Re: [PATCH] blk-mq: fix issue with shared tag queue re-running

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

On Thu, Nov 09, 2017 at 09:32:58AM -0700, Jens Axboe wrote:
> On 11/09/2017 08:30 AM, Jens Axboe wrote:
> > On 11/09/2017 03:00 AM, Ming Lei wrote:
> >> On Thu, Nov 09, 2017 at 11:41:40AM +0800, Ming Lei wrote:
> >>> On Wed, Nov 08, 2017 at 03:48:51PM -0700, Jens Axboe wrote:
> >>>> This patch attempts to make the case of hctx re-running on driver tag
> >>>> failure more robust. Without this patch, it's pretty easy to trigger a
> >>>> stall condition with shared tags. An example is using null_blk like
> >>>> this:
> >>>>
> >>>> modprobe null_blk queue_mode=2 nr_devices=4 shared_tags=1 submit_queues=1 hw_queue_depth=1
> >>>>
> >>>> which sets up 4 devices, sharing the same tag set with a depth of 1.
> >>>> Running a fio job ala:
> >>>>
> >>>> [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
> >>>>
> >>>> will inevitably end with one or more threads being stuck waiting for a
> >>>> scheduler tag. That IO is then stuck forever, until someone else
> >>>> triggers a run of the queue.
> >>>>
> >>>> Ensure that we always re-run the hardware queue, if the driver tag we
> >>>> were waiting for got freed before we added our leftover request entries
> >>>> back on the dispatch list.
> >>>>
> >>>> Signed-off-by: Jens Axboe <axboe@kernel.dk>
> >>>>
> >>>> ---
> >>>>
> >>>> 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..8dc5db40df9d 100644
> >>>> --- a/block/blk-mq.c
> >>>> +++ b/block/blk-mq.c
> >>>> @@ -998,49 +998,64 @@ bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx,
> >>>>  	return rq->tag != -1;
> >>>>  }
> >>>>  
> >>>> -static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode, int flags,
> >>>> -				void *key)
> >>>> +static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode,
> >>>> +				int flags, void *key)
> >>>>  {
> >>>>  	struct blk_mq_hw_ctx *hctx;
> >>>>  
> >>>>  	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)
> >>>>  {
> >>>> +	struct blk_mq_hw_ctx *this_hctx = *hctx;
> >>>> +	wait_queue_entry_t *wait = &this_hctx->dispatch_wait;
> >>>>  	struct sbq_wait_state *ws;
> >>>>  
> >>>> +	if (!list_empty_careful(&wait->entry))
> >>>> +		return false;
> >>>> +
> >>>> +	spin_lock(&this_hctx->lock);
> >>>> +	if (!list_empty(&wait->entry)) {
> >>>> +		spin_unlock(&this_hctx->lock);
> >>>> +		return false;
> >>>> +	}
> >>>> +
> >>>> +	ws = bt_wait_ptr(&this_hctx->tags->bitmap_tags, this_hctx);
> >>>> +	add_wait_queue(&ws->wait, wait);
> >>>> +
> >>>>  	/*
> >>>> -	 * 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.
> >>>> +	 * 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 (test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state) ||
> >>>> -	    test_and_set_bit_lock(BLK_MQ_S_TAG_WAITING, &hctx->state))
> >>>> +	if (!blk_mq_get_driver_tag(rq, hctx, false)) {
> >>>> +		spin_unlock(&this_hctx->lock);
> >>>>  		return false;
> >>>> -
> >>>> -	init_waitqueue_func_entry(&hctx->dispatch_wait, blk_mq_dispatch_wake);
> >>>> -	ws = bt_wait_ptr(&hctx->tags->bitmap_tags, hctx);
> >>>> +	}
> >>>>  
> >>>>  	/*
> >>>> -	 * 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.
> >>>> +	 * We got a tag, remove outselves from the wait queue to ensure
> >>>> +	 * someone else gets the wakeup.
> >>>>  	 */
> >>>> -	add_wait_queue(&ws->wait, &hctx->dispatch_wait);
> >>>> +	spin_lock_irq(&ws->wait.lock);
> >>>> +	list_del_init(&wait->entry);
> >>>> +	spin_unlock_irq(&ws->wait.lock);
> >>>> +	spin_unlock(&this_hctx->lock);
> >>>>  	return true;
> >>>>  }
> >>>>  
> >>>>  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 no_tag = false;
> >>>>  	int errors, queued;
> >>>>  
> >>>>  	if (list_empty(list))
> >>>> @@ -1060,22 +1075,15 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
> >>>>  		if (!blk_mq_get_driver_tag(rq, &hctx, false)) {
> >>>>  			/*
> >>>>  			 * The initial allocation attempt failed, so we need to
> >>>> -			 * rerun the hardware queue when a tag is freed.
> >>>> +			 * rerun the hardware queue when a tag is freed. The
> >>>> +			 * waitqueue takes care of that. If the queue is run
> >>>> +			 * before we add this entry back on the dispatch list,
> >>>> +			 * we'll re-run it below.
> >>>>  			 */
> >>>> -			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);
> >>>> +				no_tag = true;
> >>>>  				break;
> >>>>  			}
> >>>>  		}
> >>>> @@ -1140,10 +1148,10 @@ 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 'no_tag' is set, that means that we failed getting
> >>>> +		 * a driver tag with an I/O scheduler attached. If our dispatch
> >>>> +		 * waitqueue is no longer active, ensure that we run the queue
> >>>> +		 * AFTER adding our entries back to the list.
> >>>>  		 *
> >>>>  		 * If no I/O scheduler has been configured it is possible that
> >>>>  		 * the hardware queue got stopped and restarted before requests
> >>>> @@ -1156,7 +1164,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
> >>>>  		 *   and dm-rq.
> >>>>  		 */
> >>>>  		if (!blk_mq_sched_needs_restart(hctx) &&
> >>>> -		    !test_bit(BLK_MQ_S_TAG_WAITING, &hctx->state))
> >>>> +		    (no_tag && list_empty_careful(&hctx->dispatch_wait.entry)))
> >>>>  			blk_mq_run_hw_queue(hctx, true);
> >>>
> >>> If one rq is just completed after the check on list_empty_careful(&hctx->dispatch_wait.entry),
> >>> the queue may not be run any more. May that be an issue?
> >>
> >> Looks that can be an issue.
> >>
> >> If I revert "Revert "blk-mq: don't handle TAG_SHARED in restart"", and
> >> apply 'blk-mq: put driver tag if dispatch budget can't be got' against
> >> for-4.15/block, I still can trigger IO hang in one or two minutes easily:
> >>
> >> 1) script
> >> #!/bin/sh
> >> modprobe null_blk queue_mode=2 nr_devices=4 shared_tags=1 submit_queues=1 hw_queue_depth=1
> >> RUNTIME=10
> >>
> >> while true; do
> >> 	fio --bs=4k --size=512G  --rw=randread --norandommap --direct=1 --ioengine=libaio --iodepth=4 --runtime=$RUNTIME --group_reporting=1  --name=nullb0 --filename=/dev/nullb0 --name=nullb1 --filename=/dev/nullb1 --name=nullb2 --filename=/dev/nullb2 --name=nullb3 --filename=/dev/nullb3
> > 
> > Did you apply my patch too? I had my test case running overnight, and it
> > completed just fine. That's current for-4.15/block + the patch I posted.
> > Previously that would hang in minutes as well.
> > 
> > I'm running your test case now here, but it looks identical to mine.
> > It's been running for 5 min without issue so far, I'll leave it running
> > for an hour or so.
> 
> It's been running happily for > 1 hour now, no issues observed.

Looks there was something wrong in yesterday's test, today I can't reproduce
the issue by running latest for-4.15/block with revert a2820d1544c1, and observed
IOPS is improved >20% compared yesterday's result meantime.

Now looks the new dispatch wake works well, and it seems fine to cover
RESTART for TAG_SHARED completely. In next dev cycle, we may consider to
remove that workaround.

-- 
Ming