Re: [PATCH v1 0/2] Add timeout mechanism to qmp actions

[Zhenyu Ye]

Hi Stefan,

On 2020/9/14 21:27, Stefan Hajnoczi wrote:
>>
>> Theoretically, everything running in an iothread is asynchronous. However,
>> some 'asynchronous' actions are not non-blocking entirely, such as
>> io_submit().  This will block while the iodepth is too big and I/O pressure
>> is too high.  If we do some qmp actions, such as 'info block', at this time,
>> may cause vm soft lockup.  This series can make these qmp actions safer.
>>
>> I constructed the scene as follow:
>> 1. create a vm with 4 disks, using iothread.
>> 2. add press to the CPU on the host.  In my scene, the CPU usage exceeds 95%.
>> 3. add press to the 4 disks in the vm at the same time.  I used the fio and
>> some parameters are:
>>
>>       fio -rw=randrw -bs=1M -size=1G -iodepth=512 -ioengine=libaio -numjobs=4
>>
>> 4. do block query actions, for example, by virsh:
>>
>>      virsh qemu-monitor-command [vm name] --hmp info block
>>
>> Then the vm will soft lockup, the calltrace is:
>>
>> [  192.311393] watchdog: BUG: soft lockup - CPU#1 stuck for 42s! 
>> [kworker/1:1:33]
> 
> Hi,
> Sorry I haven't had time to investigate this myself yet.
> 
> Do you also have a QEMU backtrace when the hang occurs?
> 
> Let's find out if QEMU is stuck in the io_submit(2) syscall or whether
> there's an issue in QEMU itself that causes the softlockup (for example,
> aio_poll() with the global mutex held).

Sorry for the delay.

I traced the io_submit() within the guest. The maximum execution time exceeds 
16s:

        guest# perf trace -p `pidof -s fio` --duration 1
            14.808 (3843.082 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
          3861.336 (11470.608 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         15341.998 (479.283 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         15831.380 (3704.997 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         19547.869 (3412.839 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         22966.953 (1080.854 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         24062.689 (6939.813 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         31019.219 (532.147 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         31556.610 (3469.920 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         35038.885 (9007.175 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         44053.578 (16006.405 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         60068.944 (3068.748 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
         63138.474 (13012.499 ms): io_getevents(ctx_id: 281472924459008, 
min_nr: 511, nr: 511, events: 0x19a83150) = 511
         76191.073 (2872.657 ms): io_submit(ctx_id: 281472924459008, nr: 1, 
iocbpp: 0x19a87160          ) = 1
        ...

And in the host, the information of sys_enter_io_submit() is:

        Samples: 3K of event 'syscalls:sys_enter_io_submit', Event count 
(approx.): 3150
        Children      Self  Trace output
        -   66.70%    66.70%  ctx_id: 0xffff9c044000, nr: 0x00000001, iocbpp: 
0xffff9f7fad28
        0xffffae7f871c
        0xffffae8a27c4
        qemu_thread_start
        iothread_run
        aio_poll
        aio_dispatch_ready_handlers
        aio_dispatch_handler
        virtio_queue_host_notifier_aio_read
        virtio_queue_notify_aio_vq
        virtio_blk_data_plane_handle_output
        virtio_blk_handle_vq
        blk_io_unplug
        bdrv_io_unplug
        bdrv_io_unplug
        raw_aio_unplug
        laio_io_unplug
        syscall


When the hang occurs, the QEMU is blocked at:

        #0  0x0000ffff95762b64 in ?? () from target:/usr/lib64/libpthread.so.0
        #1  0x0000ffff9575bd88 in pthread_mutex_lock () from 
target:/usr/lib64/libpthread.so.0
        #2  0x0000aaaabb1f5948 in qemu_mutex_lock_impl (mutex=0xaaaacc8e1860,
            file=0xaaaabb4e1bd0 
"/Images/eillon/CODE/5-opensource/qemu/util/async.c", line=605)
        #3  0x0000aaaabb20acd4 in aio_context_acquire (ctx=0xaaaacc8e1800)
        #4  0x0000aaaabb105e90 in bdrv_query_image_info (bs=0xaaaacc934620,
            p_info=0xaaaaccc41e18, errp=0xffffca669118)
        #5  0x0000aaaabb105968 in bdrv_block_device_info (blk=0xaaaacdca19f0, 
bs=0xaaaacc934620,
            flat=false, errp=0xffffca6692b8)
        #6  0x0000aaaabb1063dc in bdrv_query_info (blk=0xaaaacdca19f0, 
p_info=0xaaaacd29c9a8,
            errp=0xffffca6692b8)
        #7  0x0000aaaabb106c14 in qmp_query_block (errp=0x0)
        #8  0x0000aaaabacb8e6c in hmp_info_block (mon=0xffffca6693d0, 
qdict=0xaaaacd089790)
        #9  0x0000aaaabb0068f0 in handle_hmp_command (mon=0xffffca6693d0,
            cmdline=0xaaaacc9013ca "")
        #10 0x0000aaaabaab5034 in qmp_human_monitor_command (
            command_line=0xaaaacc9013c0 "info block", has_cpu_index=false, 
cpu_index=0,
            errp=0xffffca6694d8)
        #11 0x0000aaaabb03c720 in qmp_marshal_human_monitor_command 
(args=0xffff70008070,
            ret=0xffffca669570, errp=0xffffca669568) at 
qapi/qapi-commands-misc.c:653
        #12 0x0000aaaabb18fdbc in qmp_dispatch (cmds=0xaaaabbb197c0 
<qmp_commands>,
            request=0xffff70005cc0, allow_oob=false)
        #13 0x0000aaaabb003074 in monitor_qmp_dispatch (mon=0xaaaacc962320, 
req=0xffff70005cc0)
        #14 0x0000aaaabb003488 in monitor_qmp_bh_dispatcher (data=0x0)
        #15 0x0000aaaabb209d44 in aio_bh_call (bh=0xaaaacc5f0810)
        #16 0x0000aaaabb209e50 in aio_bh_poll (ctx=0xaaaacc5ef320)
        #17 0x0000aaaabb1f1494 in aio_dispatch (ctx=0xaaaacc5ef320)
        #18 0x0000aaaabb20a330 in aio_ctx_dispatch (source=0xaaaacc5ef320, 
callback=0x0,
            user_data=0x0)
        #19 0x0000ffff95f00a7c in g_main_context_dispatch ()
           from target:/usr/lib64/libglib-2.0.so.0
        #20 0x0000aaaabb2128e8 in glib_pollfds_poll ()
        #21 0x0000aaaabb212970 in os_host_main_loop_wait (timeout=64805420)
        #22 0x0000aaaabb212a90 in main_loop_wait (nonblocking=0)
        #23 0x0000aaaabaad63d0 in qemu_main_loop ()
        #24 0x0000aaaabb188b54 in main (argc=117, argv=0xffffca669a68, 
envp=0xffffca669e18)

And the QEMU process backtrace in kernel is:
        [<0>] __switch_to+0xdc/0x140
        [<0>] futex_wait_queue_me+0xd4/0x158
        [<0>] futex_wait+0xf4/0x230
        [<0>] do_futex+0x46c/0x608
        [<0>] __arm64_sys_futex+0x13c/0x188
        [<0>] el0_svc_common+0x80/0x1b8
        [<0>] el0_svc_handler+0x38/0x78
        [<0>] el0_svc+0x10/0x14
        [<0>] 0xffffffffffffffff


Thanks,
Zhenyu