[PATCH] blk-mq: insert passthrough request into hctx->dispatch directly

[PATCH] blk-mq: insert passthrough request into hctx->dispatch directly

[Ming Lei]

For some reason, device may be in one situation which can't handle
FS request, so STS_RESOURCE is always returned and the FS request
will be added to hctx->dispatch. However passthrough request may
be required at that time for fixing the problem. If passthrough
request is added to scheduler queue, there isn't any chance for
blk-mq to dispatch it given we prioritize requests in hctx->dispatch.
Then the FS IO request may never be completed, and IO hang is caused.

So passthrough request has to be added to hctx->dispatch directly.

Fix this issue by inserting passthrough request into hctx->dispatch
directly. Then it becomes consistent with original legacy IO request
path, in which passthrough request is always added to q->queue_head.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-flush.c    |  2 +-
 block/blk-mq-sched.c | 22 +++++++++++++++-------
 block/blk-mq.c       | 16 ++++++++++------
 block/blk-mq.h       |  3 ++-
 4 files changed, 28 insertions(+), 15 deletions(-)

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 3f977c517960..5cc775bdb06a 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -412,7 +412,7 @@ void blk_insert_flush(struct request *rq)
 	 */
 	if ((policy & REQ_FSEQ_DATA) &&
 	    !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
-		blk_mq_request_bypass_insert(rq, false);
+		blk_mq_request_bypass_insert(rq, false, false);
 		return;
 	}
 
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index ca22afd47b3d..856356b1619e 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -361,13 +361,19 @@ static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx *hctx,
 				       bool has_sched,
 				       struct request *rq)
 {
-	/* dispatch flush rq directly */
-	if (rq->rq_flags & RQF_FLUSH_SEQ) {
-		spin_lock(&hctx->lock);
-		list_add(&rq->queuelist, &hctx->dispatch);
-		spin_unlock(&hctx->lock);
+	/*
+	 * dispatch flush and passthrough rq directly
+	 *
+	 * passthrough request has to be added to hctx->dispatch directly.
+	 * For some reason, device may be in one situation which can't
+	 * handle FS request, so STS_RESOURCE is always returned and the
+	 * FS request will be added to hctx->dispatch. However passthrough
+	 * request may be required at that time for fixing the problem. If
+	 * passthrough request is added to scheduler queue, there isn't any
+	 * chance to dispatch it given we prioritize requests in hctx->dispatch.
+	 */
+	if ((rq->rq_flags & RQF_FLUSH_SEQ) || blk_rq_is_passthrough(rq))
 		return true;
-	}
 
 	if (has_sched)
 		rq->rq_flags |= RQF_SORTED;
@@ -391,8 +397,10 @@ 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))
+	if (blk_mq_sched_bypass_insert(hctx, !!e, rq)) {
+		blk_mq_request_bypass_insert(rq, at_head, false);
 		goto run;
+	}
 
 	if (e && e->type->ops.insert_requests) {
 		LIST_HEAD(list);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index a12b1763508d..5f5c43ae3792 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -735,7 +735,7 @@ static void blk_mq_requeue_work(struct work_struct *work)
 		 * merge.
 		 */
 		if (rq->rq_flags & RQF_DONTPREP)
-			blk_mq_request_bypass_insert(rq, false);
+			blk_mq_request_bypass_insert(rq, false, false);
 		else
 			blk_mq_sched_insert_request(rq, true, false, false);
 	}
@@ -1677,12 +1677,16 @@ void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
  * Should only be used carefully, when the caller knows we want to
  * bypass a potential IO scheduler on the target device.
  */
-void blk_mq_request_bypass_insert(struct request *rq, bool run_queue)
+void blk_mq_request_bypass_insert(struct request *rq, bool at_head,
+				  bool run_queue)
 {
 	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
 
 	spin_lock(&hctx->lock);
-	list_add_tail(&rq->queuelist, &hctx->dispatch);
+	if (at_head)
+		list_add(&rq->queuelist, &hctx->dispatch);
+	else
+		list_add_tail(&rq->queuelist, &hctx->dispatch);
 	spin_unlock(&hctx->lock);
 
 	if (run_queue)
@@ -1849,7 +1853,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 	if (bypass_insert)
 		return BLK_STS_RESOURCE;
 
-	blk_mq_request_bypass_insert(rq, run_queue);
+	blk_mq_request_bypass_insert(rq, false, run_queue);
 	return BLK_STS_OK;
 }
 
@@ -1876,7 +1880,7 @@ static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 
 	ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false, true);
 	if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE)
-		blk_mq_request_bypass_insert(rq, true);
+		blk_mq_request_bypass_insert(rq, false, true);
 	else if (ret != BLK_STS_OK)
 		blk_mq_end_request(rq, ret);
 
@@ -1910,7 +1914,7 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
 		if (ret != BLK_STS_OK) {
 			if (ret == BLK_STS_RESOURCE ||
 					ret == BLK_STS_DEV_RESOURCE) {
-				blk_mq_request_bypass_insert(rq,
+				blk_mq_request_bypass_insert(rq, false,
 							list_empty(list));
 				break;
 			}
diff --git a/block/blk-mq.h b/block/blk-mq.h
index eaaca8fc1c28..c0fa34378eb2 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -66,7 +66,8 @@ int blk_mq_alloc_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
  */
 void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
 				bool at_head);
-void blk_mq_request_bypass_insert(struct request *rq, bool run_queue);
+void blk_mq_request_bypass_insert(struct request *rq, bool at_head,
+				  bool run_queue);
 void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
 				struct list_head *list);
 
-- 
2.20.1

Re: [PATCH] blk-mq: insert passthrough request into hctx->dispatch directly

[Christoph Hellwig]

On Sat, Feb 15, 2020 at 11:21:40AM +0800, Ming Lei wrote:
> For some reason, device may be in one situation which can't handle
> FS request, so STS_RESOURCE is always returned and the FS request
> will be added to hctx->dispatch. However passthrough request may
> be required at that time for fixing the problem. If passthrough
> request is added to scheduler queue, there isn't any chance for
> blk-mq to dispatch it given we prioritize requests in hctx->dispatch.
> Then the FS IO request may never be completed, and IO hang is caused.
> 
> So passthrough request has to be added to hctx->dispatch directly.
> 
> Fix this issue by inserting passthrough request into hctx->dispatch
> directly. Then it becomes consistent with original legacy IO request
> path, in which passthrough request is always added to q->queue_head.

Do you have a description of an actual problem this fixes?  Maybe even
a reproducer for blktests?

Re: [PATCH] blk-mq: insert passthrough request into hctx->dispatch directly

[Ming Lei]

On Wed, Feb 19, 2020 at 08:36:15AM -0800, Christoph Hellwig wrote:
> On Sat, Feb 15, 2020 at 11:21:40AM +0800, Ming Lei wrote:
> > For some reason, device may be in one situation which can't handle
> > FS request, so STS_RESOURCE is always returned and the FS request
> > will be added to hctx->dispatch. However passthrough request may
> > be required at that time for fixing the problem. If passthrough
> > request is added to scheduler queue, there isn't any chance for
> > blk-mq to dispatch it given we prioritize requests in hctx->dispatch.
> > Then the FS IO request may never be completed, and IO hang is caused.
> > 
> > So passthrough request has to be added to hctx->dispatch directly.
> > 
> > Fix this issue by inserting passthrough request into hctx->dispatch
> > directly. Then it becomes consistent with original legacy IO request
> > path, in which passthrough request is always added to q->queue_head.
> 
> Do you have a description of an actual problem this fixes?  Maybe even
> a reproducer for blktests?
> 

It is reported by one RH customer in the following test case:

	1) Start IO on Emulex FC host
	2) Fail one controller, wait 5 minutes
	3) Bring controller back online

When we trace the problem, it is found that FS request started in device_add_disk()
from scsi disk probe context stuck because scsi_queue_rq() always return
STS_BUSY via scsi_setup_fs_cmnd() -> alua_prep_fn().

The kernel ALUA state is TRANSITIONING at that time, so it is reasonable to see
BLK_TYPE_FS requests won't go anywhere because of the check in alua_prep_fn().

However, the passthrough request(TEST UNIT READY) is submitted from alua_rtpg_work
when the FS request can't be dispatched to LLD. And SCSI stack should
have been allowed to handle this passthrough rquest. But it can't reach SCSI stack
via .queue_rq() because blk-mq won't dispatch it until hctx->dispatch is
empty.

The legacy IO request code always added passthrough request into head of q->queue_head
directly instead of scheduler queue or sw queue, so no such issue.

So far not figured out one blktests test case, but the problem is real.

BTW, I just found we need the extra following change:

@@ -1301,7 +1301,7 @@ 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_init(list, &hctx->dispatch);
+               list_splice_tail_init(list, &hctx->dispatch);
                spin_unlock(&hctx->lock);

Will post V2.


Thanks,
Ming

Re: [PATCH] blk-mq: insert passthrough request into hctx->dispatch directly

[dongli.zhang]

On 2/19/20 2:10 PM, Ming Lei wrote:
> On Wed, Feb 19, 2020 at 08:36:15AM -0800, Christoph Hellwig wrote:
>> On Sat, Feb 15, 2020 at 11:21:40AM +0800, Ming Lei wrote:
>>> For some reason, device may be in one situation which can't handle
>>> FS request, so STS_RESOURCE is always returned and the FS request
>>> will be added to hctx->dispatch. However passthrough request may
>>> be required at that time for fixing the problem. If passthrough
>>> request is added to scheduler queue, there isn't any chance for
>>> blk-mq to dispatch it given we prioritize requests in hctx->dispatch.
>>> Then the FS IO request may never be completed, and IO hang is caused.
>>>
>>> So passthrough request has to be added to hctx->dispatch directly.
>>>
>>> Fix this issue by inserting passthrough request into hctx->dispatch
>>> directly. Then it becomes consistent with original legacy IO request
>>> path, in which passthrough request is always added to q->queue_head.
>>
>> Do you have a description of an actual problem this fixes?  Maybe even
>> a reproducer for blktests?
>>
> 
> It is reported by one RH customer in the following test case:
> 
> 	1) Start IO on Emulex FC host
> 	2) Fail one controller, wait 5 minutes
> 	3) Bring controller back online
> 
> When we trace the problem, it is found that FS request started in device_add_disk()
> from scsi disk probe context stuck because scsi_queue_rq() always return
> STS_BUSY via scsi_setup_fs_cmnd() -> alua_prep_fn().
> 
> The kernel ALUA state is TRANSITIONING at that time, so it is reasonable to see
> BLK_TYPE_FS requests won't go anywhere because of the check in alua_prep_fn().
> 
> However, the passthrough request(TEST UNIT READY) is submitted from alua_rtpg_work
> when the FS request can't be dispatched to LLD. And SCSI stack should
> have been allowed to handle this passthrough rquest. But it can't reach SCSI stack
> via .queue_rq() because blk-mq won't dispatch it until hctx->dispatch is
> empty.
> 
> The legacy IO request code always added passthrough request into head of q->queue_head
> directly instead of scheduler queue or sw queue, so no such issue.
> 
> So far not figured out one blktests test case, but the problem is real.
> 
> BTW, I just found we need the extra following change:
> 
> @@ -1301,7 +1301,7 @@ 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_init(list, &hctx->dispatch);
> +               list_splice_tail_init(list, &hctx->dispatch);
>                 spin_unlock(&hctx->lock);
> 

Is it fine to add to tail as the requests on dispatch would be reordered?

A, B, C and D are on the list. Suppose A is failed and the new list would become
B, C D, A?

Dongli Zhang

Re: [PATCH] blk-mq: insert passthrough request into hctx->dispatch directly

[Ming Lei]

On Wed, Feb 19, 2020 at 03:47:50PM -0800, dongli.zhang@oracle.com wrote:
> 
> 
> On 2/19/20 2:10 PM, Ming Lei wrote:
> > On Wed, Feb 19, 2020 at 08:36:15AM -0800, Christoph Hellwig wrote:
> >> On Sat, Feb 15, 2020 at 11:21:40AM +0800, Ming Lei wrote:
> >>> For some reason, device may be in one situation which can't handle
> >>> FS request, so STS_RESOURCE is always returned and the FS request
> >>> will be added to hctx->dispatch. However passthrough request may
> >>> be required at that time for fixing the problem. If passthrough
> >>> request is added to scheduler queue, there isn't any chance for
> >>> blk-mq to dispatch it given we prioritize requests in hctx->dispatch.
> >>> Then the FS IO request may never be completed, and IO hang is caused.
> >>>
> >>> So passthrough request has to be added to hctx->dispatch directly.
> >>>
> >>> Fix this issue by inserting passthrough request into hctx->dispatch
> >>> directly. Then it becomes consistent with original legacy IO request
> >>> path, in which passthrough request is always added to q->queue_head.
> >>
> >> Do you have a description of an actual problem this fixes?  Maybe even
> >> a reproducer for blktests?
> >>
> > 
> > It is reported by one RH customer in the following test case:
> > 
> > 	1) Start IO on Emulex FC host
> > 	2) Fail one controller, wait 5 minutes
> > 	3) Bring controller back online
> > 
> > When we trace the problem, it is found that FS request started in device_add_disk()
> > from scsi disk probe context stuck because scsi_queue_rq() always return
> > STS_BUSY via scsi_setup_fs_cmnd() -> alua_prep_fn().
> > 
> > The kernel ALUA state is TRANSITIONING at that time, so it is reasonable to see
> > BLK_TYPE_FS requests won't go anywhere because of the check in alua_prep_fn().
> > 
> > However, the passthrough request(TEST UNIT READY) is submitted from alua_rtpg_work
> > when the FS request can't be dispatched to LLD. And SCSI stack should
> > have been allowed to handle this passthrough rquest. But it can't reach SCSI stack
> > via .queue_rq() because blk-mq won't dispatch it until hctx->dispatch is
> > empty.
> > 
> > The legacy IO request code always added passthrough request into head of q->queue_head
> > directly instead of scheduler queue or sw queue, so no such issue.
> > 
> > So far not figured out one blktests test case, but the problem is real.
> > 
> > BTW, I just found we need the extra following change:
> > 
> > @@ -1301,7 +1301,7 @@ 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_init(list, &hctx->dispatch);
> > +               list_splice_tail_init(list, &hctx->dispatch);
> >                 spin_unlock(&hctx->lock);
> > 
> 
> Is it fine to add to tail as the requests on dispatch would be reordered?

Wrt. FS request:

Firstly we never guarantee that the request is dispatched in order.

Secondly and more importantly, request can be added into hctx->dispatch
in any order. One usual case is that request is added to hctx->dispatch
concurrently when .queue_rq() fails. On the other side, in case of not
concurrent adding to hctx->dispatch, after one request is added to
hctx->dispatch, we always dispatch request from hctx->dispatch first,
instead of dequeuing request from scheduler queue and adding them to
hctx->dispatch again after .queue_rq() fails.

> 
> A, B, C and D are on the list. Suppose A is failed and the new list would become
> B, C D, A?

Right, I don't see there is any issue in this way, do you see issues?



Thanks,
Ming

Re: [PATCH] blk-mq: insert passthrough request into hctx->dispatch directly

[Dongli Zhang]

On 2/19/20 5:45 PM, Ming Lei wrote:
> On Wed, Feb 19, 2020 at 03:47:50PM -0800, dongli.zhang@oracle.com wrote:
>>
>>
>> On 2/19/20 2:10 PM, Ming Lei wrote:
>>> On Wed, Feb 19, 2020 at 08:36:15AM -0800, Christoph Hellwig wrote:
>>>> On Sat, Feb 15, 2020 at 11:21:40AM +0800, Ming Lei wrote:
>>>>> For some reason, device may be in one situation which can't handle
>>>>> FS request, so STS_RESOURCE is always returned and the FS request
>>>>> will be added to hctx->dispatch. However passthrough request may
>>>>> be required at that time for fixing the problem. If passthrough
>>>>> request is added to scheduler queue, there isn't any chance for
>>>>> blk-mq to dispatch it given we prioritize requests in hctx->dispatch.
>>>>> Then the FS IO request may never be completed, and IO hang is caused.
>>>>>
>>>>> So passthrough request has to be added to hctx->dispatch directly.
>>>>>
>>>>> Fix this issue by inserting passthrough request into hctx->dispatch
>>>>> directly. Then it becomes consistent with original legacy IO request
>>>>> path, in which passthrough request is always added to q->queue_head.
>>>>
>>>> Do you have a description of an actual problem this fixes?  Maybe even
>>>> a reproducer for blktests?
>>>>
>>>
>>> It is reported by one RH customer in the following test case:
>>>
>>> 	1) Start IO on Emulex FC host
>>> 	2) Fail one controller, wait 5 minutes
>>> 	3) Bring controller back online
>>>
>>> When we trace the problem, it is found that FS request started in device_add_disk()
>>> from scsi disk probe context stuck because scsi_queue_rq() always return
>>> STS_BUSY via scsi_setup_fs_cmnd() -> alua_prep_fn().
>>>
>>> The kernel ALUA state is TRANSITIONING at that time, so it is reasonable to see
>>> BLK_TYPE_FS requests won't go anywhere because of the check in alua_prep_fn().
>>>
>>> However, the passthrough request(TEST UNIT READY) is submitted from alua_rtpg_work
>>> when the FS request can't be dispatched to LLD. And SCSI stack should
>>> have been allowed to handle this passthrough rquest. But it can't reach SCSI stack
>>> via .queue_rq() because blk-mq won't dispatch it until hctx->dispatch is
>>> empty.
>>>
>>> The legacy IO request code always added passthrough request into head of q->queue_head
>>> directly instead of scheduler queue or sw queue, so no such issue.
>>>
>>> So far not figured out one blktests test case, but the problem is real.
>>>
>>> BTW, I just found we need the extra following change:
>>>
>>> @@ -1301,7 +1301,7 @@ 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_init(list, &hctx->dispatch);
>>> +               list_splice_tail_init(list, &hctx->dispatch);
>>>                 spin_unlock(&hctx->lock);
>>>
>>
>> Is it fine to add to tail as the requests on dispatch would be reordered?
> 
> Wrt. FS request:
> 
> Firstly we never guarantee that the request is dispatched in order.
> 
> Secondly and more importantly, request can be added into hctx->dispatch
> in any order. One usual case is that request is added to hctx->dispatch
> concurrently when .queue_rq() fails. On the other side, in case of not
> concurrent adding to hctx->dispatch, after one request is added to
> hctx->dispatch, we always dispatch request from hctx->dispatch first,
> instead of dequeuing request from scheduler queue and adding them to
> hctx->dispatch again after .queue_rq() fails.
> 
>>
>> A, B, C and D are on the list. Suppose A is failed and the new list would become
>> B, C D, A?
> 
> Right, I don't see there is any issue in this way, do you see issues?

Thank you very much for the explanation. I do not see issue if order guarantee
in hctx->dispatch is not required.

Dongli Zhang