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
