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Wed, 11 Mar 2020 07:54:23 +0000 (UTC) Received: from ming.t460p (ovpn-8-26.pek2.redhat.com [10.72.8.26]) by smtp.corp.redhat.com (Postfix) with ESMTPS id 3B67319C58; Wed, 11 Mar 2020 07:54:14 +0000 (UTC) Date: Wed, 11 Mar 2020 15:54:03 +0800 From: Ming Lei To: Shinichiro Kawasaki Cc: Damien Le Moal , "linux-block@vger.kernel.org" , Jens Axboe Subject: Re: commit 01e99aeca397 causes longer runtime of block/004 Message-ID: <20200311075403.GA31504@ming.t460p> References: <20200307041343.GB20579@ming.t460p> <20200309000715.sqgsssrauzmmpli2@shindev.dhcp.fujisawa.hgst.com> <20200309161430.GA4871@ming.t460p> <20200310055417.o3jghx4sl5xtztci@shindev.dhcp.fujisawa.hgst.com> <20200310080744.GA7618@ming.t460p> <20200310110703.hvaek3opc67lcr74@shindev.dhcp.fujisawa.hgst.com> <20200310133724.GA25034@ming.t460p> <20200310143751.GA29474@ming.t460p> <20200311045922.swskt7kwfkawfhba@shindev.dhcp.fujisawa.hgst.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200311045922.swskt7kwfkawfhba@shindev.dhcp.fujisawa.hgst.com> User-Agent: Mutt/1.12.1 (2019-06-15) X-Scanned-By: MIMEDefang 2.84 on 10.5.11.23 Sender: linux-block-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-block@vger.kernel.org On Wed, Mar 11, 2020 at 04:59:23AM +0000, Shinichiro Kawasaki wrote: > On Mar 10, 2020 / 22:37, Ming Lei wrote: > > On Tue, Mar 10, 2020 at 09:37:24PM +0800, Ming Lei wrote: > > > On Tue, Mar 10, 2020 at 11:07:04AM +0000, Shinichiro Kawasaki wrote: > > > > On Mar 10, 2020 / 16:07, Ming Lei wrote: > > > > > On Tue, Mar 10, 2020 at 05:54:18AM +0000, Shinichiro Kawasaki wrote: > > > > > > 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. > > > > > > > > > > OK, got it. > > > > > > > > > > > > > > > > > > > > > > > > > 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 > > > > > > > > > > Looks issued flush request is too many given the flush machinery > > > > > should avoid to queue duplicated flush requests. > > > > > > > > > > I will investigate the flush code a bit. > > > > > > > > > > > > > > > > > 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. > > > > > > > > > > Indeed, not see such issue when I run the test on kvm ahci. > > > > > > > > I found that the cause of the doubled flush commands is dm-linear device I set > > > > up. I mapped two areas of single SMR drive to the dm-linear device, using dm > > > > table with two lines. My understanding is that when a flush is requested to the > > > > dm-linear device, the flush request is duplicated and forwarded to destination > > > > devices listed in dm table lines. In this case, a flush request is duplicated > > > > for two lines in dm table, and both flush requests go to the single SMR drive. > > > > > > Got it, maybe dm linear should avoid to send two flush requests to > > > single drive. > > > > > > Can you observe similar issue when running block/004 over the AHCI/SMR > > > directly? > > Yes. The runtime delta is smaller and I had not noticed that. Without the patch, > block/004 run on AHCI/SMR drive takes from 115+ seconds. With the patch, > it takes around 100 seconds. > > I also took the bpf traces and counted flush commands issued. Numbsers look > similar as those with dm-linear. > > | without patch | with patch > ------------------------+---------------+------------ > block_getrq: rwbs=FWS | 16320 | 16320 > block_rq_issue: rwbs=FF | 17051 | 5866 BTW, you should count 'block_rq_complete: rwbs=FF' instead of block_rq_issue() which may be called several times for each flush request. Anyway the effect is visible. > > > > > > > > This doubles the number of flush commands. I changed the dm table to have single > > > > line to map single area, and observed the number of flush commands (block_getrq > > > > with rwbs=FWS) got reduced to 16320, the same number as the write commands. > > > > > > > > This means that the workload I observed the longer runtime of block/004 with > > > > kernel 5.6-rc4 is really flush intensive. It has two flush commands per one > > > > write command. It might be too sensitive to flush command behavior. I'm not so > > > > sure if this is the workload worth performance care. > > > > > > The story may be something like below: > > > > > > 1) sometime, suppose there are two write request(w0, w1) in > > > dispatch queue(hctx->dispatch), and one flush request is just done, > > > so blk-mq starts to dispatch requests in hctx->dispatch via wq > > > > > > 2) a new flush request F is scheduled via requeue wq, and the flush > > > request is added to front of hctx->dispatch, which includes (F, w0, w1) > > > > > > 3) meantime, given there are 64 fio jobs, one of the jobs just run queue > > > and queue one rq from scheduler to lld, and hctx->dispatch isn't observed > > > because of timing, so sata switches to NCQ mode > > > > > > 4) step 1 or 2 starts to run queue, and try to dispatch requests(F, w0, w1). > > > .queue_rq() returns STS_RESOURCE for F because F is non-NCQ command, > > > so hctx->dispatch becomes (w0, w1, F) now. And the queue will be re-run > > > after one write request is completed, so one write IO latency is added > > > to the flush request F. > > > > > > 5) when flush request latency is increased, chance for flush request > > > merge is increased, so much less flush requests are issued to sata > > > in patched kernel, then issuing time for write requests is decreased. > > > > > > If flush request is added to tail of hctx->dispatch in step 2), one > > > extra IO latency won't be added to flush request, and flush request > > > can be completed in less time, meantime chance of flush request merge > > > is reduced much. When more flush requests are issued to sata, more time > > > is introduced to issuing write IOs. > > > > > > Anyway, this problem is NCQ specific. If it can be reproduced on > > > AHCI/SMR directly, we may need to fix it. > > > > In short, when adding flush rq to front of hctx->dispatch, it is easier > > to introduce extra time to flush rq's latency compared with adding to tail > > of dispatch queue, then chance of flush merge is increased, and less > > flush requests will be issued to driver/controller, finally this kind of > > flush workloads is improved. > > Ming, thank you very much for the description. > > > > > So maybe we can add flush rq to tail of dispatch just in case of NCQ. > > Reading the line above, I'm not yet sure how the fix will be. Do you mean > to check if sata driver is in NCQ mode, within blk_kick_flush()? We may switch to blk_kick_flush(true) directly, and I will figure out one patch. Thanks, Ming