On Fri, Mar 11, 2022 at 06:24:41AM +0000, Shinichiro Kawasaki wrote: > On Mar 10, 2022 / 05:47, Jens Axboe wrote: > > On 3/10/22 5:40 AM, Shinichiro Kawasaki wrote: > > > On Mar 10, 2022 / 18:00, Ming Lei wrote: > > >> On Thu, Mar 10, 2022 at 09:16:50AM +0000, Shinichiro Kawasaki wrote: > > >>> This issue does not look critical, but let me share it to ask comments for fix. > > >>> > > >>> When fio command with 40 jobs [1] is run for a null_blk device with memory > > >>> backing and mq-deadline scheduler, kernel reports a BUG message [2]. The > > >>> workqueue watchdog reports that kblockd blk_mq_run_work_fn keeps on running > > >>> more than 30 seconds and other work can not run. The 40 fio jobs keep on > > >>> creating many read requests to a single null_blk device, then the every time > > >>> the mq_run task calls __blk_mq_do_dispatch_sched(), it returns ret == 1 which > > >>> means more than one request was dispatched. Hence, the while loop in > > >>> blk_mq_do_dispatch_sched() does not break. > > >>> > > >>> static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) > > >>> { > > >>> int ret; > > >>> > > >>> do { > > >>> ret = __blk_mq_do_dispatch_sched(hctx); > > >>> } while (ret == 1); > > >>> > > >>> return ret; > > >>> } > > >>> > > >>> The BUG message was observed when I ran blktests block/005 with various > > >>> conditions on a system with 40 CPUs. It was observed with kernel version > > >>> v5.16-rc1 through v5.17-rc7. The trigger commit was 0a593fbbc245 ("null_blk: > > >>> poll queue support"). This commit added blk_mq_ops.map_queues callback. I > > >>> guess it changed dispatch behavior for null_blk devices and triggered the > > >>> BUG message. > > >> > > >> It is one blk-mq soft lockup issue in dispatch side, and shouldn't be related > > >> with 0a593fbbc245. > > >> > > >> If queueing requests is faster than dispatching, the issue will be triggered > > >> sooner or later, especially easy to trigger in SQ device. I am sure it can > > >> be triggered on scsi debug, even saw such report on ahci. > > > > > > Thank you for the comments. Then this is the real problem. > > > > > >> > > >>> > > >>> I'm not so sure if we really need to fix this issue. It does not seem the real > > >>> world problem since it is observed only with null_blk. The real block devices > > >>> have slower IO operation then the dispatch should stop sooner when the hardware > > >>> queue gets full. Also the 40 jobs for single device is not realistic workload. > > >>> > > >>> Having said that, it does not feel right that other works are pended during > > >>> dispatch for null_blk devices. To avoid the BUG message, I can think of two > > >>> fix approaches. First one is to break the while loop in blk_mq_do_dispatch_sched > > >>> using a loop counter [3] (or jiffies timeout check). > > >> > > >> This way could work, but the queue need to be re-run after breaking > > >> caused by max dispatch number. cond_resched() might be the simplest way, > > >> but it can't be used here because of rcu/srcu read lock. > > > > > > As far as I understand, blk_mq_run_work_fn() should return after the loop break > > > to yield the worker to other works. How about to call > > > blk_mq_delay_run_hw_queue() at the loop break? Does this re-run the dispatch? > > > > > > > > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c > > > index 55488ba978232..faa29448a72a0 100644 > > > --- a/block/blk-mq-sched.c > > > +++ b/block/blk-mq-sched.c > > > @@ -178,13 +178,19 @@ static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) > > > return !!dispatched; > > > } > > > > > > +#define MQ_DISPATCH_MAX 0x10000 > > > + > > > static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) > > > { > > > int ret; > > > + unsigned int count = MQ_DISPATCH_MAX; > > > > > > do { > > > ret = __blk_mq_do_dispatch_sched(hctx); > > > - } while (ret == 1); > > > + } while (ret == 1 && count--); > > > + > > > + if (ret == 1 && !count) > > > + blk_mq_delay_run_hw_queue(hctx, 0); > > > > > > return ret; > > > } > > > > Why not just gate it on needing to reschedule, rather than some random > > value? > > > > static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx) > > { > > int ret; > > > > do { > > ret = __blk_mq_do_dispatch_sched(hctx); > > } while (ret == 1 && !need_resched()); > > > > if (ret == 1 && need_resched()) > > blk_mq_delay_run_hw_queue(hctx, 0); > > > > return ret; > > } > > > > or something like that. > > Jens, thanks for the idea, but need_resched() check does not look working here. > I tried the code above but still the BUG message is observed. My guess is that > in the call stack below, every __blk_mq_do_dispatch_sched() call results in > might_sleep_if() call, then need_resched() does not work as expected, probably. > > __blk_mq_do_dispatch_sched > blk_mq_dispatch_rq_list > q->mq_ops->queue_rq > null_queue_rq > might_sleep_if > > Now I'm trying to find similar way as need_resched() to avoid the random number. > So far I haven't found good idea yet. Jens patch using need_resched() looks improving the situation, also the scsi_debug case won't set BLOCKING: 1) without the patch, it can be easy to trigger lockup with the following test. - modprobe scsi_debug virtual_gb=128 delay=0 dev_size_mb=2048 - fio --bs=512k --ioengine=sync --iodepth=128 --numjobs=4 --rw=randrw \ --name=sdc-sync-randrw-512k --filename=/dev/sdc --direct=1 --size=60G --runtime=120 #sdc is the created scsi_debug disk 2) with the patch, lockup still can be triggered, just not as easy as before 3) trace need_resched() from kretprobe:__blk_mq_do_dispatch_sched() in kblockd wq context, I observed that just 20~30 times resched gets set when running the above script(120sec), so maybe need_resched() isn't set during heavy IO dispatching. Thanks, Ming