_get_current();
    Error *local_err = NULL;

    trace_migration_set_incoming_channel(
        ioc, object_get_typename(OBJECT(ioc)));

    if (s->parameters.tls_creds &&
        *s->parameters.tls_creds &&
        !object_dynamic_cast(OBJECT(ioc),
                             TYPE_QIO_CHANNEL_TLS)) {
        migration_tls_channel_process_incoming(s, ioc, &local_err);
    } else {
        migration_ioc_register_yank(ioc);
        migration_ioc_process_incoming(ioc, &local_err);
    }

    if (local_err) {
        error_report_err(local_err);
    }
}


> > 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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