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

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Date:   Tue, 12 Feb 2019 09:56:56 +0800
From:   "jianchao.wang" <jianchao.w.wang@...cle.com>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] blk-mq: insert rq with DONTPREP to hctx dispatch list
 when requeue

Hi Jens

Thanks for your kindly response.

On 2/12/19 7:20 AM, Jens Axboe wrote:
> On 2/11/19 4:15 PM, Jens Axboe wrote:
>> On 2/11/19 8:59 AM, Jens Axboe wrote:
>>> On 2/10/19 10:41 PM, 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.
>>>> 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.
>>>
>>> Good catch, but how about something like this? Makes it more integrated,
>>> I think that's cleaner.
>>
>> This is probably better (and safer):
> 
> Here's the one I wanted to send, not a half done one. Maybe I'll be
> luckier this time around?
> 
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 8f5b533764ca..35e6aba52808 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -737,12 +737,21 @@ 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 is set, rq may contain some driver
> +		 * specific data. 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);
>  	}
>  
>  	while (!list_empty(&rq_list)) {
> 

The test is OK.
And I will send out the V2 based on this.

Thanks
Jianchao