Re: [Qemu-devel] [PATCH 2/2] migration: not wait RDMA_CM_EVENT_DISCONNECTED event after rdma_disconnect

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Lidong Chen (jemmy858585@gmail.com) wrote:
> When cancel migration during RDMA precopy, the source qemu main thread hangs sometime.
> 
> The backtrace is:
>     (gdb) bt
>     #0  0x00007f249eabd43d in write () from /lib64/libpthread.so.0
>     #1  0x00007f24a1ce98e4 in rdma_get_cm_event (channel=0x4675d10, event=0x7ffe2f643dd0) at src/cma.c:2189
>     #2  0x00000000007b6166 in qemu_rdma_cleanup (rdma=0x6784000) at migration/rdma.c:2296
>     #3  0x00000000007b7cae in qio_channel_rdma_close (ioc=0x3bfcc30, errp=0x0) at migration/rdma.c:2999
>     #4  0x00000000008db60e in qio_channel_close (ioc=0x3bfcc30, errp=0x0) at io/channel.c:273
>     #5  0x00000000007a8765 in channel_close (opaque=0x3bfcc30) at migration/qemu-file-channel.c:98
>     #6  0x00000000007a71f9 in qemu_fclose (f=0x527c000) at migration/qemu-file.c:334
>     #7  0x0000000000795b96 in migrate_fd_cleanup (opaque=0x3b46280) at migration/migration.c:1162
>     #8  0x000000000093a71b in aio_bh_call (bh=0x3db7a20) at util/async.c:90
>     #9  0x000000000093a7b2 in aio_bh_poll (ctx=0x3b121c0) at util/async.c:118
>     #10 0x000000000093f2ad in aio_dispatch (ctx=0x3b121c0) at util/aio-posix.c:436
>     #11 0x000000000093ab41 in aio_ctx_dispatch (source=0x3b121c0, callback=0x0, user_data=0x0)
>         at util/async.c:261
>     #12 0x00007f249f73c7aa in g_main_context_dispatch () from /lib64/libglib-2.0.so.0
>     #13 0x000000000093dc5e in glib_pollfds_poll () at util/main-loop.c:215
>     #14 0x000000000093dd4e in os_host_main_loop_wait (timeout=28000000) at util/main-loop.c:263
>     #15 0x000000000093de05 in main_loop_wait (nonblocking=0) at util/main-loop.c:522
>     #16 0x00000000005bc6a5 in main_loop () at vl.c:1944
>     #17 0x00000000005c39b5 in main (argc=56, argv=0x7ffe2f6443f8, envp=0x3ad0030) at vl.c:4752
> 
> It does not get the RDMA_CM_EVENT_DISCONNECTED event after rdma_disconnect sometime.
> I do not find out the root cause why not get RDMA_CM_EVENT_DISCONNECTED event, but
> it can be reproduced if not invoke ibv_dereg_mr to release all ram blocks which fixed
> in previous patch.

Does this happen without your other changes?
Can you give me instructions to repeat it and also say which
cards you wereusing?

> Anyway, it should not invoke rdma_get_cm_event in main thread, and the event channel
> is also destroyed in qemu_rdma_cleanup.
> 
> Signed-off-by: Lidong Chen <lidongchen@tencent.com>
> ---
>  migration/rdma.c       | 12 ++----------
>  migration/trace-events |  1 -
>  2 files changed, 2 insertions(+), 11 deletions(-)
> 
> diff --git a/migration/rdma.c b/migration/rdma.c
> index 0dd4033..92e4d30 100644
> --- a/migration/rdma.c
> +++ b/migration/rdma.c
> @@ -2275,8 +2275,7 @@ static int qemu_rdma_write(QEMUFile *f, RDMAContext *rdma,
>  
>  static void qemu_rdma_cleanup(RDMAContext *rdma)
>  {
> -    struct rdma_cm_event *cm_event;
> -    int ret, idx;
> +    int idx;
>  
>      if (rdma->cm_id && rdma->connected) {
>          if ((rdma->error_state ||
> @@ -2290,14 +2289,7 @@ static void qemu_rdma_cleanup(RDMAContext *rdma)
>              qemu_rdma_post_send_control(rdma, NULL, &head);
>          }
>  
> -        ret = rdma_disconnect(rdma->cm_id);
> -        if (!ret) {
> -            trace_qemu_rdma_cleanup_waiting_for_disconnect();
> -            ret = rdma_get_cm_event(rdma->channel, &cm_event);
> -            if (!ret) {
> -                rdma_ack_cm_event(cm_event);
> -            }
> -        }
> +        rdma_disconnect(rdma->cm_id);

I'm worried whether this change could break stuff:
The docs say for rdma_disconnect that it flushes any posted work
requests to the completion queue;  so unless we wait for the event
do we know the stuff has been flushed?   In the normal non-cancel case
I'm worried that means we could lose something.
(But I don't know the rdma/infiniband specs well enough to know if it's
really a problem).

Dave

>          trace_qemu_rdma_cleanup_disconnect();
>          rdma->connected = false;
>      }
> diff --git a/migration/trace-events b/migration/trace-events
> index d6be74b..64573ff 100644
> --- a/migration/trace-events
> +++ b/migration/trace-events
> @@ -125,7 +125,6 @@ qemu_rdma_accept_pin_state(bool pin) "%d"
>  qemu_rdma_accept_pin_verbsc(void *verbs) "Verbs context after listen: %p"
>  qemu_rdma_block_for_wrid_miss(const char *wcompstr, int wcomp, const char *gcompstr, uint64_t req) "A Wanted wrid %s (%d) but got %s (%" PRIu64 ")"
>  qemu_rdma_cleanup_disconnect(void) ""
> -qemu_rdma_cleanup_waiting_for_disconnect(void) ""
>  qemu_rdma_close(void) ""
>  qemu_rdma_connect_pin_all_requested(void) ""
>  qemu_rdma_connect_pin_all_outcome(bool pin) "%d"
> -- 
> 1.8.3.1
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK