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[66.29.188.166]) by smtp.gmail.com with ESMTPSA id g128sm8693659pfb.121.2019.02.11.15.15.45 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 11 Feb 2019 15:15:46 -0800 (PST) Subject: Re: [PATCH] blk-mq: insert rq with DONTPREP to hctx dispatch list when requeue From: Jens Axboe To: Jianchao Wang Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org References: <1549863665-1691-1-git-send-email-jianchao.w.wang@oracle.com> <937e9207-dcf7-dab5-1917-8a10303edc67@kernel.dk> Message-ID: <82deee13-a4ff-9bf5-5e01-0e4df01b7ff7@kernel.dk> Date: Mon, 11 Feb 2019 16:15:44 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.4.0 MIME-Version: 1.0 In-Reply-To: <937e9207-dcf7-dab5-1917-8a10303edc67@kernel.dk> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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): diff --git a/block/blk-mq.c b/block/blk-mq.c index 8f5b533764ca..b3908eb3881c 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_sched_insert_request(rq, true, false, false); + else + blk_mq_request_bypass_insert(rq, false); } while (!list_empty(&rq_list)) { -- Jens Axboe