Re: [PATCH] scsi: Don't wait for completion of in-flight requests

[Ming Lei <ming.lei@redhat.com>]

On Tue, Dec 03, 2024 at 09:21:04PM +0800, Qiang Ma wrote:
> On Wed, 27 Nov 2024 23:20:28 +0800
> Ming Lei <ming.lei@redhat.com> wrote:
> 
> > On Wed, Nov 27, 2024 at 10:04:37PM +0800, Qiang Ma wrote:
> > > On Tue, 26 Nov 2024 22:15:02 +0800
> > > Ming Lei <ming.lei@redhat.com> wrote:
> > >   
> > > > On Tue, Nov 26, 2024 at 07:50:08PM +0800, Qiang Ma wrote:  
> > > > > Problem:
> > > > > When the system disk uses the scsi disk bus, The main
> > > > > qemu command line includes:
> > > > > ...
> > > > > -device virtio-scsi-pci,id=scsi0 \
> > > > > -device scsi-hd,scsi-id=1,drive=drive-virtio-disk
> > > > > -drive id=drive-virtio-disk,if=none,file=/home/kvm/test.qcow2
> > > > > ...
> > > > > 
> > > > > The dmesg log is as follows::
> > > > > 
> > > > > [   50.304591][ T4382] sd 0:0:0:0: [sda] Synchronizing SCSI
> > > > > cache [   50.377002][ T4382] kexec_core: Starting new kernel  
> > 
> > Why is one new kernel started? what event triggers kernel_kexec()?
> > 
> > machine_shutdown() follows, probably the scsi controller is dead.
> > 
> 
> Yes, triggered by kexec, manually execute the following command:
> kexec -l /boot/vmlinuz-6.6.0+ --reuse-cmdline
> --initrd=/boot/initramfs-6.6.0+.img kexec -e
> 
> > > > > [   50.669775][  T194] psci: CPU1 killed (polled 0 ms)
> > > > > [   50.849665][  T194] psci: CPU2 killed (polled 0 ms)
> > > > > [   51.109625][  T194] psci: CPU3 killed (polled 0 ms)
> > > > > [   51.319594][  T194] psci: CPU4 killed (polled 0 ms)
> > > > > [   51.489667][  T194] psci: CPU5 killed (polled 0 ms)
> > > > > [   51.709582][  T194] psci: CPU6 killed (polled 0 ms)
> > > > > [   51.949508][   T10] psci: CPU7 killed (polled 0 ms)
> > > > > [   52.139499][   T10] psci: CPU8 killed (polled 0 ms)
> > > > > [   52.289426][   T10] psci: CPU9 killed (polled 0 ms)
> > > > > [   52.439552][   T10] psci: CPU10 killed (polled 0 ms)
> > > > > [   52.579525][   T10] psci: CPU11 killed (polled 0 ms)
> > > > > [   52.709501][   T10] psci: CPU12 killed (polled 0 ms)
> > > > > [   52.819509][  T194] psci: CPU13 killed (polled 0 ms)
> > > > > [   52.919509][  T194] psci: CPU14 killed (polled 0 ms)
> > > > > [  243.214009][  T115] INFO: task kworker/0:1:10 blocked for
> > > > > more than 122 seconds. [  243.214810][  T115]       Not tainted
> > > > > 6.6.0+ #1 [  243.215517][  T115] "echo 0  
> > > > > > /proc/sys/kernel/hung_task_timeout_secs" disables this
> > > > > > message. [  243.216390][  T115] task:kworker/0:1     state:D
> > > > > > stack:0 pid:10    ppid:2      flags:0x00000008  
> > > > > [  243.217299][  T115] Workqueue: events vmstat_shepherd
> > > > > [  243.217816][  T115] Call trace:
> > > > > [  243.218133][  T115]  __switch_to+0x130/0x1e8
> > > > > [  243.218568][  T115]  __schedule+0x660/0xcf8
> > > > > [  243.219013][  T115]  schedule+0x58/0xf0
> > > > > [  243.219402][  T115]  percpu_rwsem_wait+0xb0/0x1d0
> > > > > [  243.219880][  T115]  __percpu_down_read+0x40/0xe0
> > > > > [  243.220353][  T115]  cpus_read_lock+0x5c/0x70
> > > > > [  243.220795][  T115]  vmstat_shepherd+0x40/0x140
> > > > > [  243.221250][  T115]  process_one_work+0x170/0x3c0
> > > > > [  243.221726][  T115]  worker_thread+0x234/0x3b8
> > > > > [  243.222176][  T115]  kthread+0xf0/0x108
> > > > > [  243.222564][  T115]  ret_from_fork+0x10/0x20
> > > > > ...
> > > > > [  243.254080][  T115] INFO: task kworker/0:2:194 blocked for
> > > > > more than 122 seconds. [  243.254834][  T115]       Not tainted
> > > > > 6.6.0+ #1 [  243.255529][  T115] "echo 0  
> > > > > > /proc/sys/kernel/hung_task_timeout_secs" disables this
> > > > > > message. [  243.256378][  T115] task:kworker/0:2     state:D
> > > > > > stack:0 pid:194   ppid:2      flags:0x00000008  
> > > > > [  243.257284][  T115] Workqueue: events work_for_cpu_fn
> > > > > [  243.257793][  T115] Call trace:
> > > > > [  243.258111][  T115]  __switch_to+0x130/0x1e8
> > > > > [  243.258541][  T115]  __schedule+0x660/0xcf8
> > > > > [  243.258971][  T115]  schedule+0x58/0xf0
> > > > > [  243.259360][  T115]  schedule_timeout+0x280/0x2f0
> > > > > [  243.259832][  T115]  wait_for_common+0xcc/0x2d8
> > > > > [  243.260287][  T115]  wait_for_completion+0x20/0x38
> > > > > [  243.260767][  T115]  cpuhp_kick_ap+0xe8/0x278
> > > > > [  243.261207][  T115]  cpuhp_kick_ap_work+0x5c/0x188
> > > > > [  243.261688][  T115]  _cpu_down+0x120/0x378
> > > > > [  243.262103][  T115]  __cpu_down_maps_locked+0x20/0x38
> > > > > [  243.262609][  T115]  work_for_cpu_fn+0x24/0x40
> > > > > [  243.263059][  T115]  process_one_work+0x170/0x3c0
> > > > > [  243.263533][  T115]  worker_thread+0x234/0x3b8
> > > > > [  243.263981][  T115]  kthread+0xf0/0x108
> > > > > [  243.264405][  T115]  ret_from_fork+0x10/0x20
> > > > > [  243.264846][  T115] INFO: task kworker/15:2:639 blocked for
> > > > > more than 122 seconds. [  243.265602][  T115]       Not tainted
> > > > > 6.6.0+ #1 [  243.266296][  T115] "echo 0  
> > > > > > /proc/sys/kernel/hung_task_timeout_secs" disables this
> > > > > > message. [  243.267143][  T115] task:kworker/15:2    state:D
> > > > > > stack:0 pid:639   ppid:2      flags:0x00000008  
> > > > > [  243.268044][  T115] Workqueue: events_freezable_power_
> > > > > disk_events_workfn [  243.268727][  T115] Call trace:
> > > > > [  243.269051][  T115]  __switch_to+0x130/0x1e8
> > > > > [  243.269481][  T115]  __schedule+0x660/0xcf8
> > > > > [  243.269903][  T115]  schedule+0x58/0xf0
> > > > > [  243.270293][  T115]  schedule_timeout+0x280/0x2f0
> > > > > [  243.270763][  T115]  io_schedule_timeout+0x50/0x70
> > > > > [  243.271245][  T115]
> > > > > wait_for_common_io.constprop.0+0xb0/0x298
> > > > > [  243.271830][  T115]  wait_for_completion_io+0x1c/0x30
> > > > > [  243.272335][  T115]  blk_execute_rq+0x1d8/0x278
> > > > > [  243.272793][  T115]  scsi_execute_cmd+0x114/0x238
> > > > > [  243.273267][  T115]  sr_check_events+0xc8/0x310 [sr_mod]
> > > > > [  243.273808][  T115]  cdrom_check_events+0x2c/0x50 [cdrom]
> > > > > [  243.274408][  T115]  sr_block_check_events+0x34/0x48
> > > > > [sr_mod] [  243.274994][  T115]  disk_check_events+0x44/0x1b0
> > > > > [  243.275468][  T115]  disk_events_workfn+0x20/0x38
> > > > > [  243.275939][  T115]  process_one_work+0x170/0x3c0
> > > > > [  243.276410][  T115]  worker_thread+0x234/0x3b8
> > > > > [  243.276855][  T115]  kthread+0xf0/0x108
> > > > > [  243.277241][  T115]  ret_from_fork+0x10/0x20    
> > > > 
> > > > Question is why this scsi command can't be completed?
> > > > 
> > > > When blk_mq_hctx_notify_offline() is called, the CPU isn't
> > > > shutdown yet, and it still can handle interrupt of this SCSI
> > > > command.
> > > > 
> > > > 
> > > > Thanks,
> > > > Ming
> > > > 
> > > >   
> > > Sorry, at the moment I don't know why it can't be finished,
> > > just provides a solution like loop and dm-rq.
> > > 
> > > Currently see the call stack:  
> > >  => blk_mq_run_hw_queue
> > >  =>__blk_mq_sched_restart
> > >  => blk_mq_run_hw_queue
> > >  => __blk_mq_sched_restart
> > >  => __blk_mq_free_request
> > >  => blk_mq_free_request
> > >  => blk_mq_end_request
> > >  => blk_flush_complete_seq
> > >  => flush_end_io
> > >  => __blk_mq_end_request
> > >  => scsi_end_request
> > >  => scsi_io_completion
> > >  => scsi_finish_command
> > >  => scsi_complete
> > >  => blk_mq_complete_request
> > >  => scsi_done_internal
> > >  => scsi_done
> > >  => virtscsi_complete_cmd
> > >  => virtscsi_req_done
> > >  => vring_interrupt  
> > > 
> > > In finction blk_mq_run_hw_queue():
> > > __blk_mq_run_dispatch_ops(hctx->queue, false,
> > > 	need_run = !blk_queue_quiesced(hctx->queue) &&
> > > 	blk_mq_hctx_has_pending(hctx));
> > > 	
> > > 	if (!need_run)
> > > 		return;
> > > 
> > > Come here and return directly.  
> > 
> > Does blk_queue_quiesced() return true?
> > 
> 
> blk_queue_quiesced() return false
> 
> Sorry, the calltrace above is not the one that is stuck this time.
> The hang is caused by the wait_for_completion_io(), in blk_execute_rq():
> 
> blk_status_t blk_execute_rq(struct request *rq, bool at_head)
> {
> 	...
> 	rq->end_io_data = &wait;
> 	rq->end_io = blk_end_sync_rq;
> 	...
> 	blk_account_io_start(rq);
> 	blk_mq_insert_request(rq, at_head ? BLK_MQ_INSERT_AT_HEAD : 0);
> 	blk_mq_run_hw_queue(hctx, false);
> 
> 	if (blk_rq_is_poll(rq)) {
> 		blk_rq_poll_completion(rq, &wait.done);
> 	} else {
> 		...
> 		wait_for_completion_io(&wait.done);
> 	}
> }
> 
> In this case, end_io is blk_end_sync_rq, which is not executed.
> 
> The process for sending scsi commands is as follows:
> 
>      kworker/7:1-134     [007] .....    32.772727: vp_notify
>      <-virtqueue_notify kworker/7:1-134     [007] .....    32.772729:
>      <stack trace> => vp_notify
>  => virtqueue_notify
>  => virtscsi_add_cmd
>  => virtscsi_queuecommand
>  => scsi_dispatch_cmd
>  => scsi_queue_rq
>  => blk_mq_dispatch_rq_list
>  => __blk_mq_sched_dispatch_requests
>  => blk_mq_sched_dispatch_requests
>  => blk_mq_run_hw_queue
>  => blk_execute_rq
>  => scsi_execute_cmd
>  => sr_check_events
>  => cdrom_check_events
>  => sr_block_check_events
>  => disk_check_events
>  => disk_events_workfn
>  => process_one_work
>  => worker_thread
>  => kthread
>  => ret_from_fork
> 
> In qemu:
> static void virtio_scsi_handle_cmd_vq(VirtIOSCSI *s, VirtQueue *vq)
> {
> 	...
> 	do {
>         	if (suppress_notifications) {
> 			virtio_queue_set_notification(vq, 0);
> 		}
> 		...
> 	} while (ret != -EINVAL && !virtio_queue_empty(vq));
> 	...
> 	QTAILQ_FOREACH_SAFE(req, &reqs, next, next) {
> 		virtio_scsi_handle_cmd_req_submit(s, req);
> 	}
> }
> 
> virtio_queue_empty() always return true.
> 
> As a result, the virtio_scsi_handle_cmd_req_submit() was not
> executed, and the io command was never submitted.
> 
> The reason is that virtio_device_disabled returns true in
> virtio_queue_split_empty, the code is as follows:
> 
> int virtio_queue_empty(VirtQueue *vq)
> {   
> 	if (virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED)) {
> 		return virtio_queue_packed_empty(vq);
> 	} else {
> 		return virtio_queue_split_empty(vq);
> 	}
> }
> 
> static int virtio_queue_split_empty(VirtQueue *vq)
> {
> 	bool empty;
>             
> 	if (virtio_device_disabled(vq->vdev)) {
> 		return 1;
> 	...
> }
> 
> This function was introduced in the following patch:
> 
> commit 9d7bd0826f2d19f88631ad7078662668148f7b5f
> Author: Michael Roth <mdroth@linux.vnet.ibm.com>
> Date:   Tue Nov 19 18:50:03 2019 -0600
> 
>     virtio-pci: disable vring processing when bus-mastering is disabled

OK, then it is obviously one qemu issue, and it can explain why IO isn't
completed in this case.



Thanks,
Ming