[PATCH V2 RESEND 0/5] blk-mq: improvement on handling IO during CPU hotplug

[PATCH V2 RESEND 0/5] blk-mq: improvement on handling IO during CPU hotplug

[Ming Lei]

Hi,

Thomas mentioned:
    "
     That was the constraint of managed interrupts from the very beginning:
    
      The driver/subsystem has to quiesce the interrupt line and the associated
      queue _before_ it gets shutdown in CPU unplug and not fiddle with it
      until it's restarted by the core when the CPU is plugged in again.
    "

But no drivers or blk-mq do that before one hctx becomes dead(all
CPUs for one hctx are offline), and even it is worse, blk-mq stills tries
to run hw queue after hctx is dead, see blk_mq_hctx_notify_dead().

This patchset tries to address the issue by two stages:

1) add one new cpuhp state of CPUHP_AP_BLK_MQ_ONLINE

- mark the hctx as internal stopped, and drain all in-flight requests
if the hctx is going to be dead.

2) re-submit IO in the state of CPUHP_BLK_MQ_DEAD after the hctx becomes dead

- steal bios from the request, and resubmit them via generic_make_request(),
then these IO will be mapped to other live hctx for dispatch

Please comment & review, thanks!

V2:
	- patch4 & patch 5 in V1 have been merged to block tree, so remove
	  them
	- address comments from John Garry and Minwoo

Ming Lei (5):
  blk-mq: add new state of BLK_MQ_S_INTERNAL_STOPPED
  blk-mq: add blk-mq flag of BLK_MQ_F_NO_MANAGED_IRQ
  blk-mq: stop to handle IO before hctx's all CPUs become offline
  blk-mq: re-submit IO in case that hctx is dead
  blk-mq: handle requests dispatched from IO scheduler in case that hctx
    is dead

 block/blk-mq-debugfs.c     |   2 +
 block/blk-mq-tag.c         |   2 +-
 block/blk-mq-tag.h         |   2 +
 block/blk-mq.c             | 143 +++++++++++++++++++++++++++++++++----
 block/blk-mq.h             |   3 +-
 drivers/block/loop.c       |   2 +-
 drivers/md/dm-rq.c         |   2 +-
 include/linux/blk-mq.h     |   5 ++
 include/linux/cpuhotplug.h |   1 +
 9 files changed, 146 insertions(+), 16 deletions(-)

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Keith Busch <keith.busch@intel.com>
-- 
2.20.1

[PATCH V2 RESEND 1/5] blk-mq: add new state of BLK_MQ_S_INTERNAL_STOPPED

[Ming Lei]

Add a new hw queue state of BLK_MQ_S_INTERNAL_STOPPED, which prepares
for stopping hw queue before all CPUs of this hctx become offline.

We can't reuse BLK_MQ_S_STOPPED because that state can be cleared during IO
completion.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Keith Busch <keith.busch@intel.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-debugfs.c | 1 +
 block/blk-mq.h         | 3 ++-
 include/linux/blk-mq.h | 3 +++
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index b3f2ba483992..af40a02c46ee 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -213,6 +213,7 @@ static const char *const hctx_state_name[] = {
 	HCTX_STATE_NAME(STOPPED),
 	HCTX_STATE_NAME(TAG_ACTIVE),
 	HCTX_STATE_NAME(SCHED_RESTART),
+	HCTX_STATE_NAME(INTERNAL_STOPPED),
 };
 #undef HCTX_STATE_NAME
 
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 32c62c64e6c2..63717573bc16 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -176,7 +176,8 @@ static inline struct blk_mq_tags *blk_mq_tags_from_data(struct blk_mq_alloc_data
 
 static inline bool blk_mq_hctx_stopped(struct blk_mq_hw_ctx *hctx)
 {
-	return test_bit(BLK_MQ_S_STOPPED, &hctx->state);
+	return test_bit(BLK_MQ_S_STOPPED, &hctx->state) ||
+		test_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
 }
 
 static inline bool blk_mq_hw_queue_mapped(struct blk_mq_hw_ctx *hctx)
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 0bf056de5cc3..079c282e4471 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -235,6 +235,9 @@ enum {
 	BLK_MQ_S_TAG_ACTIVE	= 1,
 	BLK_MQ_S_SCHED_RESTART	= 2,
 
+	/* hw queue is internal stopped, driver do not use it */
+	BLK_MQ_S_INTERNAL_STOPPED	= 3,
+
 	BLK_MQ_MAX_DEPTH	= 10240,
 
 	BLK_MQ_CPU_WORK_BATCH	= 8,
-- 
2.20.1

[PATCH V2 RESEND 2/5] blk-mq: add blk-mq flag of BLK_MQ_F_NO_MANAGED_IRQ

[Ming Lei]

We will stop hw queue and wait for completion of in-flight requests
when one hctx is becoming dead in the following patch. This way may
cause dead-lock for some stacking blk-mq drivers, such as dm-rq and
loop.

Add blk-mq flag of BLK_MQ_F_NO_MANAGED_IRQ and mark it for dm-rq and
loop, so we needn't to wait for completion of in-flight requests of
dm-rq & loop, then the potential dead-lock can be avoided.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Keith Busch <keith.busch@intel.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-debugfs.c | 1 +
 drivers/block/loop.c   | 2 +-
 drivers/md/dm-rq.c     | 2 +-
 include/linux/blk-mq.h | 1 +
 4 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index af40a02c46ee..24fff8c90942 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -240,6 +240,7 @@ static const char *const hctx_flag_name[] = {
 	HCTX_FLAG_NAME(TAG_SHARED),
 	HCTX_FLAG_NAME(BLOCKING),
 	HCTX_FLAG_NAME(NO_SCHED),
+	HCTX_FLAG_NAME(NO_MANAGED_IRQ),
 };
 #undef HCTX_FLAG_NAME
 
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f6f77eaa7217..751a28a1d4b0 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -1999,7 +1999,7 @@ static int loop_add(struct loop_device **l, int i)
 	lo->tag_set.queue_depth = 128;
 	lo->tag_set.numa_node = NUMA_NO_NODE;
 	lo->tag_set.cmd_size = sizeof(struct loop_cmd);
-	lo->tag_set.flags = BLK_MQ_F_SHOULD_MERGE;
+	lo->tag_set.flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_NO_MANAGED_IRQ;
 	lo->tag_set.driver_data = lo;
 
 	err = blk_mq_alloc_tag_set(&lo->tag_set);
diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
index 3f8577e2c13b..5f1ff70ac029 100644
--- a/drivers/md/dm-rq.c
+++ b/drivers/md/dm-rq.c
@@ -547,7 +547,7 @@ int dm_mq_init_request_queue(struct mapped_device *md, struct dm_table *t)
 	md->tag_set->ops = &dm_mq_ops;
 	md->tag_set->queue_depth = dm_get_blk_mq_queue_depth();
 	md->tag_set->numa_node = md->numa_node_id;
-	md->tag_set->flags = BLK_MQ_F_SHOULD_MERGE;
+	md->tag_set->flags = BLK_MQ_F_SHOULD_MERGE | BLK_MQ_F_NO_MANAGED_IRQ;
 	md->tag_set->nr_hw_queues = dm_get_blk_mq_nr_hw_queues();
 	md->tag_set->driver_data = md;
 
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 079c282e4471..ee60885ec855 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -226,6 +226,7 @@ struct blk_mq_ops {
 enum {
 	BLK_MQ_F_SHOULD_MERGE	= 1 << 0,
 	BLK_MQ_F_TAG_SHARED	= 1 << 1,
+	BLK_MQ_F_NO_MANAGED_IRQ	= 1 << 2,
 	BLK_MQ_F_BLOCKING	= 1 << 5,
 	BLK_MQ_F_NO_SCHED	= 1 << 6,
 	BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,
-- 
2.20.1

[PATCH V2 RESEND 3/5] blk-mq: stop to handle IO before hctx's all CPUs become offline

[Ming Lei]

Most of blk-mq drivers depend on managed IRQ's auto-affinity to setup
up queue mapping. Thomas mentioned the following point[1]:

"
 That was the constraint of managed interrupts from the very beginning:

  The driver/subsystem has to quiesce the interrupt line and the associated
  queue _before_ it gets shutdown in CPU unplug and not fiddle with it
  until it's restarted by the core when the CPU is plugged in again.
"

However, current blk-mq implementation doesn't quiesce hw queue before
the last CPU in the hctx is shutdown. Even worse, CPUHP_BLK_MQ_DEAD is
one cpuhp state handled after the CPU is down, so there isn't any chance
to quiesce hctx for blk-mq wrt. CPU hotplug.

Add new cpuhp state of CPUHP_AP_BLK_MQ_ONLINE for blk-mq to stop queues
and wait for completion of in-flight requests.

[1] https://lore.kernel.org/linux-block/alpine.DEB.2.21.1904051331270.1802@nanos.tec.linutronix.de/

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Keith Busch <keith.busch@intel.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-tag.c         |  2 +-
 block/blk-mq-tag.h         |  2 ++
 block/blk-mq.c             | 65 ++++++++++++++++++++++++++++++++++++++
 include/linux/blk-mq.h     |  1 +
 include/linux/cpuhotplug.h |  1 +
 5 files changed, 70 insertions(+), 1 deletion(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 008388e82b5c..31828b82552b 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -325,7 +325,7 @@ static void bt_tags_for_each(struct blk_mq_tags *tags, struct sbitmap_queue *bt,
  *		true to continue iterating tags, false to stop.
  * @priv:	Will be passed as second argument to @fn.
  */
-static void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags,
+void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags,
 		busy_tag_iter_fn *fn, void *priv)
 {
 	if (tags->nr_reserved_tags)
diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index 61deab0b5a5a..321fd6f440e6 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h
@@ -35,6 +35,8 @@ extern int blk_mq_tag_update_depth(struct blk_mq_hw_ctx *hctx,
 extern void blk_mq_tag_wakeup_all(struct blk_mq_tags *tags, bool);
 void blk_mq_queue_tag_busy_iter(struct request_queue *q, busy_iter_fn *fn,
 		void *priv);
+void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags,
+		busy_tag_iter_fn *fn, void *priv);
 
 static inline struct sbq_wait_state *bt_wait_ptr(struct sbitmap_queue *bt,
 						 struct blk_mq_hw_ctx *hctx)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index ec791156e9cc..d991c122abf2 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2225,6 +2225,61 @@ int blk_mq_alloc_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 	return -ENOMEM;
 }
 
+static bool blk_mq_count_inflight_rq(struct request *rq, void *data,
+				     bool reserved)
+{
+	unsigned *count = data;
+
+	if ((blk_mq_rq_state(rq) == MQ_RQ_IN_FLIGHT))
+		(*count)++;
+
+	return true;
+}
+
+static unsigned blk_mq_tags_inflight_rqs(struct blk_mq_tags *tags)
+{
+	unsigned count = 0;
+
+	blk_mq_all_tag_busy_iter(tags, blk_mq_count_inflight_rq, &count);
+
+	return count;
+}
+
+static void blk_mq_drain_inflight_rqs(struct blk_mq_hw_ctx *hctx)
+{
+	while (1) {
+		if (!blk_mq_tags_inflight_rqs(hctx->tags))
+			break;
+		msleep(5);
+	}
+}
+
+static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
+{
+	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
+			struct blk_mq_hw_ctx, cpuhp_online);
+	unsigned prev_cpu = -1;
+
+	while (true) {
+		unsigned next_cpu = cpumask_next_and(prev_cpu, hctx->cpumask,
+				cpu_online_mask);
+
+		if (next_cpu >= nr_cpu_ids)
+			break;
+
+		/* return if there is other online CPU on this hctx */
+		if (next_cpu != cpu)
+			return 0;
+
+		prev_cpu = next_cpu;
+	}
+
+	set_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
+	blk_mq_drain_inflight_rqs(hctx);
+
+	return 0;
+}
+
 /*
  * 'cpu' is going away. splice any existing rq_list entries from this
  * software queue to the hw queue dispatch list, and ensure that it
@@ -2241,6 +2296,8 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 	ctx = __blk_mq_get_ctx(hctx->queue, cpu);
 	type = hctx->type;
 
+	clear_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
+
 	spin_lock(&ctx->lock);
 	if (!list_empty(&ctx->rq_lists[type])) {
 		list_splice_init(&ctx->rq_lists[type], &tmp);
@@ -2261,6 +2318,9 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 
 static void blk_mq_remove_cpuhp(struct blk_mq_hw_ctx *hctx)
 {
+	if (!(hctx->flags & BLK_MQ_F_NO_MANAGED_IRQ))
+		cpuhp_state_remove_instance_nocalls(CPUHP_AP_BLK_MQ_ONLINE,
+						    &hctx->cpuhp_online);
 	cpuhp_state_remove_instance_nocalls(CPUHP_BLK_MQ_DEAD,
 					    &hctx->cpuhp_dead);
 }
@@ -2320,6 +2380,9 @@ static int blk_mq_init_hctx(struct request_queue *q,
 {
 	hctx->queue_num = hctx_idx;
 
+	if (!(hctx->flags & BLK_MQ_F_NO_MANAGED_IRQ))
+		cpuhp_state_add_instance_nocalls(CPUHP_AP_BLK_MQ_ONLINE,
+				&hctx->cpuhp_online);
 	cpuhp_state_add_instance_nocalls(CPUHP_BLK_MQ_DEAD, &hctx->cpuhp_dead);
 
 	hctx->tags = set->tags[hctx_idx];
@@ -3547,6 +3610,8 @@ static int __init blk_mq_init(void)
 {
 	cpuhp_setup_state_multi(CPUHP_BLK_MQ_DEAD, "block/mq:dead", NULL,
 				blk_mq_hctx_notify_dead);
+	cpuhp_setup_state_multi(CPUHP_AP_BLK_MQ_ONLINE, "block/mq:online",
+				NULL, blk_mq_hctx_notify_online);
 	return 0;
 }
 subsys_initcall(blk_mq_init);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index ee60885ec855..a345f2cf920d 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -58,6 +58,7 @@ struct blk_mq_hw_ctx {
 
 	atomic_t		nr_active;
 
+	struct hlist_node	cpuhp_online;
 	struct hlist_node	cpuhp_dead;
 	struct kobject		kobj;
 
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 068793a619ca..bb80f52040cb 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -147,6 +147,7 @@ enum cpuhp_state {
 	CPUHP_AP_SMPBOOT_THREADS,
 	CPUHP_AP_X86_VDSO_VMA_ONLINE,
 	CPUHP_AP_IRQ_AFFINITY_ONLINE,
+	CPUHP_AP_BLK_MQ_ONLINE,
 	CPUHP_AP_ARM_MVEBU_SYNC_CLOCKS,
 	CPUHP_AP_X86_INTEL_EPB_ONLINE,
 	CPUHP_AP_PERF_ONLINE,
-- 
2.20.1

[PATCH V2 RESEND 4/5] blk-mq: re-submit IO in case that hctx is dead

[Ming Lei]

When all CPUs in one hctx are offline, we shouldn't run this hw queue
for completing request any more.

So steal bios from the request, and resubmit them, and finally free
the request in blk_mq_hctx_notify_dead().

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Keith Busch <keith.busch@intel.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c | 48 +++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 41 insertions(+), 7 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index d991c122abf2..0b35fdbd1f17 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2280,10 +2280,30 @@ static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
 	return 0;
 }
 
+static void blk_mq_resubmit_io(struct request *rq)
+{
+	struct bio_list list;
+	struct bio *bio;
+
+	bio_list_init(&list);
+	blk_steal_bios(&list, rq);
+
+	while (true) {
+		bio = bio_list_pop(&list);
+		if (!bio)
+			break;
+
+		generic_make_request(bio);
+	}
+
+	blk_mq_cleanup_rq(rq);
+	blk_mq_end_request(rq, 0);
+}
+
 /*
- * 'cpu' is going away. splice any existing rq_list entries from this
- * software queue to the hw queue dispatch list, and ensure that it
- * gets run.
+ * 'cpu' has gone away. If this hctx is dead, we can't dispatch request
+ * to the hctx any more, so steal bios from requests of this hctx, and
+ * re-submit them to the request queue, and free these requests finally.
  */
 static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 {
@@ -2291,6 +2311,8 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 	struct blk_mq_ctx *ctx;
 	LIST_HEAD(tmp);
 	enum hctx_type type;
+	bool hctx_dead;
+	struct request *rq;
 
 	hctx = hlist_entry_safe(node, struct blk_mq_hw_ctx, cpuhp_dead);
 	ctx = __blk_mq_get_ctx(hctx->queue, cpu);
@@ -2298,6 +2320,9 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 
 	clear_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
 
+	hctx_dead = cpumask_first_and(hctx->cpumask, cpu_online_mask) >=
+		nr_cpu_ids;
+
 	spin_lock(&ctx->lock);
 	if (!list_empty(&ctx->rq_lists[type])) {
 		list_splice_init(&ctx->rq_lists[type], &tmp);
@@ -2308,11 +2333,20 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 	if (list_empty(&tmp))
 		return 0;
 
-	spin_lock(&hctx->lock);
-	list_splice_tail_init(&tmp, &hctx->dispatch);
-	spin_unlock(&hctx->lock);
+	if (!hctx_dead) {
+		spin_lock(&hctx->lock);
+		list_splice_tail_init(&tmp, &hctx->dispatch);
+		spin_unlock(&hctx->lock);
+		blk_mq_run_hw_queue(hctx, true);
+		return 0;
+	}
+
+	while (!list_empty(&tmp)) {
+		rq = list_entry(tmp.next, struct request, queuelist);
+		list_del_init(&rq->queuelist);
+		blk_mq_resubmit_io(rq);
+	}
 
-	blk_mq_run_hw_queue(hctx, true);
 	return 0;
 }
 
-- 
2.20.1

[PATCH V2 RESEND 5/5] blk-mq: handle requests dispatched from IO scheduler in case that hctx is dead

[Ming Lei]

If hctx becomes dead, all in-queue IO requests aimed at this hctx have to
be re-submitted, so cover requests queued in scheduler queue.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Hannes Reinecke <hare@suse.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Keith Busch <keith.busch@intel.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c | 30 +++++++++++++++++++++++++-----
 1 file changed, 25 insertions(+), 5 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 0b35fdbd1f17..94fd47cef1bc 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2313,6 +2313,7 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 	enum hctx_type type;
 	bool hctx_dead;
 	struct request *rq;
+	struct elevator_queue *e;
 
 	hctx = hlist_entry_safe(node, struct blk_mq_hw_ctx, cpuhp_dead);
 	ctx = __blk_mq_get_ctx(hctx->queue, cpu);
@@ -2323,12 +2324,31 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 	hctx_dead = cpumask_first_and(hctx->cpumask, cpu_online_mask) >=
 		nr_cpu_ids;
 
-	spin_lock(&ctx->lock);
-	if (!list_empty(&ctx->rq_lists[type])) {
-		list_splice_init(&ctx->rq_lists[type], &tmp);
-		blk_mq_hctx_clear_pending(hctx, ctx);
+	e = hctx->queue->elevator;
+	if (!e) {
+		spin_lock(&ctx->lock);
+		if (!list_empty(&ctx->rq_lists[type])) {
+			list_splice_init(&ctx->rq_lists[type], &tmp);
+			blk_mq_hctx_clear_pending(hctx, ctx);
+		}
+		spin_unlock(&ctx->lock);
+	} else if (hctx_dead) {
+		LIST_HEAD(sched_tmp);
+
+		while ((rq = e->type->ops.dispatch_request(hctx))) {
+			if (rq->mq_hctx != hctx)
+				list_add(&rq->queuelist, &sched_tmp);
+			else
+				list_add(&rq->queuelist, &tmp);
+		}
+
+		while (!list_empty(&sched_tmp)) {
+			rq = list_entry(sched_tmp.next, struct request,
+					queuelist);
+			list_del_init(&rq->queuelist);
+			blk_mq_sched_insert_request(rq, true, true, true);
+		}
 	}
-	spin_unlock(&ctx->lock);
 
 	if (list_empty(&tmp))
 		return 0;
-- 
2.20.1

Re: [PATCH V2 RESEND 1/5] blk-mq: add new state of BLK_MQ_S_INTERNAL_STOPPED

[Hannes Reinecke]

On 10/6/19 4:45 AM, Ming Lei wrote:
> Add a new hw queue state of BLK_MQ_S_INTERNAL_STOPPED, which prepares
> for stopping hw queue before all CPUs of this hctx become offline.
> 
> We can't reuse BLK_MQ_S_STOPPED because that state can be cleared during IO
> completion.
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Keith Busch <keith.busch@intel.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq-debugfs.c | 1 +
>  block/blk-mq.h         | 3 ++-
>  include/linux/blk-mq.h | 3 +++
>  3 files changed, 6 insertions(+), 1 deletion(-)
> 
Reviewed-by: Hannes Reinecke <hare@suse.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      Teamlead Storage & Networking
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 247165 (AG München), GF: Felix Imendörffer

Re: [PATCH V2 RESEND 2/5] blk-mq: add blk-mq flag of BLK_MQ_F_NO_MANAGED_IRQ

[Hannes Reinecke]

On 10/6/19 4:45 AM, Ming Lei wrote:
> We will stop hw queue and wait for completion of in-flight requests
> when one hctx is becoming dead in the following patch. This way may
> cause dead-lock for some stacking blk-mq drivers, such as dm-rq and
> loop.
> 
> Add blk-mq flag of BLK_MQ_F_NO_MANAGED_IRQ and mark it for dm-rq and
> loop, so we needn't to wait for completion of in-flight requests of
> dm-rq & loop, then the potential dead-lock can be avoided.
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Keith Busch <keith.busch@intel.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq-debugfs.c | 1 +
>  drivers/block/loop.c   | 2 +-
>  drivers/md/dm-rq.c     | 2 +-
>  include/linux/blk-mq.h | 1 +
>  4 files changed, 4 insertions(+), 2 deletions(-)
> 
I would have preferred to queue this patch after the next one;
introducing a flag which doesn't do anything is a bit odd to me.
But anyway:

Reviewed-by: Hannes Reinecke <hare@suse.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      Teamlead Storage & Networking
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 247165 (AG München), GF: Felix Imendörffer

Re: [PATCH V2 RESEND 3/5] blk-mq: stop to handle IO before hctx's all CPUs become offline

[Hannes Reinecke]

On 10/6/19 4:45 AM, Ming Lei wrote:
> Most of blk-mq drivers depend on managed IRQ's auto-affinity to setup
> up queue mapping. Thomas mentioned the following point[1]:
> 
> "
>  That was the constraint of managed interrupts from the very beginning:
> 
>   The driver/subsystem has to quiesce the interrupt line and the associated
>   queue _before_ it gets shutdown in CPU unplug and not fiddle with it
>   until it's restarted by the core when the CPU is plugged in again.
> "
> 
> However, current blk-mq implementation doesn't quiesce hw queue before
> the last CPU in the hctx is shutdown. Even worse, CPUHP_BLK_MQ_DEAD is
> one cpuhp state handled after the CPU is down, so there isn't any chance
> to quiesce hctx for blk-mq wrt. CPU hotplug.
> 
> Add new cpuhp state of CPUHP_AP_BLK_MQ_ONLINE for blk-mq to stop queues
> and wait for completion of in-flight requests.
> 
> [1] https://lore.kernel.org/linux-block/alpine.DEB.2.21.1904051331270.1802@nanos.tec.linutronix.de/
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Keith Busch <keith.busch@intel.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq-tag.c         |  2 +-
>  block/blk-mq-tag.h         |  2 ++
>  block/blk-mq.c             | 65 ++++++++++++++++++++++++++++++++++++++
>  include/linux/blk-mq.h     |  1 +
>  include/linux/cpuhotplug.h |  1 +
>  5 files changed, 70 insertions(+), 1 deletion(-)
> 
I really don't like the zillions of 'XXX_in_flight()' helper in blk-mq;
blk_mq_queue_inflight(), blk_mq_in_flight(), blk_mq_in_flight_rw() et al.
Can't you implement your one on top of the already existing?

Otherwise:
Reviewed-by: Hannes Reinecke <hare@suse.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      Teamlead Storage & Networking
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 247165 (AG München), GF: Felix Imendörffer

Re: [PATCH V2 RESEND 4/5] blk-mq: re-submit IO in case that hctx is dead

[Hannes Reinecke]

On 10/6/19 4:45 AM, Ming Lei wrote:
> When all CPUs in one hctx are offline, we shouldn't run this hw queue
> for completing request any more.
> 
> So steal bios from the request, and resubmit them, and finally free
> the request in blk_mq_hctx_notify_dead().
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Keith Busch <keith.busch@intel.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq.c | 48 +++++++++++++++++++++++++++++++++++++++++-------
>  1 file changed, 41 insertions(+), 7 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index d991c122abf2..0b35fdbd1f17 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2280,10 +2280,30 @@ static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
>  	return 0;
>  }
>  
> +static void blk_mq_resubmit_io(struct request *rq)
> +{
> +	struct bio_list list;
> +	struct bio *bio;
> +
> +	bio_list_init(&list);
> +	blk_steal_bios(&list, rq);
> +
> +	while (true) {
> +		bio = bio_list_pop(&list);
> +		if (!bio)
> +			break;
> +
> +		generic_make_request(bio);
> +	}
> +
> +	blk_mq_cleanup_rq(rq);
> +	blk_mq_end_request(rq, 0);
> +}
> +
Hmm. Not sure if this is a good idea.
Shouldn't we call 'blk_mq_end_request()' before calling
generic_make_request()?
otherwise the cloned request might be completed before original one,
which looks a bit dodgy to me; and might lead to quite a recursion if we
have several dead cpus to content with ...

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      Teamlead Storage & Networking
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 247165 (AG München), GF: Felix Imendörffer

Re: [PATCH V2 RESEND 5/5] blk-mq: handle requests dispatched from IO scheduler in case that hctx is dead

[Hannes Reinecke]

On 10/6/19 4:45 AM, Ming Lei wrote:
> If hctx becomes dead, all in-queue IO requests aimed at this hctx have to
> be re-submitted, so cover requests queued in scheduler queue.
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Hannes Reinecke <hare@suse.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: Keith Busch <keith.busch@intel.com>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  block/blk-mq.c | 30 +++++++++++++++++++++++++-----
>  1 file changed, 25 insertions(+), 5 deletions(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 0b35fdbd1f17..94fd47cef1bc 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2313,6 +2313,7 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
>  	enum hctx_type type;
>  	bool hctx_dead;
>  	struct request *rq;
> +	struct elevator_queue *e;
>  
>  	hctx = hlist_entry_safe(node, struct blk_mq_hw_ctx, cpuhp_dead);
>  	ctx = __blk_mq_get_ctx(hctx->queue, cpu);
> @@ -2323,12 +2324,31 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
>  	hctx_dead = cpumask_first_and(hctx->cpumask, cpu_online_mask) >=
>  		nr_cpu_ids;
>  
> -	spin_lock(&ctx->lock);
> -	if (!list_empty(&ctx->rq_lists[type])) {
> -		list_splice_init(&ctx->rq_lists[type], &tmp);
> -		blk_mq_hctx_clear_pending(hctx, ctx);
> +	e = hctx->queue->elevator;
> +	if (!e) {
> +		spin_lock(&ctx->lock);
> +		if (!list_empty(&ctx->rq_lists[type])) {
> +			list_splice_init(&ctx->rq_lists[type], &tmp);
> +			blk_mq_hctx_clear_pending(hctx, ctx);
> +		}
> +		spin_unlock(&ctx->lock);
> +	} else if (hctx_dead) {
> +		LIST_HEAD(sched_tmp);
> +
> +		while ((rq = e->type->ops.dispatch_request(hctx))) {
> +			if (rq->mq_hctx != hctx)
> +				list_add(&rq->queuelist, &sched_tmp);
> +			else
> +				list_add(&rq->queuelist, &tmp);
> +		}
> +
> +		while (!list_empty(&sched_tmp)) {
> +			rq = list_entry(sched_tmp.next, struct request,
> +					queuelist);
> +			list_del_init(&rq->queuelist);
> +			blk_mq_sched_insert_request(rq, true, true, true);
> +		}
>  	}
> -	spin_unlock(&ctx->lock);
>  
>  	if (list_empty(&tmp))
>  		return 0;
> 
Reviewed-by: Hannes Reinecke <hare@suse.com>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      Teamlead Storage & Networking
hare@suse.de			                  +49 911 74053 688
SUSE Software Solutions Germany GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 247165 (AG München), GF: Felix Imendörffer

Re: [PATCH V2 RESEND 3/5] blk-mq: stop to handle IO before hctx's all CPUs become offline

[John Garry]

On 06/10/2019 03:45, Ming Lei wrote:
> +	}
> +}
> +
> +static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
> +{
> +	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
> +			struct blk_mq_hw_ctx, cpuhp_online);
> +	unsigned prev_cpu = -1;
> +
> +	while (true) {
> +		unsigned next_cpu = cpumask_next_and(prev_cpu, hctx->cpumask,
> +				cpu_online_mask);
> +
> +		if (next_cpu >= nr_cpu_ids)
> +			break;
> +
> +		/* return if there is other online CPU on this hctx */
> +		if (next_cpu != cpu)
> +			return 0;
> +
> +		prev_cpu = next_cpu;
> +	}
> +
> +	set_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
> +	blk_mq_drain_inflight_rqs(hctx);
> +

Does this do the same:

{
	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
			struct blk_mq_hw_ctx, cpuhp_online);
	cpumask_var_t tmp;

	cpumask_and(tmp, hctx->cpumask, cpu_online_mask);

	/* test if there is any other cpu online in the hctx cpu mask */
	if (cpumask_any_but(tmp, cpu) < nr_cpu_ids)
		return 0;

	set_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
	blk_mq_drain_inflight_rqs(hctx);

	return 0;
}

If so, it's more readable and concise.

Thanks,
John


BTW, You could have added my Tested-by tags...

> +	return 0;
> +}
> +

Re: [PATCH V2 RESEND 3/5] blk-mq: stop to handle IO before hctx's all CPUs become offline

[Ming Lei]

On Mon, Oct 07, 2019 at 08:23:29AM +0200, Hannes Reinecke wrote:
> On 10/6/19 4:45 AM, Ming Lei wrote:
> > Most of blk-mq drivers depend on managed IRQ's auto-affinity to setup
> > up queue mapping. Thomas mentioned the following point[1]:
> > 
> > "
> >  That was the constraint of managed interrupts from the very beginning:
> > 
> >   The driver/subsystem has to quiesce the interrupt line and the associated
> >   queue _before_ it gets shutdown in CPU unplug and not fiddle with it
> >   until it's restarted by the core when the CPU is plugged in again.
> > "
> > 
> > However, current blk-mq implementation doesn't quiesce hw queue before
> > the last CPU in the hctx is shutdown. Even worse, CPUHP_BLK_MQ_DEAD is
> > one cpuhp state handled after the CPU is down, so there isn't any chance
> > to quiesce hctx for blk-mq wrt. CPU hotplug.
> > 
> > Add new cpuhp state of CPUHP_AP_BLK_MQ_ONLINE for blk-mq to stop queues
> > and wait for completion of in-flight requests.
> > 
> > [1] https://lore.kernel.org/linux-block/alpine.DEB.2.21.1904051331270.1802@nanos.tec.linutronix.de/
> > 
> > Cc: Bart Van Assche <bvanassche@acm.org>
> > Cc: Hannes Reinecke <hare@suse.com>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Keith Busch <keith.busch@intel.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/blk-mq-tag.c         |  2 +-
> >  block/blk-mq-tag.h         |  2 ++
> >  block/blk-mq.c             | 65 ++++++++++++++++++++++++++++++++++++++
> >  include/linux/blk-mq.h     |  1 +
> >  include/linux/cpuhotplug.h |  1 +
> >  5 files changed, 70 insertions(+), 1 deletion(-)
> > 
> I really don't like the zillions of 'XXX_in_flight()' helper in blk-mq;
> blk_mq_queue_inflight(), blk_mq_in_flight(), blk_mq_in_flight_rw() et al.
> Can't you implement your one on top of the already existing?

This one returns in-flight rqs on specific tags(hctx), so far no such
kind of interface, that is why blk_mq_all_tag_busy_iter is exported out
in this patch.


Thanks,
Ming

Re: [PATCH V2 RESEND 3/5] blk-mq: stop to handle IO before hctx's all CPUs become offline

[Ming Lei]

On Mon, Oct 07, 2019 at 11:23:22AM +0100, John Garry wrote:
> On 06/10/2019 03:45, Ming Lei wrote:
> > +	}
> > +}
> > +
> > +static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
> > +{
> > +	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
> > +			struct blk_mq_hw_ctx, cpuhp_online);
> > +	unsigned prev_cpu = -1;
> > +
> > +	while (true) {
> > +		unsigned next_cpu = cpumask_next_and(prev_cpu, hctx->cpumask,
> > +				cpu_online_mask);
> > +
> > +		if (next_cpu >= nr_cpu_ids)
> > +			break;
> > +
> > +		/* return if there is other online CPU on this hctx */
> > +		if (next_cpu != cpu)
> > +			return 0;
> > +
> > +		prev_cpu = next_cpu;
> > +	}
> > +
> > +	set_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
> > +	blk_mq_drain_inflight_rqs(hctx);
> > +
> 
> Does this do the same:
> 
> {
> 	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
> 			struct blk_mq_hw_ctx, cpuhp_online);
> 	cpumask_var_t tmp;
> 
> 	cpumask_and(tmp, hctx->cpumask, cpu_online_mask);
> 
> 	/* test if there is any other cpu online in the hctx cpu mask */
> 	if (cpumask_any_but(tmp, cpu) < nr_cpu_ids)
> 		return 0;
> 
> 	set_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
> 	blk_mq_drain_inflight_rqs(hctx);
> 
> 	return 0;
> }
> 
> If so, it's more readable and concise.

Yes, but we have to allocate space for 'tmp', that is what this patch
tries to avoid, given the logic isn't too complicated.

> 
> 
> BTW, You could have added my Tested-by tags...

OK, I will add it in V3.


Thanks,
Ming

Re: [PATCH V2 RESEND 4/5] blk-mq: re-submit IO in case that hctx is dead

[Ming Lei]

On Mon, Oct 07, 2019 at 08:27:38AM +0200, Hannes Reinecke wrote:
> On 10/6/19 4:45 AM, Ming Lei wrote:
> > When all CPUs in one hctx are offline, we shouldn't run this hw queue
> > for completing request any more.
> > 
> > So steal bios from the request, and resubmit them, and finally free
> > the request in blk_mq_hctx_notify_dead().
> > 
> > Cc: Bart Van Assche <bvanassche@acm.org>
> > Cc: Hannes Reinecke <hare@suse.com>
> > Cc: Christoph Hellwig <hch@lst.de>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: Keith Busch <keith.busch@intel.com>
> > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > ---
> >  block/blk-mq.c | 48 +++++++++++++++++++++++++++++++++++++++++-------
> >  1 file changed, 41 insertions(+), 7 deletions(-)
> > 
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index d991c122abf2..0b35fdbd1f17 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -2280,10 +2280,30 @@ static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
> >  	return 0;
> >  }
> >  
> > +static void blk_mq_resubmit_io(struct request *rq)
> > +{
> > +	struct bio_list list;
> > +	struct bio *bio;
> > +
> > +	bio_list_init(&list);
> > +	blk_steal_bios(&list, rq);
> > +
> > +	while (true) {
> > +		bio = bio_list_pop(&list);
> > +		if (!bio)
> > +			break;
> > +
> > +		generic_make_request(bio);
> > +	}
> > +
> > +	blk_mq_cleanup_rq(rq);
> > +	blk_mq_end_request(rq, 0);
> > +}
> > +
> Hmm. Not sure if this is a good idea.
> Shouldn't we call 'blk_mq_end_request()' before calling
> generic_make_request()?
> otherwise the cloned request might be completed before original one,
> which looks a bit dodgy to me; and might lead to quite a recursion if we
> have several dead cpus to content with ...

Good catch, we should have freed the old empty request before calling
generic_make_request(), will fix it in V3.


Thanks,
Ming

Re: [PATCH V2 RESEND 3/5] blk-mq: stop to handle IO before hctx's all CPUs become offline

[John Garry]

On 07/10/2019 16:04, Ming Lei wrote:
> On Mon, Oct 07, 2019 at 11:23:22AM +0100, John Garry wrote:
>> On 06/10/2019 03:45, Ming Lei wrote:
>>> +	}
>>> +}
>>> +
>>> +static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
>>> +{
>>> +	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
>>> +			struct blk_mq_hw_ctx, cpuhp_online);
>>> +	unsigned prev_cpu = -1;
>>> +
>>> +	while (true) {
>>> +		unsigned next_cpu = cpumask_next_and(prev_cpu, hctx->cpumask,
>>> +				cpu_online_mask);
>>> +
>>> +		if (next_cpu >= nr_cpu_ids)
>>> +			break;
>>> +
>>> +		/* return if there is other online CPU on this hctx */
>>> +		if (next_cpu != cpu)
>>> +			return 0;
>>> +
>>> +		prev_cpu = next_cpu;
>>> +	}
>>> +
>>> +	set_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
>>> +	blk_mq_drain_inflight_rqs(hctx);
>>> +
>>
>> Does this do the same:
>>
>> {
>> 	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
>> 			struct blk_mq_hw_ctx, cpuhp_online);
>> 	cpumask_var_t tmp;
>>
>> 	cpumask_and(tmp, hctx->cpumask, cpu_online_mask);
>>
>> 	/* test if there is any other cpu online in the hctx cpu mask */
>> 	if (cpumask_any_but(tmp, cpu) < nr_cpu_ids)
>> 		return 0;
>>
>> 	set_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
>> 	blk_mq_drain_inflight_rqs(hctx);
>>
>> 	return 0;
>> }
>>
>> If so, it's more readable and concise.
>
> Yes, but we have to allocate space for 'tmp', that is what this patch
> tries to avoid,

Yeah, I forgot about the extra complications of the cpumask offstack 
stuff; but it does seem rarely used...

There is this:

{
	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
			struct blk_mq_hw_ctx, cpuhp_online);

	if ((cpumask_next_and(-1, hctx->cpumask, cpu_online_mask) ==
	     cpu) &&
	     (cpumask_next_and(cpu, hctx->cpumask, cpu_online_mask) >=
               nr_cpu_ids)) {
		set_bit(BLK_MQ_S_INTERNAL_STOPPED, &hctx->state);
		blk_mq_drain_inflight_rqs(hctx);
	}

	return 0;
}

... which looks effectively the same as yours, except a bit more 
readable (ignoring the fixable spilling of lines) to me.

Thanks,
John

 > given the logic isn't too complicated.
>
>>
>>
>> BTW, You could have added my Tested-by tags...
>
> OK, I will add it in V3.
>
>
> Thanks,
> Ming
>
> .
>