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

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

Hi Shinichiro,

On Thu, Mar 05, 2020 at 01:19:02AM +0000, Shinichiro Kawasaki wrote:
> On Mar 04, 2020 / 17:53, Ming Lei wrote:
> > 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;
> >  	}
> 
> Ming, thank you for the trial patches.
> This "patch 1" reduced the runtime, as short as rc3.
> 
> > 
> > 
> > 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);
> >  
> >  		/*
> 
> This patch 2 didn't reduce the runtime.
> 
> Wish this report helps.

Your feedback does help, then please test the following patch:

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 5cc775bdb06a..68957802f96f 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -334,7 +334,7 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
 	flush_rq->rq_disk = first_rq->rq_disk;
 	flush_rq->end_io = flush_end_io;
 
-	blk_flush_queue_rq(flush_rq, false);
+	blk_flush_queue_rq(flush_rq, true);
 }
 
 static void mq_flush_data_end_io(struct request *rq, blk_status_t error)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index d92088dec6c3..56d61b693f2e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -724,6 +724,8 @@ 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) {
+		bool at_head = !!(rq->rq_flags & RQF_SOFTBARRIER);
+
 		if (!(rq->rq_flags & (RQF_SOFTBARRIER | RQF_DONTPREP)))
 			continue;
 
@@ -735,9 +737,9 @@ static void blk_mq_requeue_work(struct work_struct *work)
 		 * merge.
 		 */
 		if (rq->rq_flags & RQF_DONTPREP)
-			blk_mq_request_bypass_insert(rq, false, false);
+			blk_mq_request_bypass_insert(rq, at_head, false);
 		else
-			blk_mq_sched_insert_request(rq, true, false, false);
+			blk_mq_sched_insert_request(rq, at_head, false, false);
 	}
 
 	while (!list_empty(&rq_list)) {

Thanks,
Ming