RE: [PATCH RFC v7 00/12] blk-mq/scsi: Provide hostwide shared tags for SCSI HBAs

Kashyap Desai
2020-06-12T18:47:44Z

> On 11/06/2020 04:07, Ming Lei wrote:
> >> Using 12x SAS SSDs on hisi_sas v3 hw. mq-deadline results are
> >> included, but it is not always an appropriate scheduler to use.
> >>
> >> Tag depth 4000 (default) 260**
> >>
> >> Baseline:
> >> none sched: 2290K IOPS 894K
> >> mq-deadline sched: 2341K IOPS 2313K
> >>
> >> Final, host_tagset=0 in LLDD*
> >> none sched: 2289K IOPS 703K
> >> mq-deadline sched: 2337K IOPS 2291K
> >>
> >> Final:
> >> none sched: 2281K IOPS 1101K
> >> mq-deadline sched: 2322K IOPS 1278K
> >>
> >> * this is relevant as this is the performance in supporting but not
> >> enabling the feature
> >> ** depth=260 is relevant as some point where we are regularly waiting
> >> for
> >> tags to be available. Figures were are a bit unstable here for
> >> testing.

John -

I tried V7 series and debug further on mq-deadline interface. This time I
have used another setup since HDD based setup is not readily available for
me.
In fact, I was able to simulate issue very easily using single scsi_device
as well. BTW, this is not an issue with this RFC, but generic issue.
Since I have converted nr_hw_queue > 1 for Broadcom product using this RFC,
It becomes noticeable now.

Problem - Using below command I see heavy CPU utilization on "
native_queued_spin_lock_slowpath". This is because kblockd work queue is
submitting IO from all the CPUs even though fio is bound to single CPU.
Lock contention from " dd_dispatch_request" is causing this issue.

numactl -C 13 fio
single.fio --iodepth=32 --bs=4k --rw=randread --ioscheduler=none --numjobs=1
--cpus_allowed_policy=split --ioscheduler=mq-deadline
--group_reporting --filename=/dev/sdd

While running above command, ideally we expect only kworker/13 to be active.
But you can see below - All the CPU is attempting submission and lots of CPU
consumption is due to lock contention.

PID USER PR NI VIRT RES SHR S %CPU %MEM TIME+ COMMAND
2726 root 0 -20 0 0 0 R 56.5 0.0 0:53.20
kworker/13:1H-k
7815 root 20 0 712404 15536 2228 R 43.2 0.0 0:05.03 fio
2792 root 0 -20 0 0 0 I 26.6 0.0 0:22.19
kworker/18:1H-k
2791 root 0 -20 0 0 0 I 19.9 0.0 0:17.17
kworker/19:1H-k
1419 root 0 -20 0 0 0 I 19.6 0.0 0:17.03
kworker/20:1H-k
2793 root 0 -20 0 0 0 I 18.3 0.0 0:15.64
kworker/21:1H-k
1424 root 0 -20 0 0 0 I 17.3 0.0 0:14.99
kworker/22:1H-k
2626 root 0 -20 0 0 0 I 16.9 0.0 0:14.68
kworker/26:1H-k
2794 root 0 -20 0 0 0 I 16.9 0.0 0:14.87
kworker/23:1H-k
2795 root 0 -20 0 0 0 I 16.9 0.0 0:14.81
kworker/24:1H-k
2797 root 0 -20 0 0 0 I 16.9 0.0 0:14.62
kworker/27:1H-k
1415 root 0 -20 0 0 0 I 16.6 0.0 0:14.44
kworker/30:1H-k
2669 root 0 -20 0 0 0 I 16.6 0.0 0:14.38
kworker/31:1H-k
2796 root 0 -20 0 0 0 I 16.6 0.0 0:14.74
kworker/25:1H-k
2799 root 0 -20 0 0 0 I 16.6 0.0 0:14.56
kworker/28:1H-k
1425 root 0 -20 0 0 0 I 16.3 0.0 0:14.21
kworker/34:1H-k
2746 root 0 -20 0 0 0 I 16.3 0.0 0:14.33
kworker/32:1H-k
2798 root 0 -20 0 0 0 I 16.3 0.0 0:14.50
kworker/29:1H-k
2800 root 0 -20 0 0 0 I 16.3 0.0 0:14.27
kworker/33:1H-k
1423 root 0 -20 0 0 0 I 15.9 0.0 0:14.10
kworker/54:1H-k
1784 root 0 -20 0 0 0 I 15.9 0.0 0:14.03
kworker/55:1H-k
2801 root 0 -20 0 0 0 I 15.9 0.0 0:14.15
kworker/35:1H-k
2815 root 0 -20 0 0 0 I 15.9 0.0 0:13.97
kworker/56:1H-k
1484 root 0 -20 0 0 0 I 15.6 0.0 0:13.90
kworker/57:1H-k
1485 root 0 -20 0 0 0 I 15.6 0.0 0:13.82
kworker/59:1H-k
1519 root 0 -20 0 0 0 I 15.6 0.0 0:13.64
kworker/62:1H-k
2315 root 0 -20 0 0 0 I 15.6 0.0 0:13.87
kworker/58:1H-k
2627 root 0 -20 0 0 0 I 15.6 0.0 0:13.69
kworker/61:1H-k
2816 root 0 -20 0 0 0 I 15.6 0.0 0:13.75
kworker/60:1H-k

I root cause this issue -

Block layer always queue IO on hctx context mapped to CPU-13, but hw queue
run from all the hctx context.
I noticed in my test hctx48 has queued all the IOs. No other hctx has queued
IO. But all the hctx is counting for "run".

# cat hctx48/queued
2087058

#cat hctx*/run
151318
30038
83110
50680
69907
60391
111239
18036
33935
91648
34582
22853
61286
19489

Below patch has fix - "Run the hctx queue for which request was completed
instead of running all the hardware queue."
If this looks valid fix, please include in V8 OR I can post separate patch
for this. Just want to have some level of review from this discussion.

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 0652acd..f52118f 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -554,6 +554,7 @@ static bool scsi_end_request(struct request *req,
blk_status_t error,
struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(req);
struct scsi_device *sdev = cmd->device;
struct request_queue *q = sdev->request_queue;
+ struct blk_mq_hw_ctx *mq_hctx = req->mq_hctx;

if (blk_update_request(req, error, bytes))
return true;
@@ -595,7 +596,8 @@ static bool scsi_end_request(struct request *req,
blk_status_t error,
!list_empty(&sdev->host->starved_list))
kblockd_schedule_work(&sdev->requeue_work);
else
- blk_mq_run_hw_queues(q, true);
+ blk_mq_run_hw_queue(mq_hctx, true);
+ //blk_mq_run_hw_queues(q, true);

percpu_ref_put(&q->q_usage_counter);
return false;

After above patch - Only kworker/13 is actively doing submission.

3858 root 0 -20 0 0 0 I 22.9 0.0 3:24.04
kworker/13:1H-k
16768 root 20 0 712008 14968 2180 R 21.6 0.0 0:03.27 fio
16769 root 20 0 712012 14968 2180 R 21.6 0.0 0:03.27 fio

Without above patch - 24 SSD driver can give 1.5M IOPS and after above patch
3.2M IOPS.

I will continue my testing.

Thanks, Kashyap

> >>
> >> A copy of the patches can be found here:
> >> https://github.com/hisilicon/kernel-dev/commits/private-topic-blk-mq-
> >> shared-tags-rfc-v7
> >>
> >> And to progress this series, we the the following to go in first, when
> >> ready:
> >> https://lore.kernel.org/linux-scsi/20200430131904.5847-1-hare@suse.de
> >> /
> > I'd suggest to add options to enable shared tags for null_blk &
> > scsi_debug in V8, so that it is easier to verify the changes without
> > real
> hardware.
> >
>
> ok, fine, I can look at including that. To stop the series getting too
> large, I
> might spin off the early patches, which are not strictly related.
>
> Thanks,
> John