Re: [PATCH V2] blk-mq: insert rq with DONTPREP to hctx dispatch list when requeue

["jianchao.wang" <jianchao.w.wang@oracle.com>]

On 2/15/19 11:14 AM, Ming Lei wrote:
> On Fri, Feb 15, 2019 at 10:34:39AM +0800, jianchao.wang wrote:
>> Hi Ming
>>
>> Thanks for your kindly response.
>>
>> On 2/15/19 10:00 AM, Ming Lei wrote:
>>> On Tue, Feb 12, 2019 at 09:56:25AM +0800, Jianchao Wang wrote:
>>>> When requeue, if RQF_DONTPREP, rq has contained some driver
>>>> specific data, so insert it to hctx dispatch list to avoid any
>>>> merge. Take scsi as example, here is the trace event log (no
>>>> io scheduler, because RQF_STARTED would prevent merging),
>>>>
>>>>    kworker/0:1H-339   [000] ...1  2037.209289: block_rq_insert: 8,0 R 4096 () 32768 + 8 [kworker/0:1H]
>>>> scsi_inert_test-1987  [000] ....  2037.220465: block_bio_queue: 8,0 R 32776 + 8 [scsi_inert_test]
>>>> scsi_inert_test-1987  [000] ...2  2037.220466: block_bio_backmerge: 8,0 R 32776 + 8 [scsi_inert_test]
>>>>    kworker/0:1H-339   [000] ....  2047.220913: block_rq_issue: 8,0 R 8192 () 32768 + 16 [kworker/0:1H]
>>>> scsi_inert_test-1996  [000] ..s1  2047.221007: block_rq_complete: 8,0 R () 32768 + 8 [0]
>>>> scsi_inert_test-1996  [000] .Ns1  2047.221045: block_rq_requeue: 8,0 R () 32776 + 8 [0]
>>>>    kworker/0:1H-339   [000] ...1  2047.221054: block_rq_insert: 8,0 R 4096 () 32776 + 8 [kworker/0:1H]
>>>>    kworker/0:1H-339   [000] ...1  2047.221056: block_rq_issue: 8,0 R 4096 () 32776 + 8 [kworker/0:1H]
>>>> scsi_inert_test-1986  [000] ..s1  2047.221119: block_rq_complete: 8,0 R () 32776 + 8 [0]
>>>>
>>>> (32768 + 8) was requeued by scsi_queue_insert and had RQF_DONTPREP.
>>>
>>> scsi_mq_requeue_cmd() does uninit the request before requeuing, but
>>> __scsi_queue_insert doesn't do that.
>>
>> Yes.
>> scsi layer use both of them.
>>
>>>
>>>
>>>> Then it was merged with (32776 + 8) and issued. Due to RQF_DONTPREP,
>>>> the sdb only contained the part of (32768 + 8), then only that part
>>>> was completed. The lucky thing was that scsi_io_completion detected
>>>> it and requeued the remaining part. So we didn't get corrupted data.
>>>> However, the requeue of (32776 + 8) is not expected.
>>>>
>>>> Suggested-by: Jens Axboe <axboe@kernel.dk>
>>>> Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
>>>> ---
>>>> V2:
>>>>  - refactor the code based on Jens' suggestion
>>>>
>>>>  block/blk-mq.c | 12 ++++++++++--
>>>>  1 file changed, 10 insertions(+), 2 deletions(-)
>>>>
>>>> diff --git a/block/blk-mq.c b/block/blk-mq.c
>>>> index 8f5b533..9437a5e 100644
>>>> --- a/block/blk-mq.c
>>>> +++ b/block/blk-mq.c
>>>> @@ -737,12 +737,20 @@ static void blk_mq_requeue_work(struct work_struct *work)
>>>>  	spin_unlock_irq(&q->requeue_lock);
>>>>  
>>>>  	list_for_each_entry_safe(rq, next, &rq_list, queuelist) {
>>>> -		if (!(rq->rq_flags & RQF_SOFTBARRIER))
>>>> +		if (!(rq->rq_flags & (RQF_SOFTBARRIER | RQF_DONTPREP)))
>>>>  			continue;
>>>>  
>>>>  		rq->rq_flags &= ~RQF_SOFTBARRIER;
>>>>  		list_del_init(&rq->queuelist);
>>>> -		blk_mq_sched_insert_request(rq, true, false, false);
>>>> +		/*
>>>> +		 * If RQF_DONTPREP, rq has contained some driver specific
>>>> +		 * data, so insert it to hctx dispatch list to avoid any
>>>> +		 * merge.
>>>> +		 */
>>>> +		if (rq->rq_flags & RQF_DONTPREP)
>>>> +			blk_mq_request_bypass_insert(rq, false);
>>>> +		else
>>>> +			blk_mq_sched_insert_request(rq, true, false, false);
>>>>  	}
>>>
>>> Suppose it is one WRITE request to zone device, this way might break
>>> the order.
>>
>> I'm not sure about this.
>> Since the request is dispatched, it should hold and zone write lock.
>> And also mq-deadline doesn't have a .requeue_request, zone write lock
>> wouldn't be released during requeue.
> 
> You are right, looks I misunderstood the zone write lock, sorry for
> the noise.
> 
>>
>> IMO, this requeue action is similar with what blk_mq_dispatch_rq_list does.
>> The latter one also issues the request to underlying driver and requeue rqs
>> on dispatch_list if get BLK_STS_SOURCE or BLK_STS_DEV_SOURCE.
>>
>> And in addition, RQF_STARTED is set by io scheduler .dispatch_request and
>> it could be stop merging as RQF_NOMERGE_FLAGS contains it. 
> 
> Yes, that is correct.
> 
> Then another question is:
> 
> Why don't always requeue request in this way so that it can be simplified
> into one code path?
> 
> 1) in block legacy code, blk_requeue_request() doesn't insert the
> request into scheduler queue, and simply put the request into
> q->queue_head.
> 
> 2) blk_mq_requeue_request() is basically run from completion context for
> handling very unusual cases(partial completion, error, timeout, ...),
> and there shouldn't have benefit to schedule/merge requeued request.

Actually, I'm also confused about questions above when I looked into the code before :)

> 
> 3) RQF_DONTPREP is like a driver private flag, and read/write by driver
> only before this patch.

Yes, indeed.
And it tells us there is driver specific data in the request.

Thanks
Jianchao