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

[858585 jemmy]

On Wed, May 9, 2018 at 2:40 AM, Dr. David Alan Gilbert
<address@hidden> wrote:
> * Lidong Chen (address@hidden) 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?

Yes, this issue also happen on v2.12.0. base on
commit 4743c23509a51bd4ee85cc272287a41917d1be35

> Can you give me instructions to repeat it and also say which
> cards you wereusing?

This issue can be reproduced by start and cancel migration.
less than 10 times, this issue will be reproduced.

The command line is:
virsh migrate --live --copy-storage-all  --undefinesource --persistent
--timeout 10800 \
 --verbose 83e0049e-1325-4f31-baf9-25231509ada1  \
qemu+ssh://9.16.46.142/system rdma://9.16.46.142

The net card i use is :
0000:3b:00.0 Ethernet controller: Mellanox Technologies MT27710 Family
[ConnectX-4 Lx]
0000:3b:00.1 Ethernet controller: Mellanox Technologies MT27710 Family
[ConnectX-4 Lx]

This issue is related to ibv_dereg_mr, if not invoke ibv_dereg_mr for
all ram block, this issue can be reproduced.
If we fixed the bugs and use ibv_dereg_mr to release all ram block,
this issue never happens.

And for the kernel part, there is a bug also cause not release ram
block when canceling live migration.
https://patchwork.kernel.org/patch/10385781/

>
>> 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 <address@hidden>
>> ---
>>  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).

In qemu_fclose function, it invoke qemu_fflush(f) before invoke f->ops->close.
so I think it's safe for normal migration case.

For the root cause why not receive RDMA_CM_EVENT_DISCONNECTED event
after rdma_disconnect,
I loop in Aviad Yehezkel<address@hidden>, Aviad will help us
find the root cause.

>
> 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 / address@hidden / Manchester, UK