linux-kernel - Re: [PATCH 4/5] blk-mq: do limited block plug for multiple queue case

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Date:	Mon, 4 May 2015 13:33:59 -0700
From:	Shaohua Li <shli@...com>
To:	Jens Axboe <axboe@...com>
CC:	Jeff Moyer <jmoyer@...hat.com>, <linux-kernel@...r.kernel.org>,
	<hch@....de>, <neilb@...e.de>
Subject: Re: [PATCH 4/5] blk-mq: do limited block plug for multiple queue case

On Mon, May 04, 2015 at 01:46:49PM -0600, Jens Axboe wrote:
> On 05/04/2015 01:40 PM, Shaohua Li wrote:
> >On Fri, May 01, 2015 at 04:16:04PM -0400, Jeff Moyer wrote:
> >>Shaohua Li <shli@...com> writes:
> >>
> >>>plug is still helpful for workload with IO merge, but it can be harmful
> >>>otherwise especially with multiple hardware queues, as there is
> >>>(supposed) no lock contention in this case and plug can introduce
> >>>latency. For multiple queues, we do limited plug, eg plug only if there
> >>>is request merge. If a request doesn't have merge with following
> >>>request, the requet will be dispatched immediately.
> >>>
> >>>This also fixes a bug. If we directly issue a request and it fails, we
> >>>use blk_mq_merge_queue_io(). But we already assigned bio to a request in
> >>>blk_mq_bio_to_request. blk_mq_merge_queue_io shouldn't run
> >>>blk_mq_bio_to_request again.
> >>
> >>Good catch.  Might've been better to split that out first for easy
> >>backport to stable kernels, but I won't hold you to that.
> >
> >It's not a severe bug, but I don't mind. Jens, please let me know if I
> >should split the patch into 2 patches.
> 
> I don't care that much for this particular case. But since one/more
> of the others need respin anyway, might be prudent to split it up in
> any case.

ok, done. I'll repost the patch 4/5. Please let me know if I should
repost 1-3.


>From 5f8117b38ba423a4c746b05d40d3647c73b89a32 Mon Sep 17 00:00:00 2001
Message-Id: <5f8117b38ba423a4c746b05d40d3647c73b89a32.1430771485.git.shli@...com>
From: Shaohua Li <shli@...com>
Date: Mon, 4 May 2015 13:26:55 -0700
Subject: [PATCH] blk-mq: avoid re-initialize request which is failed in direct
 dispatch

If we directly issue a request and it fails, we use
blk_mq_merge_queue_io(). But we already assigned bio to a request in
blk_mq_bio_to_request. blk_mq_merge_queue_io shouldn't run
blk_mq_bio_to_request again.

Signed-off-by: Shaohua Li <shli@...com>
---
 block/blk-mq.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index e1a5b9e..2a411c9 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1294,6 +1294,8 @@ static void blk_mq_make_request(struct request_queue *q, struct bio *bio)
 				blk_mq_end_request(rq, rq->errors);
 				goto done;
 			}
+			blk_mq_insert_request(rq, false, true, true);
+			return;
 		}
 	}
 
-- 
1.8.1

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