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

Ming Lei
2020-03-06T08:13:28Z

On Fri, Mar 06, 2020 at 06:06:23AM +0000, Shinichiro Kawasaki wrote:
> On Mar 05, 2020 / 10:48, Ming Lei wrote:
> > 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:
>
> Hi Ming, thank you for the patch. I applied it on top of rc4 and confirmed
> it reduces the runtime as short as rc3. Good.

Hi Shinichiro,

Thanks for your test!

Then I think the following change should make the difference actually,
you may double check that and confirm if it is that.

> @@ -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);

However, the flush request is added to tail of dispatch queue[1] for long time.
0cacba6cf825 ("blk-mq-sched: bypass the scheduler for flushes entirely")
and its predecessor(all mq scheduler start patches) changed to add flush request
to front of dispatch queue for blk-mq by ignoring 'add_queue' parameter of
blk_mq_sched_insert_flush(). That change may be by accident, and not sure it is
correct.

I guess once flush rq is added to tail of dispatch queue in block/004,
in which lots of FS request may stay in hctx->dispatch because of low
AHCI queue depth, then we may take a bit long for flush rq to be
submitted to LLD.

I'd suggest to root cause/understand the issue given it isn't obvious
correct to queue flush rq at front of dispatch queue, so could you collect
the following trace via the following script with/without the single line
patch?

#!/bin/sh

MAJ=$1
MIN=$2
MAJ=$(( $MAJ << 20 ))
DEV=$(( $MAJ | $MIN ))

/usr/share/bcc/tools/trace -t -C \
't:block:block_getrq (args->dev == '$DEV') "%s %d %d", args->rwbs, args->sector, args->nr_sector' \
't:block:block_rq_issue (args->dev == '$DEV') "%s %d %d", args->rwbs, args->sector, args->nr_sector' \
't:block:block_rq_complete (args->dev == '$DEV') "%s %d %d", args->rwbs, args->sector, args->nr_sector'

note:
1) install bcc package
2) pass the test disk(AHCI/SMR) major & minor to above script
3) start the script before running block/004

[1] 53d63e6b0dfb block: make the flush insertion use the tail of the dispatch list

Thanks,
Ming