On 11/08/2017 08:59 AM, Ming Lei wrote: > 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. This patch works for me, I don't see any more stalls after this happens. On another positive note, for my 4 device test case, it also bumps the per-device performance from (run where it actually finished): nullb0: (groupid=0, jobs=1): err= 0: pid=3540: Wed Nov 8 10:57:43 2017 read: IOPS=73.6k, BW=288MiB/s (302MB/s)(84.3GiB/300001msec) slat (nsec): min=1214, max=911980, avg=12070.86, stdev=15586.24 clat (nsec): min=1637, max=1011.2k, avg=41375.87, stdev=28103.51 lat (usec): min=7, max=1045, avg=53.56, stdev=32.30 clat percentiles (usec): | 1.00th=[ 11], 5.00th=[ 14], 10.00th=[ 17], 20.00th=[ 21], | 30.00th=[ 26], 40.00th=[ 30], 50.00th=[ 35], 60.00th=[ 41], | 70.00th=[ 47], 80.00th=[ 56], 90.00th=[ 73], 95.00th=[ 95], | 99.00th=[ 149], 99.50th=[ 161], 99.90th=[ 190], 99.95th=[ 204], | 99.99th=[ 258] bw ( KiB/s): min=223672, max=368880, per=24.76%, avg=294699.27, stdev=45893.24, samples=599 iops : min=55918, max=92220, avg=73674.76, stdev=11473.32, samples=599 lat (usec) : 2=0.01%, 10=0.89%, 20=17.13%, 50=56.13%, 100=21.42% lat (usec) : 250=4.42%, 500=0.01%, 750=0.01%, 1000=0.01% lat (msec) : 2=0.01% cpu : usr=7.84%, sys=52.94%, ctx=9763310, majf=0, minf=14 to nullb3: (groupid=0, jobs=1): err= 0: pid=3128: Wed Nov 8 11:08:18 2017 read: IOPS=215k, BW=841MiB/s (882MB/s)(246GiB/300001msec) slat (nsec): min=1335, max=739184, avg=3920.20, stdev=2561.74 clat (nsec): min=1039, max=763200, avg=14229.43, stdev=5382.08 lat (usec): min=3, max=772, avg=18.20, stdev= 6.34 clat percentiles (nsec): | 1.00th=[ 7712], 5.00th=[ 9408], 10.00th=[10048], 20.00th=[10816], | 30.00th=[12096], 40.00th=[12864], 50.00th=[13504], 60.00th=[14400], | 70.00th=[15552], 80.00th=[16768], 90.00th=[18816], 95.00th=[21632], | 99.00th=[29312], 99.50th=[31872], 99.90th=[37120], 99.95th=[41216], | 99.99th=[63232] bw ( KiB/s): min=658704, max=1012880, per=26.09%, avg=861735.73, stdev=63136.40, samples=599 iops : min=164676, max=253220, avg=215433.91, stdev=15784.07, samples=599 lat (usec) : 2=0.01%, 4=0.01%, 10=9.52%, 20=83.55%, 50=6.91% lat (usec) : 100=0.02%, 250=0.01%, 750=0.01%, 1000=0.01% cpu : usr=13.23%, sys=56.78%, ctx=33334423, majf=0, minf=14 which is a massive win on all metrics. Patch on top of current for-4.15/block diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c index 7f4a1ba532af..bb7f08415203 100644 --- a/block/blk-mq-debugfs.c +++ b/block/blk-mq-debugfs.c @@ -179,7 +179,6 @@ static const char *const hctx_state_name[] = { HCTX_STATE_NAME(STOPPED), HCTX_STATE_NAME(TAG_ACTIVE), HCTX_STATE_NAME(SCHED_RESTART), - HCTX_STATE_NAME(TAG_WAITING), HCTX_STATE_NAME(START_ON_RUN), }; #undef HCTX_STATE_NAME diff --git a/block/blk-mq.c b/block/blk-mq.c index 3d759bb8a5bb..adb6c8bceb7c 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -1005,42 +1005,53 @@ static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode, int fla hctx = container_of(wait, struct blk_mq_hw_ctx, dispatch_wait); - list_del(&wait->entry); - clear_bit_unlock(BLK_MQ_S_TAG_WAITING, &hctx->state); + list_del_init(&wait->entry); blk_mq_run_hw_queue(hctx, true); return 1; } -static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx *hctx) +static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx **hctx, + struct request *rq) { + wait_queue_entry_t *wait = &(*hctx)->dispatch_wait; struct sbq_wait_state *ws; - /* - * The TAG_WAITING bit serves as a lock protecting hctx->dispatch_wait. - * The thread which wins the race to grab this bit adds the hardware - * queue to the wait queue. - */ - if (test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state) || - test_and_set_bit_lock(BLK_MQ_S_TAG_WAITING, &hctx->state)) + if (!list_empty_careful(&wait->entry)) return false; - init_waitqueue_func_entry(&hctx->dispatch_wait, blk_mq_dispatch_wake); - ws = bt_wait_ptr(&hctx->tags->bitmap_tags, hctx); + spin_lock(&(*hctx)->lock); + if (!list_empty(&wait->entry)) { + spin_unlock(&(*hctx)->lock); + return false; + } + + init_waitqueue_func_entry(wait, blk_mq_dispatch_wake); + ws = bt_wait_ptr(&(*hctx)->tags->bitmap_tags, *hctx); + add_wait_queue(&ws->wait, wait); + spin_unlock(&(*hctx)->lock); /* - * As soon as this returns, it's no longer safe to fiddle with - * hctx->dispatch_wait, since a completion can wake up the wait queue - * and unlock the bit. + * It's possible that a tag was freed in the window between the + * allocation failure and adding the hardware queue to the wait + * queue. If we can get a tag now, remove ourselves from the + * wait queue to ensure someone else gets the wakeup. */ - add_wait_queue(&ws->wait, &hctx->dispatch_wait); - return true; + if (blk_mq_get_driver_tag(rq, hctx, false)) { + spin_lock_irq(&ws->wait.lock); + list_del_init(&wait->entry); + spin_unlock_irq(&ws->wait.lock); + return true; + } + + return false; } bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, - bool got_budget) + bool got_budget) { struct blk_mq_hw_ctx *hctx; struct request *rq, *nxt; + bool failed_tag = false; int errors, queued; if (list_empty(list)) @@ -1062,20 +1073,10 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, * The initial allocation attempt failed, so we need to * rerun the hardware queue when a tag is freed. */ - if (!blk_mq_dispatch_wait_add(hctx)) { - if (got_budget) - blk_mq_put_dispatch_budget(hctx); - break; - } - - /* - * It's possible that a tag was freed in the window - * between the allocation failure and adding the - * hardware queue to the wait queue. - */ - if (!blk_mq_get_driver_tag(rq, &hctx, false)) { + if (!blk_mq_dispatch_wait_add(&hctx, rq)) { if (got_budget) blk_mq_put_dispatch_budget(hctx); + failed_tag = true; break; } } @@ -1140,10 +1141,11 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, * it is no longer set that means that it was cleared by another * thread and hence that a queue rerun is needed. * - * If TAG_WAITING is set that means that an I/O scheduler has - * been configured and another thread is waiting for a driver - * tag. To guarantee fairness, do not rerun this hardware queue - * but let the other thread grab the driver tag. + * If 'failed_tag' is set, that means that we failed getting + * a driver tag with an I/O scheduler attached. Ensure that + * we run the queue AFTER adding our entries back to the list, + * so we don't stall if the queue run and wakeup happened + * before we got here. * * If no I/O scheduler has been configured it is possible that * the hardware queue got stopped and restarted before requests @@ -1155,8 +1157,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list, * returning BLK_STS_RESOURCE. Two exceptions are scsi-mq * and dm-rq. */ - if (!blk_mq_sched_needs_restart(hctx) && - !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state)) + if (!blk_mq_sched_needs_restart(hctx) || failed_tag) blk_mq_run_hw_queue(hctx, true); } diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h index 674641527da7..2f0036b977a9 100644 --- a/include/linux/blk-mq.h +++ b/include/linux/blk-mq.h @@ -181,8 +181,7 @@ enum { BLK_MQ_S_STOPPED = 0, BLK_MQ_S_TAG_ACTIVE = 1, BLK_MQ_S_SCHED_RESTART = 2, - BLK_MQ_S_TAG_WAITING = 3, - BLK_MQ_S_START_ON_RUN = 4, + BLK_MQ_S_START_ON_RUN = 3, BLK_MQ_MAX_DEPTH = 10240, -- Jens Axboe
