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

Shinichiro Kawasaki
2020-03-10T05:54:22Z

Ming, thank you for sharing the log files and analysis.

On Mar 10, 2020 / 03:07, Damien Le Moal wrote:
> On 2020/03/10 1:14, Ming Lei wrote:
> > On Mon, Mar 09, 2020 at 12:07:16AM +0000, Shinichiro Kawasaki wrote:
> >> On Mar 07, 2020 / 12:13, Ming Lei wrote:
> >>> On Sat, Mar 07, 2020 at 01:02:23AM +0000, Shinichiro Kawasaki wrote:
> >>>> On Mar 06, 2020 / 16:13, Ming Lei wrote:
> >>>>> 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);
> >>>>
> >>>> Yes, with this the one line change above only, the runtime was reduced.
> >>>>
> >>>>>
> >>>>> 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?
> >>>>
> >>>> Thank you for the thoughts for the correct design. I have taken the two traces,
> >>>> with and without the one liner patch above. The gzip archived trace files have
> >>>> 1.6MB size. It looks too large to post to the list. Please let me know how you
> >>>> want the trace files shared.
> >>>
> >>> I didn't thought the trace can be so big given the ios should be just
> >>> 256 * 64(1564).
> >>>
> >>> You may put the log somewhere in Internet, cloud storage, web, or
> >>> whatever. Then just provides us the link.
> >>>
> >>> Or if you can't find a place to hold it, just send to me, and I will put
> >>> it in my RH people web link.
> >>
> >> I have sent another e-mail only to you attaching the log files gziped.
> >> Your confirmation will be appreciated.
> >
> > Yeah, I got the log, and it has been put in the following link:
> >
> > http://people.redhat.com/minlei/tests/logs/blktests_block_004_perf_degrade.tar.gz
> >
> > Also I have run some analysis, and block/004 basically call pwrite() &
> > fsync() in each job.
> >
> > 1) v5.6-rc kernel
> > - write rq average latency: 0.091s
> > - flush rq average latency: 0.018s
> > - average issue times of write request: 1.978 //how many trace_block_rq_issue() is called for one rq
> > - average issue times of flush request: 1.052
> >
> > 2) v5.6-rc patched kernel
> > - write rq average latency: 0.031
> > - flush rq average latency: 0.035
> > - average issue times of write request: 1.466
> > - average issue times of flush request: 1.610
> >
> >
> > block/004 starts 64 jobs and AHCI's queue can become saturated easily,
> > then BLK_MQ_S_SCHED_RESTART should be set, so write request in dispatch
> > queue can only move on after one request is completed.
> >
> > Looks the flush request takes shorter time than normal write IO.
> > If flush request is added to front of dispatch queue, the next normal
> > write IO could be queued to lld quicker than adding to tail of dispatch
> > queue.
> > trace_block_rq_issue() is called more than one time is because of
> > AHCI or the drive's implementation. It usually means that
> > host->hostt->queuecommand() fails for several times when queuing one
> > single request. For AHCI, I understand it shouldn't fail normally given
> > we guarantee that the actual queue depth is <= either LUN or host's
> > queue depth. Maybe it depends on your SMR's implementation about handling
> > flush/write IO. Could you check why .queuecommand() fails in block/004?

I put some debug prints and confirmed that the .queuecommand function is
ata_scsi_queuecmd() and it returns SCSI_MLQUEUE_DEVICE_BUSY because
ata_std_qc_defer() returns ATA_DEFER_LINK. The comment of ata_std_qc_defer()
notes that "Non-NCQ commands cannot run with any other command, NCQ or not. As
upper layer only knows the queue depth, we are responsible for maintaining
exclusion. This function checks whether a new command @qc can be issued." Then
I guess .queuecommand() fails because is that Non-NCQ flush command and NCQ
write command are waiting the completion each other.

>
> Indeed, that is weird that queuecommand fails. There is nothing SMR specific in
> the AHCI code beside disk probe checks. So write & flush handling does not
> differ between SMR and regular disks. The same applies to the drive side. write
> and flush commands are the normal commands, no change at all. The only
> difference being the sequential write constraint which the drive honors by not
> executing the queued write command out of order. But there are no constraint for
> flush on SMR, so we get whatever the drive does, that is, totally drive dependent.
>
> I wonder if the issue may be with the particular AHCI chipset used for this test.
>
> >
> > Also can you provide queue flags via the following command?
> >
> > cat /sys/kernel/debug/block/sdN/state

The state sysfs attribute was as follows:

SAME_COMP|IO_STAT|ADD_RANDOM|INIT_DONE|WC|STATS|REGISTERED|SCSI_PASSTHROUGH|26

It didn't change before and after the block/004 run.

> >
> > I understand flush request should be slower than normal write, however
> > looks not true in this hardware.
>
> Probably due to the fact that since the writes are sequential, there is a lot of
> drive internal optimization that can be done to minimize the cost of flush
> (internal data streaming from cache to media, media-cache use, etc) That is true
> for regular disks too. And all of this is highly dependent on the hardware
> implementation.

This discussion tempted me to take closer look in the traces. And I noticed that
number of flush commands issued is different with and without the patch.

| without patch | with patch
------------------------+---------------+------------
block_getrq: rwbs=FWS | 32640 | 32640
block_rq_issue: rwbs=FF | 32101 | 7593

Without the patch, flush command is issued between two write commands. With the
patch, some write commands are executed without flush between them.

I wonder how the requeue list position of flush command (head vs tail) changes
the number of flush commands to issue.

Another weird thing is number of block_getrq traces of flush (rwds=FWS). It
doubles number of writes (256 * 64 = 16320). I will chase this further.

> Shinichiro,
>
> It may be worth trying the same run with & without Ming's patch on a machine
> with a different chipset...

Thanks for the advice. So far, I observer the long block/004 runtime on two
systems. One with Intel C220 Series SATA controller, and the other with Intel
200 Series PCH SATA controller. I will try to find other SATA controller.

--
Best Regards,
Shin'ichiro Kawasaki