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Subjectv4.15 and I/O hang with BFQ
Hi, Paolo, Ivan, Ming et al.

It looks like I've just encountered the issue Ivan has already described
in [1]. Since I'm able to reproduce it reliably in a VM, I'd like to
draw more attention to it.

First, I'm using v4.15 kernel with all pending BFQ fixes:

===
2ad909a300c4 bfq-iosched: don't call bfqg_and_blkg_put for
!CONFIG_BFQ_GROUP_IOSCHED
83c97a310f83 block, bfq: release oom-queue ref to root group on exit
5b9eb4716af1 block, bfq: put async queues for root bfq groups too
3c5529454a27 block, bfq: limit sectors served with interactive weight
raising
e6c72be3486b block, bfq: limit tags for writes and async I/O
e579b91d96ce block, bfq: increase threshold to deem I/O as random
f6cbc16aac88 block, bfq: remove superfluous check in queue-merging setup
8045d8575183 block, bfq: let a queue be merged only shortly after
starting I/O
242954975f5e block, bfq: check low_latency flag in bfq_bfqq_save_state()
8349c1bddd95 block, bfq: add missing rq_pos_tree update on rq removal
558200440cb9 block, bfq: fix occurrences of request finish method's old
name
6ed2f47ee870 block, bfq: consider also past I/O in soft real-time
detection
e5f295dd18f2 block, bfq: remove batches of confusing ifdefs
===

Next, I boot an Arch VM with this kernel and emulated USB stick
attached:

===
qemu-system-x86_64 -display gtk,gl=on -machine q35,accel=kvm -cpu
host,+vmx -enable-kvm -drive
if=pflash,format=raw,readonly,file=/mnt/vms/ovmf/code.img -drive
if=pflash,format=raw,file=/mnt/vms/ovmf/vars.img -cdrom
/mnt/vms/ovmf/shell.iso -netdev user,id=user.0 -device
virtio-net,netdev=user.0 -usb -device nec-usb-xhci,id=xhci -device
usb-tablet,bus=xhci.0 -serial stdio -m 512 -hda sda.img -hdb sdb.img
-smp 4 -drive if=none,id=stick,file=usb.img -device
usb-storage,bus=xhci.0,drive=stick
===

Within the VM itself I use udev rule to set the I/O scheduler:

===
ACTION=="add|change", KERNEL=="sd[a-z]", ATTR{queue/scheduler}="bfq"
===

Things boot and work fine until I try to create a partition on the
emulated USB stick with cfdisk. On writing a new partition table it
blocks, and I get the following stacktrace:

===
[ 225.670702] INFO: task cfdisk:416 blocked for more than 20 seconds.
[ 225.681427] Not tainted 4.15.0-pf1 #1
[ 225.685341] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[ 225.691910] cfdisk D 0 416 407 0x00000000
[ 225.700777] Call Trace:
[ 225.703654] ? __schedule+0x35f/0x1000
[ 225.706990] ? __switch_to_asm+0x40/0x70
[ 225.709943] ? __switch_to_asm+0x34/0x70
[ 225.712224] ? __switch_to_asm+0x40/0x70
[ 225.714225] ? __switch_to_asm+0x40/0x70
[ 225.716790] schedule+0x32/0xc0
[ 225.718355] io_schedule+0x12/0x40
[ 225.719747] wait_on_page_bit+0xea/0x130
[ 225.721266] ? add_to_page_cache_lru+0xe0/0xe0
[ 225.722622] __filemap_fdatawait_range+0xbf/0x110
[ 225.724625] ? preempt_count_sub+0x50/0x90
[ 225.726591] ? sync_inodes_one_sb+0x20/0x20
[ 225.727507] filemap_fdatawait_keep_errors+0x1a/0x40
[ 225.728491] iterate_bdevs+0xa7/0x140
[ 225.729304] sys_sync+0x7c/0xb0
[ 225.730095] entry_SYSCALL_64_fastpath+0x20/0x83
[ 225.732420] RIP: 0033:0x7f2631cf4a17
===

I've tried it several times with the same result. Next, I reboot the
system, change the scheduler to Kyber, and then I can create a new
partition successfully.

I also went further. While having Kyber activated to work with an
emulated USB stick properly, I create RAID10 on it, then I create LVM
PV, then VG, then LV, then XFS on top of that, then I set I/O scheduler
back to BFQ, then reboot, and on reboot mdadm gets blocked while
discovering things, producing the following stacktrace:

===
[ 41.350763] INFO: task mdadm:354 blocked for more than 20 seconds.
[ 41.356154] Not tainted 4.15.0-pf1 #1
[ 41.358674] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs"
disables this message.
[ 41.363046] mdadm D 0 354 352 0x00000100
[ 41.368700] Call Trace:
[ 41.370417] ? __schedule+0x35f/0x1000
[ 41.372668] ? blk_queue_exit+0x3e/0x60
[ 41.374816] ? generic_make_request+0x12f/0x2d0
[ 41.377363] schedule+0x32/0xc0
[ 41.380416] io_schedule+0x12/0x40
[ 41.382698] __blkdev_direct_IO_simple+0x206/0x360
[ 41.382707] ? bdget+0x120/0x120
[ 41.382714] ? blkdev_direct_IO+0x3a5/0x3f0
[ 41.382718] blkdev_direct_IO+0x3a5/0x3f0
[ 41.382724] ? current_time+0x18/0x70
[ 41.382731] ? __atime_needs_update+0x7f/0x190
[ 41.382743] ? generic_file_read_iter+0x8c/0x9d0
[ 41.382747] generic_file_read_iter+0x8c/0x9d0
[ 41.382759] ? __seccomp_filter+0x3b/0x260
[ 41.382765] __vfs_read+0xf5/0x170
[ 41.382772] vfs_read+0x91/0x130
[ 41.382778] SyS_read+0x52/0xc0
[ 41.382819] do_syscall_64+0x67/0x1a0
[ 41.382828] entry_SYSCALL64_slow_path+0x25/0x25
[ 41.382833] RIP: 0033:0x7f4b8088a3a1
[ 41.382836] RSP: 002b:00007ffdd681def8 EFLAGS: 00000246 ORIG_RAX:
0000000000000000
[ 41.382841] RAX: ffffffffffffffda RBX: 0000000000000000 RCX:
00007f4b8088a3a1
[ 41.382844] RDX: 0000000000001000 RSI: 00000000007f0000 RDI:
0000000000000003
[ 41.382846] RBP: 0000000000000003 R08: 0000000000000003 R09:
00007f4b80b55050
[ 41.382848] R10: 000000000000038b R11: 0000000000000246 R12:
00000000007eeb10
[ 41.382850] R13: 0000000000000000 R14: 00000000007f0000 R15:
00000000007eeb10
===

If I change I/O scheduler back to Kyber, the system boots okay.

Again, since I'm able to reproduce the issue reliably, let me now what
information I should also provide, and as usual I'm ready to test any
patches or suggestions.

Important note: I was unable to reproduce this on v4.14 with the same
set of BFQ patches (exactly the same case as Ivan has already
described).

Thanks.

Regards,
Oleksandr

[1] https://lkml.org/lkml/2017/12/1/80

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