[PATCH V6 0/5] blk-mq: refactor and fix on issue request directly

[PATCH V6 0/5] blk-mq: refactor and fix on issue request directly

[Jianchao Wang]

Hi Jens

Please consider this patchset for 4.21.
It refactors the code of issue request directly to unify the interface
and make the code clearer and more readable, and also fixes two defects
there.

The 1st patch refactors the code of issue request directly to unify the
helper interface which could handle all the cases.

The 2nd patch fix the issue that when queue is stopped or quiesced request
may pass through bottom device's potential io scheduler.

The 3rd patch fix the issue that hctx maybe ran on a cpu where it is not
mapped in issue directly path.

The 4th patch make blk_mq_sched_insert_requests issue requests directly
with 'bypass' false, then it needn't to handle the non-issued requests
any more.

The 5th patch replace and kill the blk_mq_request_issue_directly.

V6:
 - drop original 1st patch to address Jen's comment
 - discard the enum mq_issue_decision and blk_mq_make_decision and use
   BLK_STS_* return values directly to address Jen's comment. (1/5)
 - add 'unlikely' in blk_mq_try_issue_directly (1/5)
 - refactor the 2nd and 3rd patch based on the new 1st patch.
 - reserve the unused_cookie in 4th and 5th patch

V5:
 - rebase against Jens' for-4.21/block branch
 - adjust the order of patch04 and patch05
 - add patch06 to replace and kill the one line blk_mq_request_bypass_insert
 - comment changes

V4:
 - split the original patch 1 into two patches, 1st and 2nd patch currently
 - rename the mq_decision to mq_issue_decision
 - comment changes

V3:
 - Correct the code about the case bypass_insert is true and io scheduler
   attached. The request still need to be issued in case above. (1/4)
 - Refactor the code to make code clearer. blk_mq_make_request is introduced
   to decide insert, end or just return based on the return value of .queue_rq
   and bypass_insert (1/4) 
 - Add the 2nd patch. It introduce a new decision result which indicates to
   insert request with blk_mq_request_bypass_insert.
 - Modify the code to adapt the new patch 1.

V2:
 - Add 1st and 2nd patch to refactor the code.

Jianchao Wang(6)
  blk-mq: refactor the code of issue request directly
  blk-mq: fix issue directly case when q is stopped or quiesced
  blk-mq: ensure hctx to be ran on mapped cpu when issue directly
  blk-mq: issue directly with bypass 'false' in blk_mq_sched_insert_requests
  blk-mq: replace and kill blk_mq_request_issue_directly

block/blk-core.c     |   4 +-
block/blk-mq-sched.c |   8 ++--
block/blk-mq.c       | 126 +++++++++++++++++++++++++--------------------------
block/blk-mq.h       |   7 +--
4 files changed, 72 insertions(+), 73 deletions(-)

Thanks
Jianchao

[PATCH V6 1/5] blk-mq: refactor the code of issue request directly

[Jianchao Wang]

Merge blk_mq_try_issue_directly and __blk_mq_try_issue_directly
into one interface to unify the interfaces to issue requests
directly. The merged interface takes over the requests totally,
it could insert, end or do nothing based on the return value of
.queue_rq and 'bypass' parameter. Then caller needn't any other
handling any more.

Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
---
 block/blk-mq.c | 93 ++++++++++++++++++++++++++++------------------------------
 1 file changed, 45 insertions(+), 48 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 411be60..14b4d06 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1766,78 +1766,75 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
 	return ret;
 }
 
-static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
+static blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 						struct request *rq,
 						blk_qc_t *cookie,
-						bool bypass_insert)
+						bool bypass)
 {
 	struct request_queue *q = rq->q;
 	bool run_queue = true;
+	blk_status_t ret = BLK_STS_RESOURCE;
+	int srcu_idx;
 
+	hctx_lock(hctx, &srcu_idx);
 	/*
-	 * RCU or SRCU read lock is needed before checking quiesced flag.
+	 * hctx_lock is needed before checking quiesced flag.
 	 *
-	 * When queue is stopped or quiesced, ignore 'bypass_insert' from
-	 * blk_mq_request_issue_directly(), and return BLK_STS_OK to caller,
-	 * and avoid driver to try to dispatch again.
+	 * When queue is stopped or quiesced, ignore 'bypass', insert
+	 * and return BLK_STS_OK to caller, and avoid driver to try to
+	 * dispatch again.
 	 */
-	if (blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)) {
+	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q))) {
 		run_queue = false;
-		bypass_insert = false;
-		goto insert;
+		bypass = false;
+		goto out_unlock;
 	}
 
-	if (q->elevator && !bypass_insert)
-		goto insert;
+	/*
+	 * Bypass the potential scheduler on the bottom device.
+	 */
+	if (unlikely(q->elevator && !bypass))
+		goto out_unlock;
 
-	if (!blk_mq_get_dispatch_budget(hctx))
-		goto insert;
+	if (unlikely(!blk_mq_get_dispatch_budget(hctx)))
+		goto out_unlock;
 
-	if (!blk_mq_get_driver_tag(rq)) {
+	if (unlikely(!blk_mq_get_driver_tag(rq))) {
 		blk_mq_put_dispatch_budget(hctx);
-		goto insert;
+		goto out_unlock;
 	}
 
-	return __blk_mq_issue_directly(hctx, rq, cookie);
-insert:
-	if (bypass_insert)
-		return BLK_STS_RESOURCE;
-
-	blk_mq_sched_insert_request(rq, false, run_queue, false);
-	return BLK_STS_OK;
-}
-
-static void blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
-		struct request *rq, blk_qc_t *cookie)
-{
-	blk_status_t ret;
-	int srcu_idx;
-
-	might_sleep_if(hctx->flags & BLK_MQ_F_BLOCKING);
+	ret = __blk_mq_issue_directly(hctx, rq, cookie);
 
-	hctx_lock(hctx, &srcu_idx);
+out_unlock:
+	hctx_unlock(hctx, srcu_idx);
 
-	ret = __blk_mq_try_issue_directly(hctx, rq, cookie, false);
-	if (ret == BLK_STS_RESOURCE || ret == BLK_STS_DEV_RESOURCE)
-		blk_mq_sched_insert_request(rq, false, true, false);
-	else if (ret != BLK_STS_OK)
-		blk_mq_end_request(rq, ret);
+	switch (ret) {
+	case BLK_STS_OK:
+		break;
+	case BLK_STS_DEV_RESOURCE:
+	case BLK_STS_RESOURCE:
+		if (!bypass) {
+			blk_mq_sched_insert_request(rq, false, run_queue, false);
+			ret = BLK_STS_OK;
+		}
+		break;
+	default:
+		if (!bypass) {
+			blk_mq_end_request(rq, ret);
+			ret = BLK_STS_OK;
+		}
+		break;
+	}
 
-	hctx_unlock(hctx, srcu_idx);
+	return ret;
 }
 
 blk_status_t blk_mq_request_issue_directly(struct request *rq)
 {
-	blk_status_t ret;
-	int srcu_idx;
 	blk_qc_t unused_cookie;
-	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
 
-	hctx_lock(hctx, &srcu_idx);
-	ret = __blk_mq_try_issue_directly(hctx, rq, &unused_cookie, true);
-	hctx_unlock(hctx, srcu_idx);
-
-	return ret;
+	return blk_mq_try_issue_directly(rq->mq_hctx, rq, &unused_cookie, true);
 }
 
 void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
@@ -1958,13 +1955,13 @@ static blk_qc_t blk_mq_make_request(struct request_queue *q, struct bio *bio)
 		if (same_queue_rq) {
 			data.hctx = same_queue_rq->mq_hctx;
 			blk_mq_try_issue_directly(data.hctx, same_queue_rq,
-					&cookie);
+					&cookie, false);
 		}
 	} else if ((q->nr_hw_queues > 1 && is_sync) || (!q->elevator &&
 			!data.hctx->dispatch_busy)) {
 		blk_mq_put_ctx(data.ctx);
 		blk_mq_bio_to_request(rq, bio);
-		blk_mq_try_issue_directly(data.hctx, rq, &cookie);
+		blk_mq_try_issue_directly(data.hctx, rq, &cookie, false);
 	} else {
 		blk_mq_put_ctx(data.ctx);
 		blk_mq_bio_to_request(rq, bio);
-- 
2.7.4

[PATCH V6 2/5] blk-mq: fix issue directly case when q is stopped or quiesced

[Jianchao Wang]

When try to issue request directly, if the queue is stopped or
quiesced, 'bypass' will be ignored and return BLK_STS_OK to caller
to avoid it dispatch request again. Then the request will be
inserted with blk_mq_sched_insert_request. This is not correct
for dm-rq case where we should avoid to pass through the underlying
path's io scheduler.

To fix it, use blk_mq_request_bypass_insert to insert the request
to hctx->dispatch when we cannot pass through io scheduler but have
to insert.

Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
---
 block/blk-mq.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 14b4d06..11c52bb 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1772,7 +1772,7 @@ static blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 						bool bypass)
 {
 	struct request_queue *q = rq->q;
-	bool run_queue = true;
+	bool run_queue = true, force = false;
 	blk_status_t ret = BLK_STS_RESOURCE;
 	int srcu_idx;
 
@@ -1786,7 +1786,7 @@ static blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 	 */
 	if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q))) {
 		run_queue = false;
-		bypass = false;
+		force = true;
 		goto out_unlock;
 	}
 
@@ -1817,6 +1817,9 @@ static blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 		if (!bypass) {
 			blk_mq_sched_insert_request(rq, false, run_queue, false);
 			ret = BLK_STS_OK;
+		} else if (force) {
+			blk_mq_request_bypass_insert(rq, run_queue);
+			ret = BLK_STS_OK;
 		}
 		break;
 	default:
-- 
2.7.4

[PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly

[Jianchao Wang]

When issue request directly and the task is migrated out of the
original cpu where it allocates request, hctx could be ran on
the cpu where it is not mapped.
To fix this,
 - insert the request forcibly if BLK_MQ_F_BLOCKING is set.
 - check whether the current is mapped to the hctx, if not, insert
   forcibly.
 - invoke __blk_mq_issue_directly under preemption disabled.

Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
---
 block/blk-mq.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 11c52bb..58f15cc 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1776,6 +1776,17 @@ static blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 	blk_status_t ret = BLK_STS_RESOURCE;
 	int srcu_idx;
 
+	if (hctx->flags & BLK_MQ_F_BLOCKING) {
+		force = true;
+		goto out;
+	}
+
+	if (unlikely(!cpumask_test_cpu(get_cpu(), hctx->cpumask))) {
+		put_cpu();
+		force = true;
+		goto out;
+	}
+
 	hctx_lock(hctx, &srcu_idx);
 	/*
 	 * hctx_lock is needed before checking quiesced flag.
@@ -1808,7 +1819,8 @@ static blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 
 out_unlock:
 	hctx_unlock(hctx, srcu_idx);
-
+	put_cpu();
+out:
 	switch (ret) {
 	case BLK_STS_OK:
 		break;
-- 
2.7.4

[PATCH V6 4/5] blk-mq: issue directly with bypass 'false' in blk_mq_sched_insert_requests

[Jianchao Wang]

It is not necessary to issue request directly with bypass 'true'
in blk_mq_sched_insert_requests and handle the non-issued requests
itself. Just set bypass to 'false' and let blk_mq_try_issue_directly
handle them totally.

Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
---
 block/blk-mq-sched.c |  8 +++-----
 block/blk-mq.c       | 13 +++----------
 2 files changed, 6 insertions(+), 15 deletions(-)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 66fda19..9af57c8 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -410,12 +410,10 @@ void blk_mq_sched_insert_requests(struct blk_mq_hw_ctx *hctx,
 		 * busy in case of 'none' scheduler, and this way may save
 		 * us one extra enqueue & dequeue to sw queue.
 		 */
-		if (!hctx->dispatch_busy && !e && !run_queue_async) {
+		if (!hctx->dispatch_busy && !e && !run_queue_async)
 			blk_mq_try_issue_list_directly(hctx, list);
-			if (list_empty(list))
-				return;
-		}
-		blk_mq_insert_requests(hctx, ctx, list);
+		else
+			blk_mq_insert_requests(hctx, ctx, list);
 	}
 
 	blk_mq_run_hw_queue(hctx, run_queue_async);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 58f15cc..f41a815 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1855,21 +1855,14 @@ blk_status_t blk_mq_request_issue_directly(struct request *rq)
 void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
 		struct list_head *list)
 {
+	blk_qc_t unused_cookie;
+
 	while (!list_empty(list)) {
-		blk_status_t ret;
 		struct request *rq = list_first_entry(list, struct request,
 				queuelist);
 
 		list_del_init(&rq->queuelist);
-		ret = blk_mq_request_issue_directly(rq);
-		if (ret != BLK_STS_OK) {
-			if (ret == BLK_STS_RESOURCE ||
-					ret == BLK_STS_DEV_RESOURCE) {
-				list_add(&rq->queuelist, list);
-				break;
-			}
-			blk_mq_end_request(rq, ret);
-		}
+		blk_mq_try_issue_directly(hctx, rq, &unused_cookie, false);
 	}
 }
 
-- 
2.7.4

[PATCH V6 5/5] blk-mq: replace and kill blk_mq_request_issue_directly

[Jianchao Wang]

Replace blk_mq_request_issue_directly with blk_mq_try_issue_directly
in blk_insert_cloned_request and kill it as nobody uses it any more.

Signed-off-by: Jianchao Wang <jianchao.w.wang@oracle.com>
---
 block/blk-core.c | 4 +++-
 block/blk-mq.c   | 9 +--------
 block/blk-mq.h   | 7 ++++---
 3 files changed, 8 insertions(+), 12 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index fdc0ad2..e4eedc7 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1421,6 +1421,8 @@ static int blk_cloned_rq_check_limits(struct request_queue *q,
  */
 blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *rq)
 {
+	blk_qc_t unused_cookie;
+
 	if (blk_cloned_rq_check_limits(q, rq))
 		return BLK_STS_IOERR;
 
@@ -1436,7 +1438,7 @@ blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *
 	 * bypass a potential scheduler on the bottom device for
 	 * insert.
 	 */
-	return blk_mq_request_issue_directly(rq);
+	return blk_mq_try_issue_directly(rq->mq_hctx, rq, &unused_cookie, true);
 }
 EXPORT_SYMBOL_GPL(blk_insert_cloned_request);
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index f41a815..b5316c78 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1766,7 +1766,7 @@ static blk_status_t __blk_mq_issue_directly(struct blk_mq_hw_ctx *hctx,
 	return ret;
 }
 
-static blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
+blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 						struct request *rq,
 						blk_qc_t *cookie,
 						bool bypass)
@@ -1845,13 +1845,6 @@ static blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 	return ret;
 }
 
-blk_status_t blk_mq_request_issue_directly(struct request *rq)
-{
-	blk_qc_t unused_cookie;
-
-	return blk_mq_try_issue_directly(rq->mq_hctx, rq, &unused_cookie, true);
-}
-
 void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
 		struct list_head *list)
 {
diff --git a/block/blk-mq.h b/block/blk-mq.h
index facb6e9..f18c27c 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -61,9 +61,10 @@ void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
 void blk_mq_request_bypass_insert(struct request *rq, bool run_queue);
 void blk_mq_insert_requests(struct blk_mq_hw_ctx *hctx, struct blk_mq_ctx *ctx,
 				struct list_head *list);
-
-/* Used by blk_insert_cloned_request() to issue request directly */
-blk_status_t blk_mq_request_issue_directly(struct request *rq);
+blk_status_t blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
+						struct request *rq,
+						blk_qc_t *cookie,
+						bool bypass);
 void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
 				    struct list_head *list);
 
-- 
2.7.4

Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly

[Jens Axboe]

On 11/13/18 2:56 AM, Jianchao Wang wrote:
> When issue request directly and the task is migrated out of the
> original cpu where it allocates request, hctx could be ran on
> the cpu where it is not mapped.
> To fix this,
>  - insert the request forcibly if BLK_MQ_F_BLOCKING is set.
>  - check whether the current is mapped to the hctx, if not, insert
>    forcibly.
>  - invoke __blk_mq_issue_directly under preemption disabled.

I'm not too crazy about this one, adding a get/put_cpu() in the hot
path, and a cpumask test. The fact is that most/no drivers care
about strict placement. We always try to do so, if convenient,
since it's faster, but this seems to be doing the opposite.

I'd be more inclined to have a driver flag if it needs guaranteed
placement, using one an ops BLK_MQ_F_STRICT_CPU flag or similar.

What do you think?

-- 
Jens Axboe

Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly

[jianchao.wang]

Hi Jens

Thanks for your kindly response.

On 11/13/18 9:44 PM, Jens Axboe wrote:
> On 11/13/18 2:56 AM, Jianchao Wang wrote:
>> When issue request directly and the task is migrated out of the
>> original cpu where it allocates request, hctx could be ran on
>> the cpu where it is not mapped.
>> To fix this,
>>  - insert the request forcibly if BLK_MQ_F_BLOCKING is set.
>>  - check whether the current is mapped to the hctx, if not, insert
>>    forcibly.
>>  - invoke __blk_mq_issue_directly under preemption disabled.
> 
> I'm not too crazy about this one, adding a get/put_cpu() in the hot
> path, and a cpumask test. The fact is that most/no drivers care
> about strict placement. We always try to do so, if convenient,
> since it's faster, but this seems to be doing the opposite.
> 
> I'd be more inclined to have a driver flag if it needs guaranteed
> placement, using one an ops BLK_MQ_F_STRICT_CPU flag or similar.
> 
> What do you think?
> 

I'd inclined blk-mq should comply with a unified rule, no matter the
issuing directly path or inserting one. Then blk-mq would have a simpler
model. And also this guarantee could be a little good for drivers,
especially the case where cpu and hw queue mapping is 1:1.

Regarding with hot path, do you concern about the nvme device ?
If so, how about split a standalone path for it ?

Thanks
Jianchao

Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly

[Ming Lei]

On Wed, Nov 14, 2018 at 10:15 AM jianchao.wang
<jianchao.w.wang@oracle.com> wrote:
>
> Hi Jens
>
> Thanks for your kindly response.
>
> On 11/13/18 9:44 PM, Jens Axboe wrote:
> > On 11/13/18 2:56 AM, Jianchao Wang wrote:
> >> When issue request directly and the task is migrated out of the
> >> original cpu where it allocates request, hctx could be ran on
> >> the cpu where it is not mapped.
> >> To fix this,
> >>  - insert the request forcibly if BLK_MQ_F_BLOCKING is set.
> >>  - check whether the current is mapped to the hctx, if not, insert
> >>    forcibly.
> >>  - invoke __blk_mq_issue_directly under preemption disabled.
> >
> > I'm not too crazy about this one, adding a get/put_cpu() in the hot
> > path, and a cpumask test. The fact is that most/no drivers care
> > about strict placement. We always try to do so, if convenient,
> > since it's faster, but this seems to be doing the opposite.
> >
> > I'd be more inclined to have a driver flag if it needs guaranteed
> > placement, using one an ops BLK_MQ_F_STRICT_CPU flag or similar.
> >
> > What do you think?
> >
>
> I'd inclined blk-mq should comply with a unified rule, no matter the
> issuing directly path or inserting one. Then blk-mq would have a simpler
> model. And also this guarantee could be a little good for drivers,
> especially the case where cpu and hw queue mapping is 1:1.

I guess it is quite hard to respect this rule 100%, such as in case of
CPU hotplug.

Thanks,
Ming Lei

Re: [PATCH V6 3/5] blk-mq: ensure hctx to be ran on mapped cpu when issue directly

[jianchao.wang]

On 11/14/18 11:02 AM, Ming Lei wrote:
> On Wed, Nov 14, 2018 at 10:15 AM jianchao.wang
> <jianchao.w.wang@oracle.com> wrote:
>>
>> Hi Jens
>>
>> Thanks for your kindly response.
>>
>> On 11/13/18 9:44 PM, Jens Axboe wrote:
>>> On 11/13/18 2:56 AM, Jianchao Wang wrote:
>>>> When issue request directly and the task is migrated out of the
>>>> original cpu where it allocates request, hctx could be ran on
>>>> the cpu where it is not mapped.
>>>> To fix this,
>>>>  - insert the request forcibly if BLK_MQ_F_BLOCKING is set.
>>>>  - check whether the current is mapped to the hctx, if not, insert
>>>>    forcibly.
>>>>  - invoke __blk_mq_issue_directly under preemption disabled.
>>>
>>> I'm not too crazy about this one, adding a get/put_cpu() in the hot
>>> path, and a cpumask test. The fact is that most/no drivers care
>>> about strict placement. We always try to do so, if convenient,
>>> since it's faster, but this seems to be doing the opposite.
>>>
>>> I'd be more inclined to have a driver flag if it needs guaranteed
>>> placement, using one an ops BLK_MQ_F_STRICT_CPU flag or similar.
>>>
>>> What do you think?
>>>
>>
>> I'd inclined blk-mq should comply with a unified rule, no matter the
>> issuing directly path or inserting one. Then blk-mq would have a simpler
>> model. And also this guarantee could be a little good for drivers,
>> especially the case where cpu and hw queue mapping is 1:1.
> 
> I guess it is quite hard to respect this rule 100%, such as in case of
> CPU hotplug.
> 
Yes, it is indeed the case.

Looks like this patch is contentious.
I will drop this one and post later as a standalone one if necessary.

Thanks
Jianchao