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

Ming Lei
2020-03-05T02:48:25Z

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