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

[Jens Axboe <axboe@kernel.dk>]

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.

-- 
Jens Axboe