RE: [PATCH RFC v7 10/12] megaraid_sas: switch fusion adapters to MQ

Kashyap Desai
2020-08-06T11:17:07Z

> > Ming -
> >
> > I noted your comments.
> >
> > I have completed testing and this particular latest performance issue
> > on Volume is outstanding.
> > Currently it is 20-25% performance drop in IOPs and we want that to be
> > closed before shared host tag is enabled for <megaraid_sas> driver.
> > Just for my understanding - What will be the next steps on this ?
> >
> > I can validate any new approach/patch for this issue.
> >
>
> Hello,
>
> What do you think of the following patch?

I tested this patch. I still see IO hang.

>
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c index
> c866a4f33871..49f0fc5c7a63 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -552,8 +552,24 @@ static void scsi_run_queue_async(struct scsi_device
> *sdev)
> if (scsi_target(sdev)->single_lun ||
> !list_empty(&sdev->host->starved_list))
> kblockd_schedule_work(&sdev->requeue_work);
> - else
> - blk_mq_run_hw_queues(sdev->request_queue, true);
> + else {
> + /*
> + * smp_mb() implied in either rq->end_io or
> blk_mq_free_request
> + * is for ordering writing .device_busy in
scsi_device_unbusy()
> + * and reading sdev->restarts.
> + */
> + int old = atomic_read(&sdev->restarts);
> +
> + if (old) {
> + blk_mq_run_hw_queues(sdev->request_queue, true);
> +
> + /*
> + * ->restarts has to be kept as non-zero if there
is
> + * new budget contention comes.
> + */
> + atomic_cmpxchg(&sdev->restarts, old, 0);
> + }
> + }
> }
>
> /* Returns false when no more bytes to process, true if there are more
*/
> @@ -1612,8 +1628,34 @@ static void scsi_mq_put_budget(struct
> request_queue *q) static bool scsi_mq_get_budget(struct request_queue
*q)
> {
> struct scsi_device *sdev = q->queuedata;
> + int ret = scsi_dev_queue_ready(q, sdev);
>
> - return scsi_dev_queue_ready(q, sdev);
> + if (ret)
> + return true;
> +
> + /*
> + * If all in-flight requests originated from this LUN are
completed
> + * before setting .restarts, sdev->device_busy will be observed as
> + * zero, then blk_mq_delay_run_hw_queue() will dispatch this
request
> + * soon. Otherwise, completion of one of these request will
observe
> + * the .restarts flag, and the request queue will be run for
handling
> + * this request, see scsi_end_request().
> + */
> + atomic_inc(&sdev->restarts);
> +
> + /*
> + * Order writing .restarts and reading .device_busy, and make sure
> + * .restarts is visible to scsi_end_request(). Its pair is implied
by
> + * __blk_mq_end_request() in scsi_end_request() for ordering
> + * writing .device_busy in scsi_device_unbusy() and reading
.restarts.
> + *
> + */
> + smp_mb__after_atomic();
> +
> + if (unlikely(atomic_read(&sdev->device_busy) == 0 &&
> + !scsi_device_blocked(sdev)))
> + blk_mq_delay_run_hw_queues(sdev->request_queue,
> SCSI_QUEUE_DELAY);

Hi Ming -

There is still some race which is not handled. Take a case of IO is not
able to get budget and it has already marked <restarts> flag.
<restarts> flag will be seen non-zero in completion path and completion
path will attempt h/w queue run. (But this particular IO is still not in
s/w queue.).
Attempt of running h/w queue from completion path will not flush any IO
since there is no IO in s/w queue.

I think above code is added assuming it should manage this particular
case, but this code also does not help. If some IO in between submitted
directly to the h/w queue then sdev->device_busy will be non-zero.

If I move above section of the code into completion path, IO hang is
resolved. I also verify performance -

Multi Drive R0 1 workers per VD gives 662K prior to this patch and now
It scale to 1.1M IOPs. (90% improvement)
Multi Drive R0 4 workers per VD gives 1.9M prior to this patch and now It
scale to 3.1M IOPs. (50% improvement)

Here is modified patch -

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 6f50e5c..dcdc5f6 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -594,8 +594,26 @@ static bool scsi_end_request(struct request *req,
blk_status_t error,
if (scsi_target(sdev)->single_lun ||
!list_empty(&sdev->host->starved_list))
kblockd_schedule_work(&sdev->requeue_work);
- else
- blk_mq_run_hw_queues(q, true);
+ else {
+ /*
+ * smp_mb() implied in either rq->end_io or
blk_mq_free_request
+ * is for ordering writing .device_busy in
scsi_device_unbusy()
+ * and reading sdev->restarts.
+ */
+ int old = atomic_read(&sdev->restarts);
+
+ if (old) {
+ blk_mq_run_hw_queues(sdev->request_queue, true);
+
+ /*
+ * ->restarts has to be kept as non-zero if there
is
+ * new budget contention comes.
+ */
+ atomic_cmpxchg(&sdev->restarts, old, 0);
+ } else if (unlikely(atomic_read(&sdev->device_busy) == 0
&&
+ !scsi_device_blocked(sdev)))
+ blk_mq_delay_run_hw_queues(sdev->request_queue,
SCSI_QUEUE_DELAY);
+ }

percpu_ref_put(&q->q_usage_counter);
return false;
@@ -1615,8 +1633,31 @@ static bool scsi_mq_get_budget(struct blk_mq_hw_ctx
*hctx)
{
struct request_queue *q = hctx->queue;
struct scsi_device *sdev = q->queuedata;
+ int ret = scsi_dev_queue_ready(q, sdev);
+
+ if (ret)
+ return true;

- return scsi_dev_queue_ready(q, sdev);
+ /*
+ * If all in-flight requests originated from this LUN are
completed
+ * before setting .restarts, sdev->device_busy will be observed as
+ * zero, then blk_mq_delay_run_hw_queue() will dispatch this
request
+ * soon. Otherwise, completion of one of these request will
observe
+ * the .restarts flag, and the request queue will be run for
handling
+ * this request, see scsi_end_request().
+ */
+ atomic_inc(&sdev->restarts);
+
+ /*
+ * Order writing .restarts and reading .device_busy, and make sure
+ * .restarts is visible to scsi_end_request(). Its pair is implied
by
+ * __blk_mq_end_request() in scsi_end_request() for ordering
+ * writing .device_busy in scsi_device_unbusy() and reading
.restarts.
+ *
+ */
+ smp_mb__after_atomic();
+
+ return false;
}

static blk_status_t scsi_queue_rq(struct blk_mq_hw_ctx *hctx,
diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
index bc59090..ac45058 100644
--- a/include/scsi/scsi_device.h
+++ b/include/scsi/scsi_device.h
@@ -108,7 +108,8 @@ struct scsi_device {

atomic_t device_busy; /* commands actually active on
LLDD */
atomic_t device_blocked; /* Device returned QUEUE_FULL. */
-
+
+ atomic_t restarts;
spinlock_t list_lock;
struct list_head starved_entry;
unsigned short queue_depth; /* How deep of a queue we want */