Re: [PATCH v2 1/1] multifd: Shut down the QIO channels to avoid blocking the send threads when they are terminated.

[Daniel P . Berrangé]

On Fri, Dec 03, 2021 at 12:55:33PM +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.
> 
> (gdb) bt
> 0  0x00007f30c8dcffc0 in __pthread_clockjoin_ex () at /lib64/libpthread.so.0
> 1  0x000055cbb716084b in qemu_thread_join (thread=0x55cbb881f418) at 
> ../util/qemu-thread-posix.c:627
> 2  0x000055cbb6b54e40 in multifd_save_cleanup () at ../migration/multifd.c:542
> 3  0x000055cbb6b4de06 in migrate_fd_cleanup (s=0x55cbb8024000) at 
> ../migration/migration.c:1808
> 4  0x000055cbb6b4dfb4 in migrate_fd_cleanup_bh (opaque=0x55cbb8024000) at 
> ../migration/migration.c:1850
> 5  0x000055cbb7173ac1 in aio_bh_call (bh=0x55cbb7eb98e0) at 
> ../util/async.c:141
> 6  0x000055cbb7173bcb in aio_bh_poll (ctx=0x55cbb7ebba80) at 
> ../util/async.c:169
> 7  0x000055cbb715ba4b in aio_dispatch (ctx=0x55cbb7ebba80) at 
> ../util/aio-posix.c:381
> 8  0x000055cbb7173ffe in aio_ctx_dispatch (source=0x55cbb7ebba80, 
> callback=0x0, user_data=0x0) at ../util/async.c:311
> 9  0x00007f30c9c8cdf4 in g_main_context_dispatch () at 
> /usr/lib64/libglib-2.0.so.0
> 10 0x000055cbb71851a2 in glib_pollfds_poll () at ../util/main-loop.c:232
> 11 0x000055cbb718521c in os_host_main_loop_wait (timeout=42251070366) at 
> ../util/main-loop.c:255
> 12 0x000055cbb7185321 in main_loop_wait (nonblocking=0) at 
> ../util/main-loop.c:531
> 13 0x000055cbb6e6ba27 in qemu_main_loop () at ../softmmu/runstate.c:726
> 14 0x000055cbb6ad6fd7 in main (argc=68, argv=0x7ffc0c578888, 
> envp=0x7ffc0c578ab0) at ../softmmu/main.c:50
> 
> To make sure that the send threads could be terminated, IO channels should be
> shut down to avoid waiting IO.
> 
> Signed-off-by: Li Zhang <lizhang@suse.de>
> ---
>  migration/multifd.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/migration/multifd.c b/migration/multifd.c
> index 7c9deb1921..33f8287969 100644
> --- a/migration/multifd.c
> +++ b/migration/multifd.c
> @@ -523,6 +523,9 @@ static void multifd_send_terminate_threads(Error *err)
>          qemu_mutex_lock(&p->mutex);
>          p->quit = true;
>          qemu_sem_post(&p->sem);
> +        if (p->c) {
> +            qio_channel_shutdown(p->c, QIO_CHANNEL_SHUTDOWN_BOTH, NULL);
> +        }
>          qemu_mutex_unlock(&p->mutex);
>      }
>  }

Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>


Regards,
Daniel
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