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