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

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

On Tue, Nov 07, 2017 at 08:01:48PM -0700, Jens Axboe wrote:
> On 11/07/2017 06:03 PM, Ming Lei wrote:
> > On Tue, Nov 07, 2017 at 03:06:31PM -0700, Jens Axboe wrote:
> >> 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...
> > 
> > Thanks for running this test ans share us the result.
> > 
> >>
> >> 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.
> > 
> > Just one hint, blk-mq's RESTART won't work if no requests are pending,
> > but looks no efficient/reliable way to handle that.
> 
> Requests are pending, see above output. My assumption/theory was that
> we were racing on setting restart. Simplified version:
> 
> CPUX					CPUY
> get_driver_tag, fails
> 					complete request, put tag
> 					check restart, not set
> mark needing restart

This case should be covered by TAG_WAITING.

If get_driver_tag fails on CPUX, this hctx will be added to tags's wait
queue, then the 'complete request' on CPUY will trigger to run queue via
blk_mq_dispatch_wake().

Looks get_driver_tag() is run again after the hctx is added to tags's
wait queue, so TAG_WAITING should work fine for this case in theory.

-- 
Ming