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

Ming Lei
2017-11-08T15:59:10Z

On Wed, Nov 08, 2017 at 02:20:41PM +0800, Ming Lei wrote:
> On Tue, Nov 07, 2017 at 08:17:59PM -0700, Jens Axboe wrote:
> > On 11/07/2017 08:12 PM, Ming Lei wrote:
> > > 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.
> >
> > Right. The keyword here being 'should'.
> >
> > > 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().
> >
> > I know how it's _supposed_ to work. I'm saying that the debug info
> > indicates that we have a problem in there, which matches what the
> > TAG_WAITING should be covering for us. The TAG_WAITING may be fine and
> > the issue could be somewhere else, it just looks identical for now.
>
> One possible reason:
>
> - the request to be dispatched may not be added to hctx->dispatch
> when blk_mq_dispatch_wake() is triggered.

If we call blk_mq_dispatch_wait_add() after request is added to hctx->dispatch,
TAG_WAITING becomes basically similar with SCHED_RESTART.

Now I don't object to revert 358a3a6bccb7(blk-mq: don't handle TAG_SHARED in restart)
any more, because SCHED_RESTART works fine from this viewpoint even
though it isn't efficient. We can improve it in the future.

Also we might need to unify both TAG_WAITING and SCHED_RESTART into one generic
restart mechanism, and make it efficient. IMO, both two aren't efficient enough.

--
Ming