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* [PATCH V2] blk-mq: insert rq with DONTPREP to hctx dispatch list when requeue
@ 2019-02-12  1:56 Jianchao Wang
  2019-02-12  2:51 ` Jens Axboe
  2019-02-15  2:00 ` Ming Lei
  0 siblings, 2 replies; 6+ messages in thread
From: Jianchao Wang @ 2019-02-12  1:56 UTC (permalink / raw)
  To: axboe; +Cc: linux-block, linux-kernel

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.

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);
 	}
 
 	while (!list_empty(&rq_list)) {
-- 
2.7.4


^ permalink raw reply related	[flat|nested] 6+ messages in thread

end of thread, other threads:[~2019-02-15  3:39 UTC | newest]

Thread overview: 6+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-02-12  1:56 [PATCH V2] blk-mq: insert rq with DONTPREP to hctx dispatch list when requeue Jianchao Wang
2019-02-12  2:51 ` Jens Axboe
2019-02-15  2:00 ` Ming Lei
2019-02-15  2:34   ` jianchao.wang
2019-02-15  3:14     ` Ming Lei
2019-02-15  3:41       ` jianchao.wang

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