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

From: Li Zhang
Subject: Re: [PATCH 1/2] multifd: use qemu_sem_timedwait in multifd_recv_thread to avoid waiting forever
Date: Mon, 29 Nov 2021 14:37:29 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.13.0

On 11/29/21 12:20 PM, 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
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?

No, I tried to register 'yank' on the receive side, the receive threads are still waiting there.

It seems that on send side, 'yank' doesn't work either when the send threads are blocked.

This may be not the case to call yank. I am not quite sure about it.

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?

There is no problem with sem_sync in function multifd_load_cleanup.

But it is not called in my case, because no errors are detected on the receive side.

The problem is here:

void migration_ioc_process_incoming(QIOChannel *ioc, Error **errp)
    MigrationIncomingState *mis = migration_incoming_get_current();
    Error *local_err = NULL;
    bool start_migration;


    if (!mis->from_src_file) {


     } else {
        /* Multiple connections */
        start_migration = multifd_recv_new_channel(ioc, &local_err);
        if (local_err) {
            error_propagate(errp, local_err);
   if (start_migration) {

start_migration is always 0, and migration is not started because some receive threads are not created.

No errors are detected here and the main process works well but receive threads are all waiting for semaphore.

It's hard to know if the receive threads are not created. If we can find a way to check if any receive threads

are not created, we can kick the sem_sync and do cleanup.

From the source code, the thread will be created when QIO channel detects something by GIO watch if I understand correctly.

If nothing is detected, socket_accept_icoming_migration won't be called, the thread will not be created.

socket_start_incoming_migration_internal ->

                                          NULL, NULL,

   qio_net_listener_set_client_func_full ->

                QIO_CHANNEL(listener->sioc[i]), G_IO_IN,
                listener, (GDestroyNotify)object_unref, context);

  socket_accept_incoming_migration ->

       migration_channel_process_incoming ->

               migration_ioc_process_incoming ->

                     multifd_recv_new_channel ->

                            qemu_thread_create(&p->thread, p->name, multifd_recv_thread, p,


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