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

Kashyap Desai
2020-06-17T11:28:16Z

> On Mon, Jun 15, 2020 at 12:27:30PM +0530, Kashyap Desai wrote:
> > > >
> > > > 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);
> > >
> > > This way may cause IO hang because ->device_busy is shared by all
hctxs.
> >
> > From SCSI stack, if we attempt to run all h/w queue, is it possible
> > that block layer actually run hw_queue which has really not queued any
IO.
> > Currently, in case of mq-deadline, IOS are inserted using
> > "dd_insert_request". This function will add IOs on elevator data which
> > is per request queue and not per hctx.
> > When there is an attempt to run hctx, "blk_mq_sched_has_work" will
> > check pending work which is per request queue and not per hctx.
> > Because of this, IOs queued on only one hctx will be run from all the
> > hctx and this will create unnecessary lock contention.
>
> Deadline is supposed for HDD. slow disks, so the lock contention
shouldn't
> have been one problem given there is seldom MQ HDD. before this
patchset.
>
> Another related issue is default scheduler, I guess deadline still
should have
> been the default io sched for HDDs. attached to this kind HBA with
multiple
> reply queues and single submission queue.
>
> >
> > How about below patch - ?
> >
> > diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h index
> > 126021f..1d30bd3 100644
> > --- a/block/blk-mq-sched.h
> > +++ b/block/blk-mq-sched.h
> > @@ -74,6 +74,13 @@ static inline bool blk_mq_sched_has_work(struct
> > blk_mq_hw_ctx *hctx) {
> > struct elevator_queue *e = hctx->queue->elevator;
> >
> > + /* If current hctx has not queued any request, there is no
> > + need to
> > run.
> > + * blk_mq_run_hw_queue() on hctx which has queued IO will
handle
> > + * running specific hctx.
> > + */
> > + if (!hctx->queued)
> > + return false;
> > +
> > if (e && e->type->ops.has_work)
> > return e->type->ops.has_work(hctx);
>
> ->queued is increased only and not decreased just for debug purpose so
> ->far, so
> it can't be relied for this purpose.

Thanks. I overlooked that that it is only incremental counter.

>
> One approach is to add one similar counter, and maintain it by
scheduler's
> insert/dispatch callback.

I tried below and I see performance is on expected range.

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index fdcc2c1..ea201d0 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -485,6 +485,7 @@ void blk_mq_sched_insert_request(struct request *rq,
bool at_head,

list_add(&rq->queuelist, &list);
e->type->ops.insert_requests(hctx, &list, at_head);
+ atomic_inc(&hctx->elevator_queued);
} else {
spin_lock(&ctx->lock);
__blk_mq_insert_request(hctx, rq, at_head);
@@ -511,8 +512,10 @@ void blk_mq_sched_insert_requests(struct
blk_mq_hw_ctx *hctx,
percpu_ref_get(&q->q_usage_counter);

e = hctx->queue->elevator;
- if (e && e->type->ops.insert_requests)
+ if (e && e->type->ops.insert_requests) {
e->type->ops.insert_requests(hctx, list, false);
+ atomic_inc(&hctx->elevator_queued);
+ }
else {
/*
* try to issue requests directly if the hw queue isn't
diff --git a/block/blk-mq-sched.h b/block/blk-mq-sched.h
index 126021f..946b47a 100644
--- a/block/blk-mq-sched.h
+++ b/block/blk-mq-sched.h
@@ -74,6 +74,13 @@ static inline bool blk_mq_sched_has_work(struct
blk_mq_hw_ctx *hctx)
{
struct elevator_queue *e = hctx->queue->elevator;

+ /* If current hctx has not queued any request, there is no need to
run.
+ * blk_mq_run_hw_queue() on hctx which has queued IO will handle
+ * running specific hctx.
+ */
+ if (!atomic_read(&hctx->elevator_queued))
+ return false;
+
if (e && e->type->ops.has_work)
return e->type->ops.has_work(hctx);

diff --git a/block/blk-mq.c b/block/blk-mq.c
index f73a2f9..48f1824 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -517,8 +517,10 @@ void blk_mq_free_request(struct request *rq)
struct blk_mq_hw_ctx *hctx = rq->mq_hctx;

if (rq->rq_flags & RQF_ELVPRIV) {
- if (e && e->type->ops.finish_request)
+ if (e && e->type->ops.finish_request) {
e->type->ops.finish_request(rq);
+ atomic_dec(&hctx->elevator_queued);
+ }
if (rq->elv.icq) {
put_io_context(rq->elv.icq->ioc);
rq->elv.icq = NULL;
@@ -2571,6 +2573,7 @@ blk_mq_alloc_hctx(struct request_queue *q, struct
blk_mq_tag_set *set,
goto free_hctx;

atomic_set(&hctx->nr_active, 0);
+ atomic_set(&hctx->elevator_queued, 0);
if (node == NUMA_NO_NODE)
node = set->numa_node;
hctx->numa_node = node;
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 66711c7..ea1ddb1 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -139,6 +139,10 @@ struct blk_mq_hw_ctx {
* shared across request queues.
*/
atomic_t nr_active;
+ /**
+ * @elevator_queued: Number of queued requests on hctx.
+ */
+ atomic_t elevator_queued;

/** @cpuhp_online: List to store request if CPU is going to die */
struct hlist_node cpuhp_online;

>
> Thanks,
> Ming