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

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

* 858585 jemmy (jemmy858585@gmail.com) wrote:
> On Wed, May 9, 2018 at 2:40 AM, Dr. David Alan Gilbert
> <dgilbert@redhat.com> wrote:
> > * 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?
> 
> 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.

Maybe that is the right fix; I can imagine that the RDMA code doesn't
like closing down if there are still ramblocks registered that
potentially could have incoming DMA?

> 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/

OK, that's a pain; which threads are doing the dereg - is some stuff
in the migration thread and some stuff in the main thread on cleanup?

Dave

> >
> >> 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).
> 
> 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<aviadye@mellanox.com>, 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 / dgilbert@redhat.com / Manchester, UK
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK