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

[Li Zhang]

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);
     }
 }
-- 
2.31.1