Re: [PATCH] SCSI: don't get target/host busy_count in scsi_mq_get_budget()

Jens Axboe
2017-11-07T17:36:04Z

On 11/07/2017 10:10 AM, Jens Axboe wrote:
> On 11/07/2017 09:29 AM, Jens Axboe wrote:
>> On 11/07/2017 09:20 AM, Bart Van Assche wrote:
>>> On Tue, 2017-11-07 at 10:11 +0800, Ming Lei wrote:
>>>> If you can reproduce, please provide me at least the following log
>>>> first:
>>>>
>>>> find /sys/kernel/debug/block -name tags | xargs cat | grep busy
>>>>
>>>> If any pending requests arn't completed, please provide the related
>>>> info in dbgfs about where is the request.
>>>
>>> Every time I ran the above or a similar command its output was empty. I
>>> assume that's because the hang usually occurs in a phase where these debugfs
>>> attributes either have not yet been created or have already disappeared.
>>
>> Bart, do you still see a hang with the patch that fixes the tag leak when
>> we fail to get a dispatch budget?
>>
>> https://marc.info/?l=linux-block&m=151004881411480&w=2
>>
>> I've run a lot of stability testing here, and I haven't run into any
>> issues. This is with shared tags as well. So if you still see the failure
>> case with the current tree AND the above patch, then I'll try and get
>> a test case setup that hits it too so we can get to the bottom of this.
>
> Ming, I managed to reproduce the hang using null_blk. Note this is
> WITHOUT the patch mentioned above, running with that now.
>
> # modprobe null_blk queue_mode=2 nr_devices=4 shared_tags=1 submit_queues=1 hw_queue_depth=1
>
> and using this fio file:
>
> [global]
> bs=4k
> rw=randread
> norandommap
> direct=1
> ioengine=libaio
> iodepth=4
>
> [nullb0]
> filename=/dev/nullb0
> [nullb1]
> filename=/dev/nullb1
> [nullb2]
> filename=/dev/nullb2
> [nullb3]
> filename=/dev/nullb3
>
> it seemed to keep running, but it hung when exiting. The troublesome
> device was nullb1:
>
> [ 492.513374] INFO: task fio:3263 blocked for more than 120 seconds.
> [ 492.520782] Not tainted 4.14.0-rc7+ #499
> [ 492.526247] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
> [ 492.535904] fio D13208 3263 3211 0x00000000
> [ 492.542535] Call Trace:
> [ 492.545764] __schedule+0x279/0x720
> [ 492.550151] schedule+0x33/0x90
> [ 492.554145] io_schedule+0x16/0x40
> [ 492.558435] blk_mq_get_tag+0x148/0x250
> [ 492.563211] ? finish_wait+0x90/0x90
> [ 492.567693] blk_mq_get_request+0xf0/0x3e0
> [ 492.572760] blk_mq_make_request+0xe2/0x690
> [ 492.577913] generic_make_request+0xfc/0x2f0
> [ 492.583177] submit_bio+0x64/0x120
> [ 492.587475] ? set_page_dirty_lock+0x4b/0x60
> [ 492.592736] ? submit_bio+0x64/0x120
> [ 492.597309] ? bio_set_pages_dirty+0x55/0x60
> [ 492.602570] blkdev_direct_IO+0x388/0x3c0
> [ 492.607546] ? free_ioctx_users+0xe0/0xe0
> [ 492.612511] ? blk_mq_dispatch_rq_list+0x238/0x3a0
> [ 492.618353] ? _raw_spin_unlock+0xe/0x20
> [ 492.623227] generic_file_read_iter+0xb3/0xa00
> [ 492.628682] ? generic_file_read_iter+0xb3/0xa00
> [ 492.634334] ? security_file_permission+0x9b/0xc0
> [ 492.640114] blkdev_read_iter+0x35/0x40
> [ 492.644877] aio_read+0xc5/0x120
> [ 492.648973] ? aio_read_events+0x24c/0x340
> [ 492.654124] ? __might_sleep+0x4a/0x80
> [ 492.658800] do_io_submit+0x47c/0x5e0
> [ 492.663373] ? do_io_submit+0x47c/0x5e0
> [ 492.668234] SyS_io_submit+0x10/0x20
> [ 492.672715] ? SyS_io_submit+0x10/0x20
> [ 492.677394] entry_SYSCALL_64_fastpath+0x13/0x94
> [ 492.683039] RIP: 0033:0x7f83d1871717
> [ 492.687521] RSP: 002b:00007ffd38fe5a88 EFLAGS: 00000202 ORIG_RAX: 00000000000000d1
> [ 492.696969] RAX: ffffffffffffffda RBX: 00007f83b6972b50 RCX: 00007f83d1871717
> [ 492.705423] RDX: 0000000001f41418 RSI: 0000000000000001 RDI: 00007f83e4d36000
> [ 492.713889] RBP: 0000000000000001 R08: 0000000000000001 R09: 0000000001f3f2e0
> [ 492.722352] R10: 0000000000001000 R11: 0000000000000202 R12: 00007ffd38fe5be0
> [ 492.730827] R13: 00007f83b6972b01 R14: 00007f83b69824b8 R15: 00007f83b6982368
>
> and if we look at the debug entries, it's waiting on a scheduler tag:
>
> sched_tags:nr_tags=2
> sched_tags:nr_reserved_tags=0
> sched_tags:active_queues=0
> sched_tags:bitmap_tags:
> sched_tags:depth=2
> sched_tags:busy=2
> sched_tags:bits_per_word=64
> sched_tags:map_nr=1
> sched_tags:alloc_hint={0, 0, 0, 0, 0, 1, 0, 0, 1, 0, 1, 1, 0, 0, 0, 0, 1, 0, 0, 1, 1, 1, 0, 0, 0, 1, 0, 1, 0, 0, 1, 0, 0, 0, 0, 0, 0, 1, 1, 0, 1, 1, 1, 1, 0, 0, 1, 1, 0, 0, 0, 1, 1, 1, 1, 0, 1, 0, 1, 1, 0, 0, 1, 0}
> sched_tags:wake_batch=1
> sched_tags:wake_index=4
> sched_tags:ws={
> sched_tags: {.wait_cnt=-102, .wait=inactive},
> sched_tags: {.wait_cnt=-126, .wait=inactive},
> sched_tags: {.wait_cnt=-72, .wait=inactive},
> sched_tags: {.wait_cnt=-96, .wait=inactive},
> sched_tags: {.wait_cnt=-134, .wait=inactive},
> sched_tags: {.wait_cnt=-116, .wait=inactive},
> sched_tags: {.wait_cnt=-90, .wait=inactive},
> sched_tags: {.wait_cnt=-115, .wait=active},
> sched_tags:}
> sched_tags:round_robin=0
> sched_tags_bitmap:00000000: 03
>
> with SCHED_RESTART being set:
>
> state:SCHED_RESTART
>
> and with the driver tags being idle:
>
> tags:nr_tags=1
> tags:nr_reserved_tags=0
> tags:active_queues=0
> tags:bitmap_tags:
> tags:depth=1
> tags:busy=0
> tags:bits_per_word=64
> tags:map_nr=1
> tags:alloc_hint={0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0}
> tags:wake_batch=1
> tags:wake_index=3
> tags:ws={
> tags: {.wait_cnt=-48, .wait=inactive},
> tags: {.wait_cnt=-39, .wait=inactive},
> tags: {.wait_cnt=-50, .wait=inactive},
> tags: {.wait_cnt=-47, .wait=inactive},
> tags: {.wait_cnt=-25, .wait=inactive},
> tags: {.wait_cnt=-24, .wait=inactive},
> tags: {.wait_cnt=-47, .wait=inactive},
> tags: {.wait_cnt=-47, .wait=inactive},
> tags:}

Unsurprisingly (since this is sched_tags starvation) this still happens
with the patch. Same trace as above. Note that dispatch has two requests
sitting ready:

ffff881ff5ab1000 {.op=READ, .cmd_flags=, .rq_flags=STARTED|IO_STAT, .atomic_flags=COMPLETE, .tag=-1, .internal_tag=0}
ffff881ff5ab1240 {.op=READ, .cmd_flags=, .rq_flags=STARTED|IO_STAT, .atomic_flags=COMPLETE, .tag=-1, .internal_tag=1}

while we're blocked trying to get a new sched tag off the regular
blk_mq_make_request() path. It looks like a missing restart of the hctx.

--
Jens Axboe