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. -- Ming
