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SubjectRe: [PATCH] blk-mq: insert rq with DONTPREP to hctx dispatch list when requeue
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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)) {
--
Jens Axboe

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