Re: commit 01e99aeca397 causes longer runtime of block/004

[Ming Lei <ming.lei@redhat.com>]

On Wed, Mar 04, 2020 at 06:11:37AM +0000, Shinichiro Kawasaki wrote:
> On Mar 04, 2020 / 11:46, Ming Lei wrote:
> > On Wed, Mar 04, 2020 at 02:38:43AM +0000, Shinichiro Kawasaki wrote:
> > > I noticed that blktests block/004 takes longer runtime with 5.6-rc4 than
> > > 5.6-rc3, and found that the commit 01e99aeca397 ("blk-mq: insert passthrough
> > > request into hctx->dispatch directly") triggers it.
> > > 
> > > The longer runtime was observed with dm-linear device which maps SATA SMR HDD
> > > connected via AHCI. It was not observed with dm-linear on SAS/SATA SMR HDDs
> > > connected via SAS-HBA. Not observed with dm-linear on non-SMR HDDs either.
> > > 
> > > Before the commit, block/004 took around 130 seconds. After the commit, it takes
> > > around 300 seconds. I need to dig in further details to understand why the
> > > commit makes the test case longer.
> > > 
> > > The test case block/004 does "flush intensive workload". Is this longer runtime
> > > expected?
> > 
> > The following patch might address this issue:
> > 
> > https://lore.kernel.org/linux-block/20200207190416.99928-1-sqazi@google.com/#t
> > 
> > Please test and provide us the result.
> > 
> > thanks,
> > Ming
> >
> 
> Hi Ming,
> 
> I applied the patch to 5.6-rc4 but I observed the longer runtime of block/004.
> Still it takes around 300 seconds.

Hello Shinichiro,

block/004 only sends 1564 sync randwrite, and seems 130s has been slow
enough.

There are two related effect in that commit for your issue:

1) 'at_head' is applied in blk_mq_sched_insert_request() for flush
request

2) all IO is added back to tail of hctx->dispatch after .queue_rq()
returns STS_RESOURCE

Seems it is more related with 2) given you can't reproduce the issue on 
SAS.

So please test the following two patches, and see which one makes a
difference for you.

BTW, both two looks not reasonable, just for narrowing down the issue.

1) patch 1

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 856356b1619e..86137c75283c 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -398,7 +398,7 @@ void blk_mq_sched_insert_request(struct request *rq, bool at_head,
 	WARN_ON(e && (rq->tag != -1));
 
 	if (blk_mq_sched_bypass_insert(hctx, !!e, rq)) {
-		blk_mq_request_bypass_insert(rq, at_head, false);
+		blk_mq_request_bypass_insert(rq, true, false);
 		goto run;
 	}


2) patch 2
diff --git a/block/blk-mq.c b/block/blk-mq.c
index d92088dec6c3..447d5cb39832 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1286,7 +1286,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
 			q->mq_ops->commit_rqs(hctx);
 
 		spin_lock(&hctx->lock);
-		list_splice_tail_init(list, &hctx->dispatch);
+		list_splice_init(list, &hctx->dispatch);
 		spin_unlock(&hctx->lock);
 
 		/*


Thanks,
Ming