Re: [Qemu-devel] postcopy migration hangs while loading virtio state

[Christian Borntraeger]

On 04/24/2017 12:53 PM, Dr. David Alan Gilbert wrote:
> * Christian Borntraeger (address@hidden) wrote:
>> David, Juan,
>> 
>> I can trigger a hang of postcopy migration (when I do it early) so
>> that both sides are in paused state. Looks like one thread is still
>> loading vmstates for virtio and this load accesses guest memory and
>> triggers a userfault.
> 
> It's perfectly legal for the destination to cause a userfault as it
> loads the virtio queue - the virtio queue should be being loaded by
> the main thread from the buffer while the 'listening' thread is
> waiting for the incoming page data.
> 
> Can you turn on the following tracing please: destination: 
> postcopy_ram_fault_thread_request postcopy_place_page 
> postcopy_place_page_zero
> 
> source: migrate_handle_rp_req_pages ram_save_queue_pages
> 
> You should see: virtio does the access userfault generates a fault 
> postcopy_ram_fault_thread_request sends the request to the source
> 
> the source sees migrate_handle_rp_req_pages queues it with
> ram_save_queue_pages
> 
> the destination sees the page arrive and postcopy_place_page or
> postcopy_place_page_zero
> 
> some of that might disappear if the page was already on it's way.


the last event on the source are
[..]
address@hidden:postcopy_place_page host=0x3ff92246000
address@hidden:postcopy_place_page host=0x3ff92247000
address@hidden:migrate_handle_rp_req_pages in s390.ram at 41d9000 len 1000
address@hidden:ram_save_queue_pages s390.ram: start: 41d9000 len: 100


On the target a see lots of

address@hidden:postcopy_place_page_zero host=0x3ff9befa000
address@hidden:postcopy_place_page host=0x3ff9befb000
address@hidden:postcopy_place_page host=0x3ff9befc000
address@hidden:postcopy_place_page host=0x3ff9befd000
address@hidden:postcopy_place_page_zero host=0x3ff9befe000
address@hidden:postcopy_place_page host=0x3ff9beff000

So we have about 2 seconds of traffic going on after that request,
I assume its precopy related.

Looking back on the target history there was
address@hidden:postcopy_ram_fault_thread_request Request for HVA=3ff618d9000 
rb=s390.ram offset=41d9000

In fact it seems to be the first and only request:

# cat /tmp/test0.trace | grep -v postcopy_place_page

address@hidden:postcopy_ram_fault_thread_request Request for HVA=3ff618d9000 
rb=s390.ram offset=41d9000

> 
> Dave
> 
>> Thread 1 (Thread 0x3ffa2f45f00 (LWP 21122)): #0  0x0000000001017130
>> in lduw_he_p (ptr=0x3ff498d9002) at
>> /root/qemu/include/qemu/bswap.h:317 <---- according to the host
>> kernel this threads hangs in handle_userfault. #1
>> 0x0000000001017342 in lduw_le_p (ptr=0x3ff498d9002) at
>> /root/qemu/include/qemu/bswap.h:359 #2  0x0000000001025840 in
>> address_space_lduw_internal_cached (cache=0x283491d0, addr=2,
>> attrs=..., result=0x0, endian=DEVICE_LITTLE_ENDIAN) at
>> /root/qemu/memory_ldst.inc.c:284 #3  0x00000000010259a6 in
>> address_space_lduw_le_cached (cache=0x283491d0, addr=2, attrs=...,
>> result=0x0) at /root/qemu/memory_ldst.inc.c:315 #4
>> 0x0000000001025ad6 in lduw_le_phys_cached (cache=0x283491d0,
>> addr=2) at /root/qemu/memory_ldst.inc.c:334 #5  0x0000000001116c10
>> in virtio_lduw_phys_cached (vdev=0x28275090, cache=0x283491d0,
>> pa=2) at /root/qemu/include/hw/virtio/virtio-access.h:166 #6
>> 0x0000000001117940 in vring_used_idx (vq=0x3ffa2e1c090) at
>> /root/qemu/hw/virtio/virtio.c:263 #7  0x000000000111daea in
>> virtio_load (vdev=0x28275090, f=0x28353600, version_id=2) at
>> /root/qemu/hw/virtio/virtio.c:2168 #8  0x000000000111d0cc in
>> virtio_device_get (f=0x28353600, opaque=0x28275090, size=0,
>> field=0x16adf38 <__compound_literal.0>) at
>> /root/qemu/hw/virtio/virtio.c:1975 #9  0x00000000012a7f50 in
>> vmstate_load_state (f=0x28353600, vmsd=0x14db480
>> <vmstate_virtio_blk>, opaque=0x28275090, version_id=2) at
>> /root/qemu/migration/vmstate.c:128 #10 0x00000000010cbc08 in
>> vmstate_load (f=0x28353600, se=0x28279e60, version_id=2) at
>> /root/qemu/migration/savevm.c:732 #11 0x00000000010ced6e in
>> qemu_loadvm_section_start_full (f=0x28353600, mis=0x17169a8
>> <mis_current>) at /root/qemu/migration/savevm.c:1890 #12
>> 0x00000000010cf00c in qemu_loadvm_state_main (f=0x28353600,
>> mis=0x17169a8 <mis_current>) at /root/qemu/migration/savevm.c:1947 
>> #13 0x00000000010ce5bc in loadvm_handle_cmd_packaged (mis=0x17169a8
>> <mis_current>) at /root/qemu/migration/savevm.c:1707 #14
>> 0x00000000010ce87e in loadvm_process_command (f=0x282c7d20) at
>> /root/qemu/migration/savevm.c:1770 #15 0x00000000010cf056 in
>> qemu_loadvm_state_main (f=0x282c7d20, mis=0x17169a8 <mis_current>)
>> at /root/qemu/migration/savevm.c:1960 #16 0x00000000010cf2b6 in
>> qemu_loadvm_state (f=0x282c7d20) at
>> /root/qemu/migration/savevm.c:2020 #17 0x000000000129d234 in
>> process_incoming_migration_co (opaque=0x282c7d20) at
>> /root/qemu/migration/migration.c:404 #18 0x00000000014b6ca2 in
>> coroutine_trampoline (i0=0, i1=674533168) at
>> /root/qemu/util/coroutine-ucontext.c:79 #19 0x000003ff8de530ea in
>> __makecontext_ret () from target:/lib64/libc.so.6
>> 
>> Thread 8 (Thread 0x3ff40b38910 (LWP 21165)): #0  0x000003ff8df04336
>> in syscall () from target:/lib64/libc.so.6 #1  0x00000000014933ce
>> in qemu_futex_wait (f=0x17169b0 <mis_current+8>, val=4294967295) at
>> /root/qemu/include/qemu/futex.h:26 #2  0x00000000014935d0 in
>> qemu_event_wait (ev=0x17169b0 <mis_current+8>) at
>> /root/qemu/util/qemu-thread-posix.c:399 #3  0x00000000010ce094 in
>> postcopy_ram_listen_thread (opaque=0x282c7d20) at
>> /root/qemu/migration/savevm.c:1532 #4  0x000003ff8e007bc4 in
>> start_thread () from target:/lib64/libpthread.so.0 #5
>> 0x000003ff8df0a9f2 in thread_start () from target:/lib64/libc.so.6
>> 
>> Thread 7 (Thread 0x3ff41338910 (LWP 21164)): #0  0x000003ff8defd72c
>> in ?? () from target:/lib64/libc.so.6 #1  0x00000000012af920 in
>> postcopy_ram_fault_thread (opaque=0x17169a8 <mis_current>) at
>> /root/qemu/migration/postcopy-ram.c:437 #2  0x000003ff8e007bc4 in
>> start_thread () from target:/lib64/libpthread.so.0 #3
>> 0x000003ff8df0a9f2 in thread_start () from target:/lib64/libc.so.6
>> 
>> Thread 6 (Thread 0x3ff41b38910 (LWP 21163)): #0  0x000003ff8e00d450
>> in pthread_cond_wait@@GLIBC_2.3.2 () from
>> target:/lib64/libpthread.so.0 #1  0x0000000001492d66 in
>> qemu_cond_wait (cond=0x28302408, mutex=0x283023e0) at
>> /root/qemu/util/qemu-thread-posix.c:133 #2  0x00000000010c6080 in
>> do_data_decompress (opaque=0x283023d8) at
>> /root/qemu/migration/ram.c:2277 #3  0x000003ff8e007bc4 in
>> start_thread () from target:/lib64/libpthread.so.0 #4
>> 0x000003ff8df0a9f2 in thread_start () from target:/lib64/libc.so.6
>> 
>> Thread 5 (Thread 0x3ff42338910 (LWP 21162)): #0  0x000003ff8e00d450
>> in pthread_cond_wait@@GLIBC_2.3.2 () from
>> target:/lib64/libpthread.so.0 #1  0x0000000001492d66 in
>> qemu_cond_wait (cond=0x28302390, mutex=0x28302368) at
>> /root/qemu/util/qemu-thread-posix.c:133 #2  0x00000000010c6080 in
>> do_data_decompress (opaque=0x28302360) at
>> /root/qemu/migration/ram.c:2277 #3  0x000003ff8e007bc4 in
>> start_thread () from target:/lib64/libpthread.so.0 #4
>> 0x000003ff8df0a9f2 in thread_start () from target:/lib64/libc.so.6
>> 
>> Thread 4 (Thread 0x3ff89287910 (LWP 21148)): #0  0x000003ff8e010abc
>> in __lll_lock_wait () from target:/lib64/libpthread.so.0 #1
>> 0x000003ff8e00bcb6 in __pthread_mutex_cond_lock () from
>> target:/lib64/libpthread.so.0 #2  0x000003ff8e00d4ee in
>> pthread_cond_wait@@GLIBC_2.3.2 () from
>> target:/lib64/libpthread.so.0 #3  0x0000000001492d66 in
>> qemu_cond_wait (cond=0x2831eb00, mutex=0x1711688
>> <qemu_global_mutex>) at /root/qemu/util/qemu-thread-posix.c:133 #4
>> 0x000000000107720e in qemu_kvm_wait_io_event (cpu=0x282e09b0) at
>> /root/qemu/cpus.c:1085 #5  0x00000000010773aa in
>> qemu_kvm_cpu_thread_fn (arg=0x282e09b0) at /root/qemu/cpus.c:1123 
>> #6  0x000003ff8e007bc4 in start_thread () from
>> target:/lib64/libpthread.so.0 #7  0x000003ff8df0a9f2 in
>> thread_start () from target:/lib64/libc.so.6
>> 
>> Thread 3 (Thread 0x3ff8a7ff910 (LWP 21145)): #0  0x000003ff8defd832
>> in ppoll () from target:/lib64/libc.so.6 #1  0x000000000148b092 in
>> qemu_poll_ns (fds=0x3ff840008c0, nfds=1, timeout=-1) at
>> /root/qemu/util/qemu-timer.c:322 #2  0x000000000148f46a in aio_poll
>> (ctx=0x2827e7b0, blocking=true) at /root/qemu/util/aio-posix.c:622 
>> #3  0x00000000011a78f6 in iothread_run (opaque=0x2827d740) at
>> /root/qemu/iothread.c:59 #4  0x000003ff8e007bc4 in start_thread ()
>> from target:/lib64/libpthread.so.0 #5  0x000003ff8df0a9f2 in
>> thread_start () from target:/lib64/libc.so.6
>> 
>> Thread 2 (Thread 0x3ff8afff910 (LWP 21142)): #0  0x000003ff8df04336
>> in syscall () from target:/lib64/libc.so.6 #1  0x00000000014933ce
>> in qemu_futex_wait (f=0x1b2b9b4 <rcu_call_ready_event>,
>> val=4294967295) at /root/qemu/include/qemu/futex.h:26 #2
>> 0x00000000014935d0 in qemu_event_wait (ev=0x1b2b9b4
>> <rcu_call_ready_event>) at /root/qemu/util/qemu-thread-posix.c:399 
>> #3  0x00000000014b3e40 in call_rcu_thread (opaque=0x0) at
>> /root/qemu/util/rcu.c:249 #4  0x000003ff8e007bc4 in start_thread ()
>> from target:/lib64/libpthread.so.0 #5  0x000003ff8df0a9f2 in
>> thread_start () from target:/lib64/libc.so.6
>> 
>> 
>> Looks like that this <qemu:arg value='-drive'/> <qemu:arg
>> value='driver=null-aio,id=null1,if=none,size=100G'/> <qemu:arg
>> value='-device'/> <qemu:arg
>> value='virtio-blk-ccw,scsi=off,drive=null1,serial=null1,num-queues=4'/>
>>
>>
>> is necessary to trigger the hang, with nun-qeues=1 everything seems to work 
>> fine, so it might
>> be a virtio issue after all?
>> 
>> Any idea?
>> 
> -- Dr. David Alan Gilbert / address@hidden / Manchester, UK
>