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

[Jens Axboe <axboe@kernel.dk>]

On 11/07/2017 10:36 AM, Jens Axboe wrote:
> 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.

Just to keep everyone in the loop, this bug is not new to
for-4.15/block, nor is it new to the current 4.41-rc or 4.13. So it's
probably different to what Bart is hitting, but it's a bug none the
less...

To me, this looks like a race between marking the queue as needing a
restart, and the driver tag being released and re-running the queue. If
I do:

# echo run > /sys/kernel/debug/block/nullb1/state

then it starts just fine. So there must be a race between marking the
need for a restart (when getting the driver tag fails), and when one of
the shared tag queues finishes a request and releases the driver tag.

-- 
Jens Axboe