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

[Fam Zheng]

On 2020-09-17 15:36, Zhenyu Ye wrote:
> 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

Hi Zhenyu,

Did you by any chance capture the kernel stack where io_submit is stuck?

Thanks,
Fam