Hi Kevin, On 2020/8/10 23:38, Kevin Wolf wrote: > Am 10.08.2020 um 16:52 hat Zhenyu Ye geschrieben: >> Before doing qmp actions, we need to lock the qemu_global_mutex, >> so the qmp actions should not take too long time. >> >> Unfortunately, some qmp actions need to acquire aio context and >> this may take a long time. The vm will soft lockup if this time >> is too long. > > Do you have a specific situation in mind where getting the lock of an > AioContext can take a long time? I know that the main thread can > block for considerable time, but QMP commands run in the main thread, so > this patch doesn't change anything for this case. It would be effective > if an iothread blocks, but shouldn't everything running in an iothread > be asynchronous and therefore keep the AioContext lock only for a short > time? >
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] [ 192.314241] Kernel panic - not syncing: softlockup: hung tasks [ 192.316370] CPU: 1 PID: 33 Comm: kworker/1:1 Kdump: loaded Tainted: G OEL 4.19.36+ #16 [ 192.318765] Hardware name: QEMU KVM Virtual Machine, BIOS 0.0.0 02/06/2015 [ 192.325638] Workqueue: events drm_fb_helper_dirty_work [ 192.roorr327238] Call trace: [ 192.331528] dump_backtrace+0x0/0x198 [ 192.332695] show_stack+0x24/0x30 [ 192.334219] dump_stack+0xa4/0xcc [ 192.335578] panic+0x12c/0x314 [ 192.336565] watchdog_timoot_fn+0x3e4/0x3e8 [ 192.339984] __hrtimer_run_queues+0x114/0x358 [ 192.341576] hrtimer_interrupt+0x104/0x2d8 [ 192.343247] arch_timer_handler_virt+0x38/0x58 [ 192.345074] handle_percpu_devid_irq+0x90/0x248 [ 192.347238] generic_handle_irq+0x34/0x50 [ 192.349536] __handle_domain_irq+0x68/0xc0 [ 192.351193] gic_handle_irq+0x6c/0x150 [ 192.352639] el1_irq+0xb8/0x140 [ 192.353855] vp_notify+0x28/0x38 [virtio_pci] [ 192.355646] virtqueue_kick+0x3c/0x78 [virtio_ring] [ 192.357539] virtio_gpu_queue_ctrl_buffer_locked+0x180/0x248 [virtio_gpu] [ 192.359869] virtio_gpu_queue_ctrl_buffer+0x50/0x78 [virtio_gpu] [ 192.361456] virtio_gpu_cmd_resource_flush+0x8c/0xb0 [virtio_gpu] [ 192.363422] virtio_gpu_surface_dirty+0x60/0x110 [virtio_gpu] [ 192.365215] virtio_gpu_framebuffer_surface_dirty+0x34/0x48 [virtio_gpu] [ 192.367452] drm_fb_helper_dirty_work+0x178/0x1c0 [ 192.368912] process_one_work+0x1b4/0x3f8 [ 192.370192] worker_thread+0x54/0x470 [ 192.371370] kthread+0x134/0x138 [ 192.379241] ret_from_fork+0x10/0x18 [ 192.382688] kernel fault(0x5) notification starting on CPU 1 [ 192.385059] kernel fault(0x5) notification finished on CPU 1 [ 192.387311] SMP: stopping secondary CPUs [ 192.391024] Kernel Offset: disabled [ 192.392111] CPU features: 0x0,a1806008 [ 192.393306] Memory Limit: none [ 192.396701] Starting crashdump kernel... [ 192.397821] Bye! This problem can be avoided after this series applied. Thanks, Zhenyu