[PATCH V11 00/12] blk-mq: improvement CPU hotplug

[PATCH V11 00/12] blk-mq: improvement 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 inactive(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

Thanks John Garry for running lots of tests on arm64 with this patchset
and co-working on investigating all kinds of issues.

Thanks Christoph's review on V7 & V8.

Please consider it for v5.8.

https://github.com/ming1/linux/commits/v5.7-rc-blk-mq-improve-cpu-hotplug

V11:
	- drop new callback from blk_mq_all_tag_busy_iter, add new helper
	of blk_mq_all_tag_iter (5/12), as suggested by Bart
	- fix request allocation hang in case of queue freeze(11/12), as
	reported by Bart

V10:
	- fix double bio complete in request resubmission(10/11)
	- add tested-by tag

V9:
	- add Reviewed-by tag
	- document more on memory barrier usage between getting driver tag
	and handling cpu offline(7/11)
	- small code cleanup as suggested by Chritoph(7/11)
	- rebase against for-5.8/block(1/11, 2/11)
V8:
	- add patches to share code with blk_rq_prep_clone
	- code re-organization as suggested by Christoph, most of them are
	in 04/11, 10/11
	- add reviewed-by tag

V7:
	- fix updating .nr_active in get_driver_tag
	- add hctx->cpumask check in cpuhp handler
	- only drain requests which tag is >= 0
	- pass more aggressive cpuhotplug&io test

V6:
	- simplify getting driver tag, so that we can drain in-flight
	  requests correctly without using synchronize_rcu()
	- handle re-submission of flush & passthrough request correctly

V5:
	- rename BLK_MQ_S_INTERNAL_STOPPED as BLK_MQ_S_INACTIVE
	- re-factor code for re-submit requests in cpu dead hotplug handler
	- address requeue corner case

V4:
	- resubmit IOs in dispatch list in case that this hctx is dead 

V3:
	- re-organize patch 2 & 3 a bit for addressing Hannes's comment
	- fix patch 4 for avoiding potential deadlock, as found by Hannes

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


*** BLURB HERE ***

Ming Lei (12):
  block: clone nr_integrity_segments and write_hint in blk_rq_prep_clone
  block: add helper for copying request
  blk-mq: mark blk_mq_get_driver_tag as static
  blk-mq: assign rq->tag in blk_mq_get_driver_tag
  blk-mq: add blk_mq_all_tag_iter
  blk-mq: prepare for draining IO when hctx's all CPUs are offline
  blk-mq: stop to handle IO and drain IO before hctx becomes inactive
  block: add blk_end_flush_machinery
  blk-mq: add blk_mq_hctx_handle_dead_cpu for handling cpu dead
  block: add request allocation flag of BLK_MQ_REQ_FORCE
  blk-mq: re-submit IO in case that hctx is inactive
  block: deactivate hctx when the hctx is actually inactive

 block/blk-core.c           |  32 +++-
 block/blk-flush.c          | 141 ++++++++++++---
 block/blk-mq-debugfs.c     |   2 +
 block/blk-mq-tag.c         |  33 +++-
 block/blk-mq-tag.h         |   2 +
 block/blk-mq.c             | 356 +++++++++++++++++++++++++++++--------
 block/blk-mq.h             |  22 ++-
 block/blk.h                |  11 +-
 drivers/block/loop.c       |   2 +-
 drivers/md/dm-rq.c         |   2 +-
 include/linux/blk-mq.h     |  14 ++
 include/linux/cpuhotplug.h |   1 +
 12 files changed, 494 insertions(+), 124 deletions(-)

-- 
2.25.2

[PATCH V11 01/12] block: clone nr_integrity_segments and write_hint in blk_rq_prep_clone

[Ming Lei]

So far blk_rq_prep_clone() is only used for setup one underlying cloned
request from dm-rq request. block intetrity can be enabled for both dm-rq
and the underlying queues, so it is reasonable to clone rq's
nr_integrity_segments. Also write_hint is from bio, it should have been
cloned too.

So clone nr_integrity_segments and write_hint in blk_rq_prep_clone.

Cc: John Garry <john.garry@huawei.com>
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: Mike Snitzer <snitzer@redhat.com>
Cc: dm-devel@redhat.com
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-core.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index cf5b2163edfe..08ee92baa451 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1669,8 +1669,12 @@ int blk_rq_prep_clone(struct request *rq, struct request *rq_src,
 		rq->rq_flags |= RQF_SPECIAL_PAYLOAD;
 		rq->special_vec = rq_src->special_vec;
 	}
+#ifdef CONFIG_BLK_DEV_INTEGRITY
+	rq->nr_integrity_segments = rq_src->nr_integrity_segments;
+#endif
 	rq->nr_phys_segments = rq_src->nr_phys_segments;
 	rq->ioprio = rq_src->ioprio;
+	rq->write_hint = rq_src->write_hint;
 
 	return 0;
 
-- 
2.25.2

[PATCH V11 02/12] block: add helper for copying request

[Ming Lei]

Add one new helper of blk_rq_copy_request() to copy request, and the helper
will be used in this patch for re-submitting request, so make it as a block
layer internal helper.

Cc: John Garry <john.garry@huawei.com>
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: Mike Snitzer <snitzer@redhat.com>
Cc: dm-devel@redhat.com
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-core.c | 31 ++++++++++++++++++-------------
 block/blk.h      |  2 ++
 2 files changed, 20 insertions(+), 13 deletions(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 08ee92baa451..ffb1579fd4da 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1620,6 +1620,23 @@ void blk_rq_unprep_clone(struct request *rq)
 }
 EXPORT_SYMBOL_GPL(blk_rq_unprep_clone);
 
+void blk_rq_copy_request(struct request *rq, struct request *rq_src)
+{
+	/* Copy attributes of the original request to the clone request. */
+	rq->__sector = blk_rq_pos(rq_src);
+	rq->__data_len = blk_rq_bytes(rq_src);
+	if (rq_src->rq_flags & RQF_SPECIAL_PAYLOAD) {
+		rq->rq_flags |= RQF_SPECIAL_PAYLOAD;
+		rq->special_vec = rq_src->special_vec;
+	}
+#ifdef CONFIG_BLK_DEV_INTEGRITY
+	rq->nr_integrity_segments = rq_src->nr_integrity_segments;
+#endif
+	rq->nr_phys_segments = rq_src->nr_phys_segments;
+	rq->ioprio = rq_src->ioprio;
+	rq->write_hint = rq_src->write_hint;
+}
+
 /**
  * blk_rq_prep_clone - Helper function to setup clone request
  * @rq: the request to be setup
@@ -1662,19 +1679,7 @@ int blk_rq_prep_clone(struct request *rq, struct request *rq_src,
 			rq->bio = rq->biotail = bio;
 	}
 
-	/* Copy attributes of the original request to the clone request. */
-	rq->__sector = blk_rq_pos(rq_src);
-	rq->__data_len = blk_rq_bytes(rq_src);
-	if (rq_src->rq_flags & RQF_SPECIAL_PAYLOAD) {
-		rq->rq_flags |= RQF_SPECIAL_PAYLOAD;
-		rq->special_vec = rq_src->special_vec;
-	}
-#ifdef CONFIG_BLK_DEV_INTEGRITY
-	rq->nr_integrity_segments = rq_src->nr_integrity_segments;
-#endif
-	rq->nr_phys_segments = rq_src->nr_phys_segments;
-	rq->ioprio = rq_src->ioprio;
-	rq->write_hint = rq_src->write_hint;
+	blk_rq_copy_request(rq, rq_src);
 
 	return 0;
 
diff --git a/block/blk.h b/block/blk.h
index e5cd350ca379..faf616cb0463 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -120,6 +120,8 @@ static inline void blk_rq_bio_prep(struct request *rq, struct bio *bio,
 		rq->rq_disk = bio->bi_disk;
 }
 
+void blk_rq_copy_request(struct request *rq, struct request *rq_src);
+
 #ifdef CONFIG_BLK_DEV_INTEGRITY
 void blk_flush_integrity(void);
 bool __bio_integrity_endio(struct bio *);
-- 
2.25.2

[PATCH V11 03/12] blk-mq: mark blk_mq_get_driver_tag as static

[Ming Lei]

Now all callers of blk_mq_get_driver_tag are in blk-mq.c, so mark
it as static.

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: John Garry <john.garry@huawei.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c | 2 +-
 block/blk-mq.h | 1 -
 2 files changed, 1 insertion(+), 2 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 9ee695bdf873..53c6e7678c14 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1015,7 +1015,7 @@ static inline unsigned int queued_to_index(unsigned int queued)
 	return min(BLK_MQ_MAX_DISPATCH_ORDER - 1, ilog2(queued) + 1);
 }
 
-bool blk_mq_get_driver_tag(struct request *rq)
+static bool blk_mq_get_driver_tag(struct request *rq)
 {
 	struct blk_mq_alloc_data data = {
 		.q = rq->q,
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 10bfdfb494fa..e7d1da4b1f73 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -44,7 +44,6 @@ bool blk_mq_dispatch_rq_list(struct request_queue *, struct list_head *, bool);
 void blk_mq_add_to_requeue_list(struct request *rq, bool at_head,
 				bool kick_requeue_list);
 void blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list);
-bool blk_mq_get_driver_tag(struct request *rq);
 struct request *blk_mq_dequeue_from_ctx(struct blk_mq_hw_ctx *hctx,
 					struct blk_mq_ctx *start);
 
-- 
2.25.2

[PATCH V11 04/12] blk-mq: assign rq->tag in blk_mq_get_driver_tag

[Ming Lei]

Especially for none elevator, rq->tag is assigned after the request is
allocated, so there isn't any way to figure out if one request is in
being dispatched. Also the code path wrt. driver tag becomes a bit
difference between none and io scheduler.

When one hctx becomes inactive, we have to prevent any request from
being dispatched to LLD. And get driver tag provides one perfect chance
to do that. Meantime we can drain any such requests by checking if
rq->tag is assigned.

So only assign rq->tag until blk_mq_get_driver_tag() is called.

This way also simplifies code of dealing with driver tag a lot.

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: John Garry <john.garry@huawei.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-flush.c | 18 ++----------
 block/blk-mq.c    | 75 ++++++++++++++++++++++++-----------------------
 block/blk-mq.h    | 21 +++++++------
 block/blk.h       |  5 ----
 4 files changed, 51 insertions(+), 68 deletions(-)

diff --git a/block/blk-flush.c b/block/blk-flush.c
index c7f396e3d5e2..977edf95d711 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -236,13 +236,8 @@ static void flush_end_io(struct request *flush_rq, blk_status_t error)
 		error = fq->rq_status;
 
 	hctx = flush_rq->mq_hctx;
-	if (!q->elevator) {
-		blk_mq_tag_set_rq(hctx, flush_rq->tag, fq->orig_rq);
-		flush_rq->tag = -1;
-	} else {
-		blk_mq_put_driver_tag(flush_rq);
-		flush_rq->internal_tag = -1;
-	}
+	flush_rq->internal_tag = -1;
+	blk_mq_put_driver_tag(flush_rq);
 
 	running = &fq->flush_queue[fq->flush_running_idx];
 	BUG_ON(fq->flush_pending_idx == fq->flush_running_idx);
@@ -317,14 +312,7 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
 	flush_rq->mq_ctx = first_rq->mq_ctx;
 	flush_rq->mq_hctx = first_rq->mq_hctx;
 
-	if (!q->elevator) {
-		fq->orig_rq = first_rq;
-		flush_rq->tag = first_rq->tag;
-		blk_mq_tag_set_rq(flush_rq->mq_hctx, first_rq->tag, flush_rq);
-	} else {
-		flush_rq->internal_tag = first_rq->internal_tag;
-	}
-
+	flush_rq->internal_tag = first_rq->internal_tag;
 	flush_rq->cmd_flags = REQ_OP_FLUSH | REQ_PREFLUSH;
 	flush_rq->cmd_flags |= (flags & REQ_DRV) | (flags & REQ_FAILFAST_MASK);
 	flush_rq->rq_flags |= RQF_FLUSH_SEQ;
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 53c6e7678c14..80d25e1d792a 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -276,18 +276,8 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
 	struct request *rq = tags->static_rqs[tag];
 	req_flags_t rq_flags = 0;
 
-	if (data->flags & BLK_MQ_REQ_INTERNAL) {
-		rq->tag = -1;
-		rq->internal_tag = tag;
-	} else {
-		if (data->hctx->flags & BLK_MQ_F_TAG_SHARED) {
-			rq_flags = RQF_MQ_INFLIGHT;
-			atomic_inc(&data->hctx->nr_active);
-		}
-		rq->tag = tag;
-		rq->internal_tag = -1;
-		data->hctx->tags->rqs[rq->tag] = rq;
-	}
+	rq->internal_tag = tag;
+	rq->tag = -1;
 
 	/* csd/requeue_work/fifo_time is initialized before use */
 	rq->q = data->q;
@@ -471,14 +461,18 @@ static void __blk_mq_free_request(struct request *rq)
 	struct request_queue *q = rq->q;
 	struct blk_mq_ctx *ctx = rq->mq_ctx;
 	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
-	const int sched_tag = rq->internal_tag;
 
 	blk_pm_mark_last_busy(rq);
 	rq->mq_hctx = NULL;
-	if (rq->tag != -1)
-		blk_mq_put_tag(hctx->tags, ctx, rq->tag);
-	if (sched_tag != -1)
-		blk_mq_put_tag(hctx->sched_tags, ctx, sched_tag);
+
+	if (hctx->sched_tags) {
+		if (rq->tag >= 0)
+			blk_mq_put_tag(hctx->tags, ctx, rq->tag);
+		blk_mq_put_tag(hctx->sched_tags, ctx, rq->internal_tag);
+	} else {
+		blk_mq_put_tag(hctx->tags, ctx, rq->internal_tag);
+        }
+
 	blk_mq_sched_restart(hctx);
 	blk_queue_exit(q);
 }
@@ -526,7 +520,7 @@ inline void __blk_mq_end_request(struct request *rq, blk_status_t error)
 		blk_stat_add(rq, now);
 	}
 
-	if (rq->internal_tag != -1)
+	if (rq->q->elevator && rq->internal_tag != -1)
 		blk_mq_sched_completed_request(rq, now);
 
 	blk_account_io_done(rq, now);
@@ -1015,33 +1009,40 @@ static inline unsigned int queued_to_index(unsigned int queued)
 	return min(BLK_MQ_MAX_DISPATCH_ORDER - 1, ilog2(queued) + 1);
 }
 
-static bool blk_mq_get_driver_tag(struct request *rq)
+static bool __blk_mq_get_driver_tag(struct request *rq)
 {
 	struct blk_mq_alloc_data data = {
-		.q = rq->q,
-		.hctx = rq->mq_hctx,
-		.flags = BLK_MQ_REQ_NOWAIT,
-		.cmd_flags = rq->cmd_flags,
+		.q		= rq->q,
+		.hctx		= rq->mq_hctx,
+		.flags		= BLK_MQ_REQ_NOWAIT,
+		.cmd_flags	= rq->cmd_flags,
 	};
-	bool shared;
 
-	if (rq->tag != -1)
-		return true;
+	if (data.hctx->sched_tags) {
+		if (blk_mq_tag_is_reserved(data.hctx->sched_tags,
+				rq->internal_tag))
+			data.flags |= BLK_MQ_REQ_RESERVED;
+		rq->tag = blk_mq_get_tag(&data);
+	} else {
+		rq->tag = rq->internal_tag;
+	}
 
-	if (blk_mq_tag_is_reserved(data.hctx->sched_tags, rq->internal_tag))
-		data.flags |= BLK_MQ_REQ_RESERVED;
+	if (rq->tag == -1)
+		return false;
 
-	shared = blk_mq_tag_busy(data.hctx);
-	rq->tag = blk_mq_get_tag(&data);
-	if (rq->tag >= 0) {
-		if (shared) {
-			rq->rq_flags |= RQF_MQ_INFLIGHT;
-			atomic_inc(&data.hctx->nr_active);
-		}
-		data.hctx->tags->rqs[rq->tag] = rq;
+	if (blk_mq_tag_busy(data.hctx)) {
+		rq->rq_flags |= RQF_MQ_INFLIGHT;
+		atomic_inc(&data.hctx->nr_active);
 	}
+	data.hctx->tags->rqs[rq->tag] = rq;
+	return true;
+}
 
-	return rq->tag != -1;
+static bool blk_mq_get_driver_tag(struct request *rq)
+{
+	if (rq->tag != -1)
+		return true;
+	return __blk_mq_get_driver_tag(rq);
 }
 
 static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode,
diff --git a/block/blk-mq.h b/block/blk-mq.h
index e7d1da4b1f73..d0c72d7d07c8 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -196,26 +196,25 @@ static inline bool blk_mq_get_dispatch_budget(struct blk_mq_hw_ctx *hctx)
 	return true;
 }
 
-static inline void __blk_mq_put_driver_tag(struct blk_mq_hw_ctx *hctx,
-					   struct request *rq)
+static inline void blk_mq_put_driver_tag(struct request *rq)
 {
-	blk_mq_put_tag(hctx->tags, rq->mq_ctx, rq->tag);
+	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
+	int tag = rq->tag;
+
+	if (tag < 0)
+		return;
+
 	rq->tag = -1;
 
+	if (hctx->sched_tags)
+		blk_mq_put_tag(hctx->tags, rq->mq_ctx, tag);
+
 	if (rq->rq_flags & RQF_MQ_INFLIGHT) {
 		rq->rq_flags &= ~RQF_MQ_INFLIGHT;
 		atomic_dec(&hctx->nr_active);
 	}
 }
 
-static inline void blk_mq_put_driver_tag(struct request *rq)
-{
-	if (rq->tag == -1 || rq->internal_tag == -1)
-		return;
-
-	__blk_mq_put_driver_tag(rq->mq_hctx, rq);
-}
-
 static inline void blk_mq_clear_mq_map(struct blk_mq_queue_map *qmap)
 {
 	int cpu;
diff --git a/block/blk.h b/block/blk.h
index faf616cb0463..002104739465 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -26,11 +26,6 @@ struct blk_flush_queue {
 	struct list_head	flush_data_in_flight;
 	struct request		*flush_rq;
 
-	/*
-	 * flush_rq shares tag with this rq, both can't be active
-	 * at the same time
-	 */
-	struct request		*orig_rq;
 	struct lock_class_key	key;
 	spinlock_t		mq_flush_lock;
 };
-- 
2.25.2

[PATCH V11 05/12] blk-mq: add blk_mq_all_tag_iter

[Ming Lei]

Now request is thought as in-flight only when its state is updated as
MQ_RQ_IN_FLIGHT, which is done by driver via blk_mq_start_request().

Actually from blk-mq's view, one rq can be thought as in-flight
after its tag is >= 0.

Add one new function of blk_mq_all_tag_iter so that we can iterate
every in-flight request, and this way is more flexible since caller
can decide to handle request according to rq's state.

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: John Garry <john.garry@huawei.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-tag.c | 33 +++++++++++++++++++++++++++++----
 block/blk-mq-tag.h |  2 ++
 2 files changed, 31 insertions(+), 4 deletions(-)

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 586c9d6e904a..c27c6dfc7d36 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -257,6 +257,7 @@ struct bt_tags_iter_data {
 	busy_tag_iter_fn *fn;
 	void *data;
 	bool reserved;
+	bool iterate_all;
 };
 
 static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
@@ -274,7 +275,7 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
 	 * test and set the bit before assining ->rqs[].
 	 */
 	rq = tags->rqs[bitnr];
-	if (rq && blk_mq_request_started(rq))
+	if (rq && (iter_data->iterate_all || blk_mq_request_started(rq)))
 		return iter_data->fn(rq, iter_data->data, reserved);
 
 	return true;
@@ -294,13 +295,15 @@ static bool bt_tags_iter(struct sbitmap *bitmap, unsigned int bitnr, void *data)
  *		bitmap_tags member of struct blk_mq_tags.
  */
 static void bt_tags_for_each(struct blk_mq_tags *tags, struct sbitmap_queue *bt,
-			     busy_tag_iter_fn *fn, void *data, bool reserved)
+			     busy_tag_iter_fn *fn, void *data, bool reserved,
+			     bool iterate_all)
 {
 	struct bt_tags_iter_data iter_data = {
 		.tags = tags,
 		.fn = fn,
 		.data = data,
 		.reserved = reserved,
+		.iterate_all = iterate_all,
 	};
 
 	if (tags->rqs)
@@ -321,8 +324,30 @@ static void blk_mq_all_tag_busy_iter(struct blk_mq_tags *tags,
 		busy_tag_iter_fn *fn, void *priv)
 {
 	if (tags->nr_reserved_tags)
-		bt_tags_for_each(tags, &tags->breserved_tags, fn, priv, true);
-	bt_tags_for_each(tags, &tags->bitmap_tags, fn, priv, false);
+		bt_tags_for_each(tags, &tags->breserved_tags, fn, priv, true,
+				 false);
+	bt_tags_for_each(tags, &tags->bitmap_tags, fn, priv, false, false);
+}
+
+/**
+ * blk_mq_all_tag_iter - iterate over all requests in a tag map
+ * @tags:	Tag map to iterate over.
+ * @fn:		Pointer to the function that will be called for each
+ *		request. @fn will be called as follows: @fn(rq, @priv,
+ *		reserved) where rq is a pointer to a request. 'reserved'
+ *		indicates whether or not @rq is a reserved request. Return
+ *		true to continue iterating tags, false to stop.
+ * @priv:	Will be passed as second argument to @fn.
+ *
+ * It is the caller's responsibility to check rq's state in @fn.
+ */
+void blk_mq_all_tag_iter(struct blk_mq_tags *tags, busy_tag_iter_fn *fn,
+		void *priv)
+{
+	if (tags->nr_reserved_tags)
+		bt_tags_for_each(tags, &tags->breserved_tags, fn, priv, true,
+				 true);
+	bt_tags_for_each(tags, &tags->bitmap_tags, fn, priv, false, true);
 }
 
 /**
diff --git a/block/blk-mq-tag.h b/block/blk-mq-tag.h
index 2b8321efb682..d19546e8246b 100644
--- a/block/blk-mq-tag.h
+++ b/block/blk-mq-tag.h
@@ -34,6 +34,8 @@ 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_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)
-- 
2.25.2

[PATCH V11 06/12] blk-mq: prepare for draining IO when hctx's all CPUs are 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.

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 from
dm-rq & loop, then the potential dead-lock can be avoided.

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

Cc: John Garry <john.garry@huawei.com>
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>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-debugfs.c     |  1 +
 block/blk-mq.c             | 19 +++++++++++++++++++
 drivers/block/loop.c       |  2 +-
 drivers/md/dm-rq.c         |  2 +-
 include/linux/blk-mq.h     |  4 ++++
 include/linux/cpuhotplug.h |  1 +
 6 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 96b7a35c898a..ddec58743e88 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -239,6 +239,7 @@ 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/block/blk-mq.c b/block/blk-mq.c
index 80d25e1d792a..25d2cbe9c716 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2300,6 +2300,16 @@ int blk_mq_alloc_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 	return -ENOMEM;
 }
 
+static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
+{
+	return 0;
+}
+
+static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
+{
+	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
@@ -2336,6 +2346,9 @@ 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);
 }
@@ -2395,6 +2408,9 @@ 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];
@@ -3649,6 +3665,9 @@ 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",
+				blk_mq_hctx_notify_online,
+				blk_mq_hctx_notify_offline);
 	return 0;
 }
 subsys_initcall(blk_mq_init);
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index da693e6a834e..784f2e038b55 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -2037,7 +2037,7 @@ 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 d7307795439a..ddd2cb6ed21c 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -140,6 +140,8 @@ struct blk_mq_hw_ctx {
 	 */
 	atomic_t		nr_active;
 
+	/** @cpuhp_online: List to store request if CPU is going to die */
+	struct hlist_node	cpuhp_online;
 	/** @cpuhp_dead: List to store request if some CPU die. */
 	struct hlist_node	cpuhp_dead;
 	/** @kobj: Kernel object for sysfs. */
@@ -391,6 +393,8 @@ struct blk_mq_ops {
 enum {
 	BLK_MQ_F_SHOULD_MERGE	= 1 << 0,
 	BLK_MQ_F_TAG_SHARED	= 1 << 1,
+	/* set when device's interrupt affinity isn't managed by kernel */
+	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,
diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 77d70b633531..24b3a77810b6 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -152,6 +152,7 @@ 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,
-- 
2.25.2

[PATCH V11 07/12] blk-mq: stop to handle IO and drain IO before hctx becomes inactive

[Ming Lei]

Before one CPU becomes offline, check if it is the last online CPU of hctx.
If yes, mark this hctx as inactive, meantime wait for completion of all
in-flight IOs originated from this hctx. Meantime check if this hctx has
become inactive in blk_mq_get_driver_tag(), if yes, release the
allocated tag.

This way guarantees that there isn't any inflight IO before shutdowning
the managed IRQ line when all CPUs of this IRQ line is offline.

Cc: John Garry <john.garry@huawei.com>
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>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq-debugfs.c |   1 +
 block/blk-mq.c         | 117 +++++++++++++++++++++++++++++++++++++----
 include/linux/blk-mq.h |   3 ++
 3 files changed, 110 insertions(+), 11 deletions(-)

diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index ddec58743e88..dc66cb689d2f 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(INACTIVE),
 };
 #undef HCTX_STATE_NAME
 
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 25d2cbe9c716..171bbf2fbc56 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1038,11 +1038,36 @@ static bool __blk_mq_get_driver_tag(struct request *rq)
 	return true;
 }
 
-static bool blk_mq_get_driver_tag(struct request *rq)
+static bool blk_mq_get_driver_tag(struct request *rq, bool direct_issue)
 {
 	if (rq->tag != -1)
 		return true;
-	return __blk_mq_get_driver_tag(rq);
+
+	if (!__blk_mq_get_driver_tag(rq))
+		return false;
+	/*
+	 * In case that direct issue IO process is migrated to other CPU
+	 * which may not belong to this hctx, add one memory barrier so we
+	 * can order driver tag assignment and checking BLK_MQ_S_INACTIVE.
+	 * Otherwise, barrier() is enough given both setting BLK_MQ_S_INACTIVE
+	 * and driver tag assignment are run on the same CPU because
+	 * BLK_MQ_S_INACTIVE is only set after the last CPU of this hctx is
+	 * becoming offline.
+	 *
+	 * Process migration might happen after the check on current processor
+	 * id, smp_mb() is implied by processor migration, so no need to worry
+	 * about it.
+	 */
+	if (unlikely(direct_issue && rq->mq_ctx->cpu != raw_smp_processor_id()))
+		smp_mb();
+	else
+		barrier();
+
+	if (unlikely(test_bit(BLK_MQ_S_INACTIVE, &rq->mq_hctx->state))) {
+		blk_mq_put_driver_tag(rq);
+		return false;
+	}
+	return true;
 }
 
 static int blk_mq_dispatch_wake(wait_queue_entry_t *wait, unsigned mode,
@@ -1091,7 +1116,7 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx *hctx,
 		 * Don't clear RESTART here, someone else could have set it.
 		 * At most this will cost an extra queue run.
 		 */
-		return blk_mq_get_driver_tag(rq);
+		return blk_mq_get_driver_tag(rq, false);
 	}
 
 	wait = &hctx->dispatch_wait;
@@ -1117,7 +1142,7 @@ static bool blk_mq_mark_tag_wait(struct blk_mq_hw_ctx *hctx,
 	 * allocation failure and adding the hardware queue to the wait
 	 * queue.
 	 */
-	ret = blk_mq_get_driver_tag(rq);
+	ret = blk_mq_get_driver_tag(rq, false);
 	if (!ret) {
 		spin_unlock(&hctx->dispatch_wait_lock);
 		spin_unlock_irq(&wq->lock);
@@ -1232,7 +1257,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
 			break;
 		}
 
-		if (!blk_mq_get_driver_tag(rq)) {
+		if (!blk_mq_get_driver_tag(rq, false)) {
 			/*
 			 * The initial allocation attempt failed, so we need to
 			 * rerun the hardware queue when a tag is freed. The
@@ -1264,7 +1289,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
 			bd.last = true;
 		else {
 			nxt = list_first_entry(list, struct request, queuelist);
-			bd.last = !blk_mq_get_driver_tag(nxt);
+			bd.last = !blk_mq_get_driver_tag(nxt, false);
 		}
 
 		ret = q->mq_ops->queue_rq(hctx, &bd);
@@ -1891,7 +1916,7 @@ static blk_status_t __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
 	if (!blk_mq_get_dispatch_budget(hctx))
 		goto insert;
 
-	if (!blk_mq_get_driver_tag(rq)) {
+	if (!blk_mq_get_driver_tag(rq, true)) {
 		blk_mq_put_dispatch_budget(hctx);
 		goto insert;
 	}
@@ -2300,13 +2325,80 @@ int blk_mq_alloc_rqs(struct blk_mq_tag_set *set, struct blk_mq_tags *tags,
 	return -ENOMEM;
 }
 
-static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
+struct count_inflight_data {
+	unsigned count;
+	struct blk_mq_hw_ctx *hctx;
+};
+
+static bool blk_mq_count_inflight_rq(struct request *rq, void *data,
+				     bool reserved)
 {
-	return 0;
+	struct count_inflight_data *count_data = data;
+
+	/*
+	 * Can't check rq's state because it is updated to MQ_RQ_IN_FLIGHT
+	 * in blk_mq_start_request(), at that time we can't prevent this rq
+	 * from being issued.
+	 *
+	 * So check if driver tag is assigned, if yes, count this rq as
+	 * inflight.
+	 */
+	if (rq->tag >= 0 && rq->mq_hctx == count_data->hctx)
+		count_data->count++;
+
+	return true;
+}
+
+static unsigned blk_mq_tags_inflight_rqs(struct blk_mq_hw_ctx *hctx)
+{
+	struct count_inflight_data count_data = {
+		.hctx	= hctx,
+	};
+
+	blk_mq_all_tag_iter(hctx->tags, blk_mq_count_inflight_rq, &count_data);
+	return count_data.count;
+}
+
+static inline bool blk_mq_last_cpu_in_hctx(unsigned int cpu,
+		struct blk_mq_hw_ctx *hctx)
+{
+	if (cpumask_next_and(-1, hctx->cpumask, cpu_online_mask) != cpu)
+		return false;
+	if (cpumask_next_and(cpu, hctx->cpumask, cpu_online_mask) < nr_cpu_ids)
+		return false;
+	return true;
 }
 
 static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
 {
+	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
+			struct blk_mq_hw_ctx, cpuhp_online);
+
+	if (!cpumask_test_cpu(cpu, hctx->cpumask))
+		return 0;
+
+	if (!blk_mq_last_cpu_in_hctx(cpu, hctx))
+		return 0;
+
+	/*
+	 * Order setting BLK_MQ_S_INACTIVE versus checking rq->tag and rqs[tag],
+	 * in blk_mq_tags_inflight_rqs.  It pairs with the smp_mb() in
+	 * blk_mq_get_driver_tag.
+	 */
+	set_bit(BLK_MQ_S_INACTIVE, &hctx->state);
+	smp_mb__after_atomic();
+	while (blk_mq_tags_inflight_rqs(hctx))
+		msleep(5);
+	return 0;
+}
+
+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);
+
+	if (cpumask_test_cpu(cpu, hctx->cpumask))
+		clear_bit(BLK_MQ_S_INACTIVE, &hctx->state);
 	return 0;
 }
 
@@ -2317,12 +2409,15 @@ static int blk_mq_hctx_notify_offline(unsigned int cpu, struct hlist_node *node)
  */
 static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 {
-	struct blk_mq_hw_ctx *hctx;
+	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
+			struct blk_mq_hw_ctx, cpuhp_dead);
 	struct blk_mq_ctx *ctx;
 	LIST_HEAD(tmp);
 	enum hctx_type type;
 
-	hctx = hlist_entry_safe(node, struct blk_mq_hw_ctx, cpuhp_dead);
+	if (!cpumask_test_cpu(cpu, hctx->cpumask))
+		return 0;
+
 	ctx = __blk_mq_get_ctx(hctx->queue, cpu);
 	type = hctx->type;
 
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index ddd2cb6ed21c..c2ea0a6e5b56 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -404,6 +404,9 @@ enum {
 	BLK_MQ_S_TAG_ACTIVE	= 1,
 	BLK_MQ_S_SCHED_RESTART	= 2,
 
+	/* hw queue is inactive after all its CPUs become offline */
+	BLK_MQ_S_INACTIVE	= 3,
+
 	BLK_MQ_MAX_DEPTH	= 10240,
 
 	BLK_MQ_CPU_WORK_BATCH	= 8,
-- 
2.25.2

[PATCH V11 08/12] block: add blk_end_flush_machinery

[Ming Lei]

Flush requests aren't same with normal FS request:

1) one dedicated per-hctx flush rq is pre-allocated for sending flush request

2) flush request si issued to hardware via one machinary so that flush merge
can be applied

We can't simply re-submit flush rqs via blk_steal_bios(), so add
blk_end_flush_machinery to collect flush requests which needs to
be resubmitted:

- if one flush command without DATA is enough, send one flush, complete this
kind of requests

- otherwise, add the request into a list and let caller re-submit it.

Cc: John Garry <john.garry@huawei.com>
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>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-flush.c | 123 +++++++++++++++++++++++++++++++++++++++++++---
 block/blk.h       |   4 ++
 2 files changed, 120 insertions(+), 7 deletions(-)

diff --git a/block/blk-flush.c b/block/blk-flush.c
index 977edf95d711..745d878697ed 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -170,10 +170,11 @@ static void blk_flush_complete_seq(struct request *rq,
 	unsigned int cmd_flags;
 
 	BUG_ON(rq->flush.seq & seq);
-	rq->flush.seq |= seq;
+	if (!error)
+		rq->flush.seq |= seq;
 	cmd_flags = rq->cmd_flags;
 
-	if (likely(!error))
+	if (likely(!error && !fq->flush_queue_terminating))
 		seq = blk_flush_cur_seq(rq);
 	else
 		seq = REQ_FSEQ_DONE;
@@ -200,9 +201,15 @@ static void blk_flush_complete_seq(struct request *rq,
 		 * normal completion and end it.
 		 */
 		BUG_ON(!list_empty(&rq->queuelist));
-		list_del_init(&rq->flush.list);
-		blk_flush_restore_request(rq);
-		blk_mq_end_request(rq, error);
+
+		/* Terminating code will end the request from flush queue */
+		if (likely(!fq->flush_queue_terminating)) {
+			list_del_init(&rq->flush.list);
+			blk_flush_restore_request(rq);
+			blk_mq_end_request(rq, error);
+		} else {
+			list_move_tail(&rq->flush.list, pending);
+		}
 		break;
 
 	default:
@@ -279,7 +286,8 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
 	struct request *flush_rq = fq->flush_rq;
 
 	/* C1 described at the top of this file */
-	if (fq->flush_pending_idx != fq->flush_running_idx || list_empty(pending))
+	if (fq->flush_pending_idx != fq->flush_running_idx ||
+			list_empty(pending) || fq->flush_queue_terminating)
 		return;
 
 	/* C2 and C3
@@ -331,7 +339,7 @@ static void mq_flush_data_end_io(struct request *rq, blk_status_t error)
 	struct blk_flush_queue *fq = blk_get_flush_queue(q, ctx);
 
 	if (q->elevator) {
-		WARN_ON(rq->tag < 0);
+		WARN_ON(rq->tag < 0 && !fq->flush_queue_terminating);
 		blk_mq_put_driver_tag(rq);
 	}
 
@@ -503,3 +511,104 @@ void blk_free_flush_queue(struct blk_flush_queue *fq)
 	kfree(fq->flush_rq);
 	kfree(fq);
 }
+
+static void __blk_end_queued_flush(struct blk_flush_queue *fq,
+		unsigned int queue_idx, struct list_head *resubmit_list,
+		struct list_head *flush_list)
+{
+	struct list_head *queue = &fq->flush_queue[queue_idx];
+	struct request *rq, *nxt;
+
+	list_for_each_entry_safe(rq, nxt, queue, flush.list) {
+		unsigned int seq = blk_flush_cur_seq(rq);
+
+		list_del_init(&rq->flush.list);
+		blk_flush_restore_request(rq);
+		if (!blk_rq_sectors(rq) || seq == REQ_FSEQ_POSTFLUSH )
+			list_add_tail(&rq->queuelist, flush_list);
+		else
+			list_add_tail(&rq->queuelist, resubmit_list);
+	}
+}
+
+static void blk_end_queued_flush(struct blk_flush_queue *fq,
+		struct list_head *resubmit_list, struct list_head *flush_list)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&fq->mq_flush_lock, flags);
+	__blk_end_queued_flush(fq, 0, resubmit_list, flush_list);
+	__blk_end_queued_flush(fq, 1, resubmit_list, flush_list);
+	spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
+}
+
+/* complete requests which just requires one flush command */
+static void blk_complete_flush_requests(struct blk_flush_queue *fq,
+		struct list_head *flush_list)
+{
+	struct block_device *bdev;
+	struct request *rq;
+	int error = -ENXIO;
+
+	if (list_empty(flush_list))
+		return;
+
+	rq = list_first_entry(flush_list, struct request, queuelist);
+
+	/* Send flush via one active hctx so we can move on */
+	bdev = bdget_disk(rq->rq_disk, 0);
+	if (bdev) {
+		error = blkdev_issue_flush(bdev, GFP_KERNEL, NULL);
+		bdput(bdev);
+	}
+
+	while (!list_empty(flush_list)) {
+		rq = list_first_entry(flush_list, struct request, queuelist);
+		list_del_init(&rq->queuelist);
+		blk_mq_end_request(rq, error);
+	}
+}
+
+/*
+ * Called when this hctx is inactive and all CPUs of this hctx is dead,
+ * otherwise don't reuse this function.
+ *
+ * Terminate this hw queue's flush machinery, and try to complete flush
+ * IO requests if possible, such as any flush IO without data, or flush
+ * data IO in POSTFLUSH stage. Otherwise, add the flush IOs into @list
+ * and let caller to re-submit them.
+ */
+void blk_end_flush_machinery(struct blk_mq_hw_ctx *hctx,
+		struct list_head *in, struct list_head *out)
+{
+	LIST_HEAD(resubmit_list);
+	LIST_HEAD(flush_list);
+	struct blk_flush_queue *fq = hctx->fq;
+	struct request *rq, *nxt;
+	unsigned long flags;
+
+	spin_lock_irqsave(&fq->mq_flush_lock, flags);
+	fq->flush_queue_terminating = 1;
+	spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
+
+	/* End inflight flush requests */
+	list_for_each_entry_safe(rq, nxt, in, queuelist) {
+		WARN_ON(!(rq->rq_flags & RQF_FLUSH_SEQ));
+		list_del_init(&rq->queuelist);
+		rq->end_io(rq, BLK_STS_AGAIN);
+	}
+
+	/* End queued requests */
+	blk_end_queued_flush(fq, &resubmit_list, &flush_list);
+
+	/* Send flush and complete requests which just need one flush req */
+	blk_complete_flush_requests(fq, &flush_list);
+
+	spin_lock_irqsave(&fq->mq_flush_lock, flags);
+	/* reset flush queue so that it is ready to work next time */
+	fq->flush_pending_idx = fq->flush_running_idx = 0;
+	fq->flush_queue_terminating = 0;
+	spin_unlock_irqrestore(&fq->mq_flush_lock, flags);
+
+	list_splice_init(&resubmit_list, out);
+}
diff --git a/block/blk.h b/block/blk.h
index 002104739465..79aaf976d15d 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -20,6 +20,7 @@ struct blk_flush_queue {
 	unsigned int		flush_queue_delayed:1;
 	unsigned int		flush_pending_idx:1;
 	unsigned int		flush_running_idx:1;
+	unsigned int		flush_queue_terminating:1;
 	blk_status_t 		rq_status;
 	unsigned long		flush_pending_since;
 	struct list_head	flush_queue[2];
@@ -453,4 +454,7 @@ int bio_add_hw_page(struct request_queue *q, struct bio *bio,
 		struct page *page, unsigned int len, unsigned int offset,
 		unsigned int max_sectors, bool *same_page);
 
+void blk_end_flush_machinery(struct blk_mq_hw_ctx *hctx,
+		struct list_head *in, struct list_head *out);
+
 #endif /* BLK_INTERNAL_H */
-- 
2.25.2

[PATCH V11 09/12] blk-mq: add blk_mq_hctx_handle_dead_cpu for handling cpu dead

[Ming Lei]

Add helper of blk_mq_hctx_handle_dead_cpu for handling cpu dead,
and prepare for handling inactive hctx.

No functional change.

Cc: John Garry <john.garry@huawei.com>
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>
Reviewed-by: Hannes Reinecke <hare@suse.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c | 29 +++++++++++++++++------------
 1 file changed, 17 insertions(+), 12 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 171bbf2fbc56..7c640482fb24 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2402,22 +2402,13 @@ static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
 	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
- * gets run.
- */
-static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
+static void blk_mq_hctx_handle_dead_cpu(struct blk_mq_hw_ctx *hctx,
+		unsigned int cpu)
 {
-	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
-			struct blk_mq_hw_ctx, cpuhp_dead);
 	struct blk_mq_ctx *ctx;
 	LIST_HEAD(tmp);
 	enum hctx_type type;
 
-	if (!cpumask_test_cpu(cpu, hctx->cpumask))
-		return 0;
-
 	ctx = __blk_mq_get_ctx(hctx->queue, cpu);
 	type = hctx->type;
 
@@ -2429,13 +2420,27 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 	spin_unlock(&ctx->lock);
 
 	if (list_empty(&tmp))
-		return 0;
+		return;
 
 	spin_lock(&hctx->lock);
 	list_splice_tail_init(&tmp, &hctx->dispatch);
 	spin_unlock(&hctx->lock);
 
 	blk_mq_run_hw_queue(hctx, true);
+}
+
+/*
+ * '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.
+ */
+static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
+{
+	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
+			struct blk_mq_hw_ctx, cpuhp_dead);
+
+	if (cpumask_test_cpu(cpu, hctx->cpumask))
+		blk_mq_hctx_handle_dead_cpu(hctx, cpu);
 	return 0;
 }
 
-- 
2.25.2

[PATCH V11 10/12] block: add request allocation flag of BLK_MQ_REQ_FORCE

[Ming Lei]

When one hctx becomes inactive, there may be requests allocated from
this hctx, we can't queue them to the inactive hctx, one approach is
to re-submit them via one active hctx.

However, the request queue may have been started to freeze, and allocating
request becomes not possible. Add flag of BLK_MQ_REQ_FORCE to allow block
layer to allocate request in this case becasue the queue won't be frozen
completely before all requests allocated from inactive hctx are completed.

The similar approach has been applied in commit 8dc765d438f1 ("SCSI: fix queue
cleanup race before queue initialization is done").

This way can help on other request dependency case too, such as, storage
device side has some problem, and IO request can't be queued to device
successfully, and passthrough request is required to fix the device problem.
If queue freeze just comes before allocating passthrough request, hang will be
triggered in queue freeze process, IO process and context for allocating
passthrough request forever. See commit 01e99aeca397 ("blk-mq: insert passthrough
request into hctx->dispatch directly") for background of this kind of issue.

Cc: John Garry <john.garry@huawei.com>
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>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-core.c       | 5 +++++
 include/linux/blk-mq.h | 7 +++++++
 2 files changed, 12 insertions(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index ffb1579fd4da..82be15c1fde4 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -430,6 +430,11 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
 		if (success)
 			return 0;
 
+		if (flags & BLK_MQ_REQ_FORCE) {
+			percpu_ref_get(&q->q_usage_counter);
+			return 0;
+		}
+
 		if (flags & BLK_MQ_REQ_NOWAIT)
 			return -EBUSY;
 
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index c2ea0a6e5b56..7d7aa5305a67 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -448,6 +448,13 @@ enum {
 	BLK_MQ_REQ_INTERNAL	= (__force blk_mq_req_flags_t)(1 << 2),
 	/* set RQF_PREEMPT */
 	BLK_MQ_REQ_PREEMPT	= (__force blk_mq_req_flags_t)(1 << 3),
+
+	/*
+	 * force to allocate request and caller has to make sure queue
+	 * won't be frozen completely during allocation, and this flag
+	 * is only applied after queue freeze is started
+	 */
+	BLK_MQ_REQ_FORCE	= (__force blk_mq_req_flags_t)(1 << 4),
 };
 
 struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
-- 
2.25.2

[PATCH V11 11/12] blk-mq: re-submit IO in case that hctx is inactive

[Ming Lei]

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

So allocate request from one live hctx, and clone & resubmit the request,
either it is from sw queue or scheduler queue.

Cc: John Garry <john.garry@huawei.com>
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>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c | 116 ++++++++++++++++++++++++++++++++++++++++++++++---
 1 file changed, 111 insertions(+), 5 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 7c640482fb24..c9a3e48a1ebc 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2402,6 +2402,109 @@ static int blk_mq_hctx_notify_online(unsigned int cpu, struct hlist_node *node)
 	return 0;
 }
 
+static void blk_mq_resubmit_end_rq(struct request *rq, blk_status_t error)
+{
+	struct request *orig_rq = rq->end_io_data;
+
+	blk_mq_cleanup_rq(orig_rq);
+	blk_mq_end_request(orig_rq, error);
+
+	blk_put_request(rq);
+}
+
+static void blk_mq_resubmit_rq(struct request *rq)
+{
+	struct request *nrq;
+	unsigned int flags = 0;
+	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
+	struct blk_mq_tags *tags = rq->q->elevator ? hctx->sched_tags :
+		hctx->tags;
+	bool reserved = blk_mq_tag_is_reserved(tags, rq->internal_tag);
+
+	if (rq->rq_flags & RQF_PREEMPT)
+		flags |= BLK_MQ_REQ_PREEMPT;
+	if (reserved)
+		flags |= BLK_MQ_REQ_RESERVED;
+	/*
+	 * Queue freezing might be in-progress, and wait freeze can't be
+	 * done now because we have request not completed yet, so mark this
+	 * allocation as BLK_MQ_REQ_FORCE for avoiding this allocation &
+	 * freeze hung forever.
+	 */
+	flags |= BLK_MQ_REQ_FORCE;
+
+	/* avoid allocation failure by clearing NOWAIT */
+	nrq = blk_get_request(rq->q, rq->cmd_flags & ~REQ_NOWAIT, flags);
+	if (!nrq)
+		return;
+
+	blk_rq_copy_request(nrq, rq);
+
+	nrq->timeout = rq->timeout;
+	nrq->rq_disk = rq->rq_disk;
+	nrq->part = rq->part;
+
+	memcpy(blk_mq_rq_to_pdu(nrq), blk_mq_rq_to_pdu(rq),
+			rq->q->tag_set->cmd_size);
+
+	nrq->end_io = blk_mq_resubmit_end_rq;
+	nrq->end_io_data = rq;
+	nrq->bio = rq->bio;
+	nrq->biotail = rq->biotail;
+
+	/* bios ownership has been transfered to new request */
+	rq->bio = rq->biotail = NULL;
+	rq->__data_len = 0;
+
+	if (blk_insert_cloned_request(nrq->q, nrq) != BLK_STS_OK)
+		blk_mq_request_bypass_insert(nrq, false, true);
+}
+
+static void blk_mq_hctx_deactivate(struct blk_mq_hw_ctx *hctx)
+{
+	LIST_HEAD(sched);
+	LIST_HEAD(re_submit);
+	LIST_HEAD(flush_in);
+	LIST_HEAD(flush_out);
+	struct request *rq, *nxt;
+	struct elevator_queue *e = hctx->queue->elevator;
+
+	if (!e) {
+		blk_mq_flush_busy_ctxs(hctx, &re_submit);
+	} else {
+		while ((rq = e->type->ops.dispatch_request(hctx))) {
+			if (rq->mq_hctx != hctx)
+				list_add(&rq->queuelist, &sched);
+			else
+				list_add(&rq->queuelist, &re_submit);
+		}
+	}
+	while (!list_empty(&sched)) {
+		rq = list_first_entry(&sched, struct request, queuelist);
+		list_del_init(&rq->queuelist);
+		blk_mq_sched_insert_request(rq, true, true, true);
+	}
+
+	/* requests in dispatch list have to be re-submitted too */
+	spin_lock(&hctx->lock);
+	list_splice_tail_init(&hctx->dispatch, &re_submit);
+	spin_unlock(&hctx->lock);
+
+	/* blk_end_flush_machinery will cover flush request */
+	list_for_each_entry_safe(rq, nxt, &re_submit, queuelist) {
+		if (rq->rq_flags & RQF_FLUSH_SEQ)
+			list_move(&rq->queuelist, &flush_in);
+	}
+	blk_end_flush_machinery(hctx, &flush_in, &flush_out);
+	list_splice_tail(&flush_out, &re_submit);
+
+	while (!list_empty(&re_submit)) {
+		rq = list_first_entry(&re_submit, struct request, queuelist);
+		list_del_init(&rq->queuelist);
+		blk_mq_resubmit_rq(rq);
+	}
+}
+
 static void blk_mq_hctx_handle_dead_cpu(struct blk_mq_hw_ctx *hctx,
 		unsigned int cpu)
 {
@@ -2430,17 +2533,20 @@ static void blk_mq_hctx_handle_dead_cpu(struct blk_mq_hw_ctx *hctx,
 }
 
 /*
- * '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 inactive, we can't dispatch request
+ * to the hctx any more, so clone and re-submit requests from this hctx
  */
 static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 {
 	struct blk_mq_hw_ctx *hctx = hlist_entry_safe(node,
 			struct blk_mq_hw_ctx, cpuhp_dead);
 
-	if (cpumask_test_cpu(cpu, hctx->cpumask))
-		blk_mq_hctx_handle_dead_cpu(hctx, cpu);
+	if (cpumask_test_cpu(cpu, hctx->cpumask)) {
+		if (test_bit(BLK_MQ_S_INACTIVE, &hctx->state))
+			blk_mq_hctx_deactivate(hctx);
+		else
+			blk_mq_hctx_handle_dead_cpu(hctx, cpu);
+	}
 	return 0;
 }
 
-- 
2.25.2

[PATCH V11 12/12] block: deactivate hctx when the hctx is actually inactive

[Ming Lei]

Run queue on dead CPU still may be triggered in some corner case,
such as one request is requeued after CPU hotplug is handled.

So handle this corner case during run queue.

Cc: John Garry <john.garry@huawei.com>
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>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 block/blk-mq.c | 30 ++++++++++--------------------
 1 file changed, 10 insertions(+), 20 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c9a3e48a1ebc..0f1afa2ab5e3 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -43,6 +43,8 @@
 static void blk_mq_poll_stats_start(struct request_queue *q);
 static void blk_mq_poll_stats_fn(struct blk_stat_callback *cb);
 
+static void blk_mq_hctx_deactivate(struct blk_mq_hw_ctx *hctx);
+
 static int blk_mq_poll_stats_bkt(const struct request *rq)
 {
 	int ddir, sectors, bucket;
@@ -1400,28 +1402,16 @@ static void __blk_mq_run_hw_queue(struct blk_mq_hw_ctx *hctx)
 	int srcu_idx;
 
 	/*
-	 * We should be running this queue from one of the CPUs that
-	 * are mapped to it.
-	 *
-	 * There are at least two related races now between setting
-	 * hctx->next_cpu from blk_mq_hctx_next_cpu() and running
-	 * __blk_mq_run_hw_queue():
-	 *
-	 * - hctx->next_cpu is found offline in blk_mq_hctx_next_cpu(),
-	 *   but later it becomes online, then this warning is harmless
-	 *   at all
-	 *
-	 * - hctx->next_cpu is found online in blk_mq_hctx_next_cpu(),
-	 *   but later it becomes offline, then the warning can't be
-	 *   triggered, and we depend on blk-mq timeout handler to
-	 *   handle dispatched requests to this hctx
+	 * BLK_MQ_S_INACTIVE may not deal with some requeue corner case:
+	 * one request is requeued after cpu unplug is handled, so check
+	 * if the hctx is actually inactive. If yes, deactive it and
+	 * re-submit all requests in the queue.
 	 */
 	if (!cpumask_test_cpu(raw_smp_processor_id(), hctx->cpumask) &&
-		cpu_online(hctx->next_cpu)) {
-		printk(KERN_WARNING "run queue from wrong CPU %d, hctx %s\n",
-			raw_smp_processor_id(),
-			cpumask_empty(hctx->cpumask) ? "inactive": "active");
-		dump_stack();
+	    cpumask_next_and(-1, hctx->cpumask, cpu_online_mask) >=
+	    nr_cpu_ids) {
+		blk_mq_hctx_deactivate(hctx);
+		return;
 	}
 
 	/*
-- 
2.25.2

Re: [PATCH V11 06/12] blk-mq: prepare for draining IO when hctx's all CPUs are offline

[Hannes Reinecke]

On 5/13/20 5:47 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.
> 
> 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 from
> dm-rq & loop, then the potential dead-lock can be avoided.
> 
> [1] https://lore.kernel.org/linux-block/alpine.DEB.2.21.1904051331270.1802@nanos.tec.linutronix.de/
> 
> Cc: John Garry <john.garry@huawei.com>
> 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>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>   block/blk-mq-debugfs.c     |  1 +
>   block/blk-mq.c             | 19 +++++++++++++++++++
>   drivers/block/loop.c       |  2 +-
>   drivers/md/dm-rq.c         |  2 +-
>   include/linux/blk-mq.h     |  4 ++++
>   include/linux/cpuhotplug.h |  1 +
>   6 files changed, 27 insertions(+), 2 deletions(-)
> 
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@suse.de                               +49 911 74053 688
SUSE Software Solutions GmbH, Maxfeldstr. 5, 90409 Nürnberg
HRB 36809 (AG Nürnberg), Geschäftsführer: Felix Imendörffer

Re: [PATCH V11 00/12] blk-mq: improvement CPU hotplug

[John Garry]

On 13/05/2020 04:47, Ming Lei wrote:
> 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 inactive(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
> 
> Thanks John Garry for running lots of tests on arm64 with this patchset
> and co-working on investigating all kinds of issues.
> 
> Thanks Christoph's review on V7 & V8.
> 
> Please consider it for v5.8.
> 
> https://github.com/ming1/linux/commits/v5.7-rc-blk-mq-improve-cpu-hotplug
> 
> V11:
> 	- drop new callback from blk_mq_all_tag_busy_iter, add new helper
> 	of blk_mq_all_tag_iter (5/12), as suggested by Bart
> 	- fix request allocation hang in case of queue freeze(11/12), as
> 	reported by Bart
> 
> V10:
> 	- fix double bio complete in request resubmission(10/11)
> 	- add tested-by tag
> 
> V9:
> 	- add Reviewed-by tag
> 	- document more on memory barrier usage between getting driver tag
> 	and handling cpu offline(7/11)
> 	- small code cleanup as suggested by Chritoph(7/11)
> 	- rebase against for-5.8/block(1/11, 2/11)
> V8:
> 	- add patches to share code with blk_rq_prep_clone
> 	- code re-organization as suggested by Christoph, most of them are
> 	in 04/11, 10/11
> 	- add reviewed-by tag
> 
> V7:
> 	- fix updating .nr_active in get_driver_tag
> 	- add hctx->cpumask check in cpuhp handler
> 	- only drain requests which tag is >= 0
> 	- pass more aggressive cpuhotplug&io test
> 
> V6:
> 	- simplify getting driver tag, so that we can drain in-flight
> 	  requests correctly without using synchronize_rcu()
> 	- handle re-submission of flush & passthrough request correctly
> 
> V5:
> 	- rename BLK_MQ_S_INTERNAL_STOPPED as BLK_MQ_S_INACTIVE
> 	- re-factor code for re-submit requests in cpu dead hotplug handler
> 	- address requeue corner case
> 
> V4:
> 	- resubmit IOs in dispatch list in case that this hctx is dead
> 
> V3:
> 	- re-organize patch 2 & 3 a bit for addressing Hannes's comment
> 	- fix patch 4 for avoiding potential deadlock, as found by Hannes
> 
> V2:
> 	- patch4 & patch 5 in V1 have been merged to block tree, so remove
> 	  them
> 	- address comments from John Garry and Minwoo
> 
> 
> *** BLURB HERE ***

:)

So my tested-by tags have been dropped. I'll test again, since the 
changes are non-trivial.

Tip commit of 
https://github.com/ming1/linux/commits/v5.7-rc-blk-mq-improve-cpu-hotplug 
at this moment is b55e97a4

> 
> Ming Lei (12):
>    block: clone nr_integrity_segments and write_hint in blk_rq_prep_clone
>    block: add helper for copying request
>    blk-mq: mark blk_mq_get_driver_tag as static
>    blk-mq: assign rq->tag in blk_mq_get_driver_tag
>    blk-mq: add blk_mq_all_tag_iter
>    blk-mq: prepare for draining IO when hctx's all CPUs are offline
>    blk-mq: stop to handle IO and drain IO before hctx becomes inactive
>    block: add blk_end_flush_machinery
>    blk-mq: add blk_mq_hctx_handle_dead_cpu for handling cpu dead
>    block: add request allocation flag of BLK_MQ_REQ_FORCE
>    blk-mq: re-submit IO in case that hctx is inactive
>    block: deactivate hctx when the hctx is actually inactive
> 
>   block/blk-core.c           |  32 +++-
>   block/blk-flush.c          | 141 ++++++++++++---
>   block/blk-mq-debugfs.c     |   2 +
>   block/blk-mq-tag.c         |  33 +++-
>   block/blk-mq-tag.h         |   2 +
>   block/blk-mq.c             | 356 +++++++++++++++++++++++++++++--------
>   block/blk-mq.h             |  22 ++-
>   block/blk.h                |  11 +-
>   drivers/block/loop.c       |   2 +-
>   drivers/md/dm-rq.c         |   2 +-
>   include/linux/blk-mq.h     |  14 ++
>   include/linux/cpuhotplug.h |   1 +
>   12 files changed, 494 insertions(+), 124 deletions(-)
> 

Re: [PATCH V11 11/12] blk-mq: re-submit IO in case that hctx is inactive

[John Garry]

On 13/05/2020 04:48, Ming Lei wrote:
> +static void blk_mq_resubmit_rq(struct request *rq)
> +{
> +	struct request *nrq;
> +	unsigned int flags = 0;
> +	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> +	struct blk_mq_tags *tags = rq->q->elevator ? hctx->sched_tags :
> +		hctx->tags;
> +	bool reserved = blk_mq_tag_is_reserved(tags, rq->internal_tag);
> +
> +	if (rq->rq_flags & RQF_PREEMPT)
> +		flags |= BLK_MQ_REQ_PREEMPT;
> +	if (reserved)
> +		flags |= BLK_MQ_REQ_RESERVED;
> +	/*
> +	 * Queue freezing might be in-progress, and wait freeze can't be
> +	 * done now because we have request not completed yet, so mark this
> +	 * allocation as BLK_MQ_REQ_FORCE for avoiding this allocation &
> +	 * freeze hung forever.
> +	 */
> +	flags |= BLK_MQ_REQ_FORCE;
> +

So setting this flag triggers this WARN:

[  101.308666] Modules linked in:
[  101.311710] CPU: 23 PID: 1491 Comm: bash Not tainted 
5.7.0-rc2-00106-g63430d85fea8 #337
[  101.319698] Hardware name: Huawei Taishan 2280 /D05, BIOS Hisilicon 
D05 IT21 Nemo 2.0 RC0 04/18/2018
[  101.328816] pstate: 60000005 (nZCv daif -PAN -UAO)
[  101.333593] pc : blk_get_request+0xa4/0xac
[  101.337676] lr : blk_get_request+0xa4/0xac
[  101.341758] sp : ffff800021773aa0
[  101.345059] x29: ffff800021773aa0 x28: 0000000000000004
[  101.350357] x27: 0000000000000004 x26: 0000000000000004
[  101.355655] x25: 0000000000000000 x24: ffff800010414b20
[  101.360953] x23: 0000000000000008 x22: ffff001fb0ecf900
[  101.366251] x21: ffff001fb0a42f40 x20: 0000000000004000
[  101.371549] x19: 0000000000000010 x18: 0000000000000000
[  101.376846] x17: 0000000000000000 x16: 0000000000000000
[  101.382144] x15: 0000000000000000 x14: 0000000000000000
[  101.387441] x13: 0000000000000000 x12: 0000000000000000
[  101.392739] x11: 000000000000064f x10: 0000000000000008
[  101.398036] x9 : ffff8000118f1c28 x8 : 2074736575716572
[  101.403334] x7 : 5f7465675f6b6c62 x6 : ffff041febee21d0
[  101.408632] x5 : 0000000000000000 x4 : 0000000000000000
[  101.413930] x3 : 0000000000000000 x2 : ffff041febee9080
[  101.419229] x1 : 0000000100000000 x0 : 000000000000001a
[  101.424527] Call trace:
[  101.426961]  blk_get_request+0xa4/0xac
[  101.430698]  blk_mq_hctx_deactivate+0x270/0x3e4
[  101.435215]  blk_mq_hctx_notify_dead+0x198/0x1b4
[  101.439821]  cpuhp_invoke_callback+0x170/0x1e0
[  101.444253]  _cpu_down+0x100/0x238
[  101.447642]  cpu_down+0x40/0x68
[  101.450770]  cpu_device_down+0x14/0x1c
[  101.454508]  cpu_subsys_offline+0xc/0x14
[  101.458417]  device_offline+0x98/0xc4
[  101.462065]  online_store+0x3c/0x88
[  101.465541]  dev_attr_store+0x14/0x24
[  101.469192]  sysfs_kf_write+0x44/0x4c
[  101.472840]  kernfs_fop_write+0xfc/0x208
[  101.476751]  __vfs_write+0x18/0x3c
[  101.480140]  vfs_write+0xb4/0x1b8
[  101.483442]  ksys_write+0x4c/0xac
[  101.486743]  __arm64_sys_write+0x1c/0x24
[  101.490654]  el0_svc_common.constprop.3+0xb8/0x170
[  101.495431]  do_el0_svc+0x70/0x88
[  101.498734]  el0_sync_handler+0xf0/0x12c
[  101.502643]  el0_sync+0x140/0x180
[  101.505944] ---[ end trace 137fed615521bd97 ]--

(the series tests ok, apart from that)

Thanks,
John

> +	/* avoid allocation failure by clearing NOWAIT */
> +	nrq = blk_get_request(rq->q, rq->cmd_flags & ~REQ_NOWAIT, flags);
> +	if (!nrq)
> +		return;

[PATCH V12 10/12] block: add request allocation flag of BLK_MQ_REQ_FORCE

[Ming Lei]

From 1151afd4c11997c2769c385586097bf4f1cf60ce Mon Sep 17 00:00:00 2001
From: Ming Lei <ming.lei@redhat.com>
Date: Mon, 11 May 2020 15:43:28 +0800
Subject: [PATCH V12 10/12] block: add request allocation flag of
 BLK_MQ_REQ_FORCE

When one hctx becomes inactive, there may be requests allocated from
this hctx, we can't queue them to the inactive hctx, one approach is
to re-submit them via one active hctx.

However, the request queue may have been started to freeze, and allocating
request becomes not possible. Add flag of BLK_MQ_REQ_FORCE to allow block
layer to allocate request in this case becasue the queue won't be frozen
completely before all requests allocated from inactive hctx are completed.

The similar approach has been applied in commit 8dc765d438f1 ("SCSI: fix queue
cleanup race before queue initialization is done").

This way can help on other request dependency case too, such as, storage
device side has some problem, and IO request can't be queued to device
successfully, and passthrough request is required to fix the device problem.
If queue freeze just comes before allocating passthrough request, hang will be
triggered in queue freeze process, IO process and context for allocating
passthrough request forever. See commit 01e99aeca397 ("blk-mq: insert passthrough
request into hctx->dispatch directly") for background of this kind of issue.

Cc: John Garry <john.garry@huawei.com>
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>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
V12:
	- one line change for not warning on BLK_MQ_REQ_FORCE

 block/blk-core.c       | 8 +++++++-
 include/linux/blk-mq.h | 7 +++++++
 2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index ffb1579fd4da..c4e306f0e6fd 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -430,6 +430,11 @@ int blk_queue_enter(struct request_queue *q, blk_mq_req_flags_t flags)
 		if (success)
 			return 0;
 
+		if (flags & BLK_MQ_REQ_FORCE) {
+			percpu_ref_get(&q->q_usage_counter);
+			return 0;
+		}
+
 		if (flags & BLK_MQ_REQ_NOWAIT)
 			return -EBUSY;
 
@@ -617,7 +622,8 @@ struct request *blk_get_request(struct request_queue *q, unsigned int op,
 	struct request *req;
 
 	WARN_ON_ONCE(op & REQ_NOWAIT);
-	WARN_ON_ONCE(flags & ~(BLK_MQ_REQ_NOWAIT | BLK_MQ_REQ_PREEMPT));
+	WARN_ON_ONCE(flags & ~(BLK_MQ_REQ_NOWAIT | BLK_MQ_REQ_PREEMPT |
+				BLK_MQ_REQ_FORCE));
 
 	req = blk_mq_alloc_request(q, op, flags);
 	if (!IS_ERR(req) && q->mq_ops->initialize_rq_fn)
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index c2ea0a6e5b56..7d7aa5305a67 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -448,6 +448,13 @@ enum {
 	BLK_MQ_REQ_INTERNAL	= (__force blk_mq_req_flags_t)(1 << 2),
 	/* set RQF_PREEMPT */
 	BLK_MQ_REQ_PREEMPT	= (__force blk_mq_req_flags_t)(1 << 3),
+
+	/*
+	 * force to allocate request and caller has to make sure queue
+	 * won't be frozen completely during allocation, and this flag
+	 * is only applied after queue freeze is started
+	 */
+	BLK_MQ_REQ_FORCE	= (__force blk_mq_req_flags_t)(1 << 4),
 };
 
 struct request *blk_mq_alloc_request(struct request_queue *q, unsigned int op,
-- 
2.25.2

Re: [PATCH V11 00/12] blk-mq: improvement CPU hotplug

[Ming Lei]

On Wed, May 13, 2020 at 08:34:01AM +0100, John Garry wrote:
> On 13/05/2020 04:47, Ming Lei wrote:
> > 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 inactive(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
> > 
> > Thanks John Garry for running lots of tests on arm64 with this patchset
> > and co-working on investigating all kinds of issues.
> > 
> > Thanks Christoph's review on V7 & V8.
> > 
> > Please consider it for v5.8.
> > 
> > https://github.com/ming1/linux/commits/v5.7-rc-blk-mq-improve-cpu-hotplug
> > 
> > V11:
> > 	- drop new callback from blk_mq_all_tag_busy_iter, add new helper
> > 	of blk_mq_all_tag_iter (5/12), as suggested by Bart
> > 	- fix request allocation hang in case of queue freeze(11/12), as
> > 	reported by Bart
> > 
> > V10:
> > 	- fix double bio complete in request resubmission(10/11)
> > 	- add tested-by tag
> > 
> > V9:
> > 	- add Reviewed-by tag
> > 	- document more on memory barrier usage between getting driver tag
> > 	and handling cpu offline(7/11)
> > 	- small code cleanup as suggested by Chritoph(7/11)
> > 	- rebase against for-5.8/block(1/11, 2/11)
> > V8:
> > 	- add patches to share code with blk_rq_prep_clone
> > 	- code re-organization as suggested by Christoph, most of them are
> > 	in 04/11, 10/11
> > 	- add reviewed-by tag
> > 
> > V7:
> > 	- fix updating .nr_active in get_driver_tag
> > 	- add hctx->cpumask check in cpuhp handler
> > 	- only drain requests which tag is >= 0
> > 	- pass more aggressive cpuhotplug&io test
> > 
> > V6:
> > 	- simplify getting driver tag, so that we can drain in-flight
> > 	  requests correctly without using synchronize_rcu()
> > 	- handle re-submission of flush & passthrough request correctly
> > 
> > V5:
> > 	- rename BLK_MQ_S_INTERNAL_STOPPED as BLK_MQ_S_INACTIVE
> > 	- re-factor code for re-submit requests in cpu dead hotplug handler
> > 	- address requeue corner case
> > 
> > V4:
> > 	- resubmit IOs in dispatch list in case that this hctx is dead
> > 
> > V3:
> > 	- re-organize patch 2 & 3 a bit for addressing Hannes's comment
> > 	- fix patch 4 for avoiding potential deadlock, as found by Hannes
> > 
> > V2:
> > 	- patch4 & patch 5 in V1 have been merged to block tree, so remove
> > 	  them
> > 	- address comments from John Garry and Minwoo
> > 
> > 
> > *** BLURB HERE ***
> 
> :)
> 
> So my tested-by tags have been dropped. I'll test again, since the changes
> are non-trivial.
> 
> Tip commit of
> https://github.com/ming1/linux/commits/v5.7-rc-blk-mq-improve-cpu-hotplug at
> this moment is b55e97a4

Hi John,

I just sent out V12 of patch 10/12, which can kill the warning on BLK_MQ_REQ_FORCE,
and the warning is harmless actually. And the above git branch has been
updated too.

Please retest and update with us.

Thanks, 
Ming

Re: [PATCH V11 00/12] blk-mq: improvement CPU hotplug

[John Garry]

On 13/05/2020 11:37, Ming Lei wrote:
>> Tip commit of
>> https://github.com/ming1/linux/commits/v5.7-rc-blk-mq-improve-cpu-hotplug  at
>> this moment is b55e97a4
> Hi John,
> 
> I just sent out V12 of patch 10/12, which can kill the warning on BLK_MQ_REQ_FORCE,
> and the warning is harmless actually. And the above git branch has been
> updated too.
> 
> Please retest and update with us.

ok, so this looks better - no issue after 12 from my normal 50 test 
loops, as opposed to an issue with loop 1/50 previously. I figure it's 
ok to send v12 series proper.

Thanks,
John

Re: [PATCH V11 05/12] blk-mq: add blk_mq_all_tag_iter

[Christoph Hellwig]

On Wed, May 13, 2020 at 11:47:56AM +0800, Ming Lei wrote:
> Now request is thought as in-flight only when its state is updated as
> MQ_RQ_IN_FLIGHT, which is done by driver via blk_mq_start_request().

This should probably mention who considers it it in flight.

> Actually from blk-mq's view, one rq can be thought as in-flight
> after its tag is >= 0.

> Add one new function of blk_mq_all_tag_iter so that we can iterate
> every in-flight request, and this way is more flexible since caller
> can decide to handle request according to rq's state.

Maybe I'd reword the whole commit log as:

"Add a new blk_mq_all_tag_iter that iterates all requets which have a tag
assigned to them.  This is more flexible than the existing iterator
iterator as it allows the caller to check the request state and act
upon it."

Otherwise looks good:

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH V11 06/12] blk-mq: prepare for draining IO when hctx's all CPUs are offline

[Christoph Hellwig]

I think the flag should be inverted, indicated managed irqs if set.
And we should fine a a way to automatically set it from the managed
IRQ blk_mq_*_map_queues helpers instead of leaving the decisions to
the driver author that is most likely going to get it wrong, especially
for SCSI, where the actual driver can't even get at the current flag.

Re: [PATCH V11 07/12] blk-mq: stop to handle IO and drain IO before hctx becomes inactive

[Christoph Hellwig]

> +	if (unlikely(direct_issue && rq->mq_ctx->cpu != raw_smp_processor_id()))
> +		smp_mb();
> +	else
> +		barrier();

Independ of the fact that I still think this is horrible and hacky,
I also still don't see what the barrier() is trying to help with.

Re: [PATCH V11 08/12] block: add blk_end_flush_machinery

[Christoph Hellwig]

> +		struct list_head *flush_list)
> +{
> +	struct block_device *bdev;
> +	struct request *rq;
> +	int error = -ENXIO;
> +
> +	if (list_empty(flush_list))
> +		return;
> +
> +	rq = list_first_entry(flush_list, struct request, queuelist);

> +	/* Send flush via one active hctx so we can move on */
> +	bdev = bdget_disk(rq->rq_disk, 0);
> +	if (bdev) {
> +		error = blkdev_issue_flush(bdev, GFP_KERNEL, NULL);
> +		bdput(bdev);
> +	}

As mentioned before:  please don't add pointless bdget calls, as we
don't even need the block_device.  Just open code the trivial bits of
blkdev_issue_flushblkdev_issue_flush that are needed.

Re: [PATCH V11 09/12] blk-mq: add blk_mq_hctx_handle_dead_cpu for handling cpu dead

[Christoph Hellwig]

On Wed, May 13, 2020 at 11:48:00AM +0800, Ming Lei wrote:
> Add helper of blk_mq_hctx_handle_dead_cpu for handling cpu dead,
> and prepare for handling inactive hctx.

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>

Re: [PATCH V11 11/12] blk-mq: re-submit IO in case that hctx is inactive

[Christoph Hellwig]

Use of the BLK_MQ_REQ_FORCE is pretty bogus here..

> +	if (rq->rq_flags & RQF_PREEMPT)
> +		flags |= BLK_MQ_REQ_PREEMPT;
> +	if (reserved)
> +		flags |= BLK_MQ_REQ_RESERVED;
> +	/*
> +	 * Queue freezing might be in-progress, and wait freeze can't be
> +	 * done now because we have request not completed yet, so mark this
> +	 * allocation as BLK_MQ_REQ_FORCE for avoiding this allocation &
> +	 * freeze hung forever.
> +	 */
> +	flags |= BLK_MQ_REQ_FORCE;
> +
> +	/* avoid allocation failure by clearing NOWAIT */
> +	nrq = blk_get_request(rq->q, rq->cmd_flags & ~REQ_NOWAIT, flags);
> +	if (!nrq)
> +		return;

blk_get_request returns an ERR_PTR.

But I'd rather avoid the magic new BLK_MQ_REQ_FORCE hack when we can
just open code it and document what is going on:

static struct blk_mq_tags *blk_mq_rq_tags(struct request *rq)
{
	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;

	if (rq->q->elevator)
		return hctx->sched_tags;
	return hctx->tags;
}

static void blk_mq_resubmit_rq(struct request *rq)
{
	struct blk_mq_alloc_data alloc_data = {
		.cmd_flags	= rq->cmd_flags & ~REQ_NOWAIT;
	};
	struct request *nrq;

	if (rq->rq_flags & RQF_PREEMPT)
		alloc_data.flags |= BLK_MQ_REQ_PREEMPT;
	if (blk_mq_tag_is_reserved(blk_mq_rq_tags(rq), rq->internal_tag))
		alloc_data.flags |= BLK_MQ_REQ_RESERVED;

	/*
	 * We must still be able to finish a resubmission due to a hotplug
	 * even even if a queue freeze is in progress.
	 */
	percpu_ref_get(&q->q_usage_counter);
	nrq = blk_mq_get_request(rq->q, NULL, &alloc_data);
	blk_queue_exit(q);

	if (!nrq)
		return; // XXX: warn?
	if (nrq->q->mq_ops->initialize_rq_fn)
		rq->mq_ops->initialize_rq_fn(nrq);

	blk_rq_copy_request(nrq, rq);
	...

Re: [PATCH V11 11/12] blk-mq: re-submit IO in case that hctx is inactive

[Bart Van Assche]

On 2020-05-13 05:21, Christoph Hellwig wrote:
> Use of the BLK_MQ_REQ_FORCE is pretty bogus here..
> 
>> +	if (rq->rq_flags & RQF_PREEMPT)
>> +		flags |= BLK_MQ_REQ_PREEMPT;
>> +	if (reserved)
>> +		flags |= BLK_MQ_REQ_RESERVED;
>> +	/*
>> +	 * Queue freezing might be in-progress, and wait freeze can't be
>> +	 * done now because we have request not completed yet, so mark this
>> +	 * allocation as BLK_MQ_REQ_FORCE for avoiding this allocation &
>> +	 * freeze hung forever.
>> +	 */
>> +	flags |= BLK_MQ_REQ_FORCE;
>> +
>> +	/* avoid allocation failure by clearing NOWAIT */
>> +	nrq = blk_get_request(rq->q, rq->cmd_flags & ~REQ_NOWAIT, flags);
>> +	if (!nrq)
>> +		return;
> 
> blk_get_request returns an ERR_PTR.
> 
> But I'd rather avoid the magic new BLK_MQ_REQ_FORCE hack when we can
> just open code it and document what is going on:
> 
> static struct blk_mq_tags *blk_mq_rq_tags(struct request *rq)
> {
> 	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> 
> 	if (rq->q->elevator)
> 		return hctx->sched_tags;
> 	return hctx->tags;
> }
> 
> static void blk_mq_resubmit_rq(struct request *rq)
> {
> 	struct blk_mq_alloc_data alloc_data = {
> 		.cmd_flags	= rq->cmd_flags & ~REQ_NOWAIT;
> 	};
> 	struct request *nrq;
> 
> 	if (rq->rq_flags & RQF_PREEMPT)
> 		alloc_data.flags |= BLK_MQ_REQ_PREEMPT;
> 	if (blk_mq_tag_is_reserved(blk_mq_rq_tags(rq), rq->internal_tag))
> 		alloc_data.flags |= BLK_MQ_REQ_RESERVED;
> 
> 	/*
> 	 * We must still be able to finish a resubmission due to a hotplug
> 	 * even even if a queue freeze is in progress.
> 	 */
> 	percpu_ref_get(&q->q_usage_counter);
> 	nrq = blk_mq_get_request(rq->q, NULL, &alloc_data);
> 	blk_queue_exit(q);
> 
> 	if (!nrq)
> 		return; // XXX: warn?
> 	if (nrq->q->mq_ops->initialize_rq_fn)
> 		rq->mq_ops->initialize_rq_fn(nrq);
> 
> 	blk_rq_copy_request(nrq, rq);
> 	...

I don't like this because the above code allows allocation of requests
and tags while a request queue is frozen. I'm concerned that this will
break code that assumes that no tags are allocated while a request queue
is frozen. If a request queue has a single hardware queue with 64 tags,
if the above code allocates tag 40 and if blk_mq_tag_update_depth()
reduces the queue depth to 32, will nrq become a dangling pointer?

Thanks,

Bart.

Re: [PATCH V11 06/12] blk-mq: prepare for draining IO when hctx's all CPUs are offline

[Ming Lei]

On Wed, May 13, 2020 at 01:58:32PM +0200, Christoph Hellwig wrote:
> I think the flag should be inverted, indicated managed irqs if set.
> And we should fine a a way to automatically set it from the managed
> IRQ blk_mq_*_map_queues helpers instead of leaving the decisions to

blk_mq_*_map_queues doesn't tell us if managed IRQ is used, only the
driver knows that.

> the driver author that is most likely going to get it wrong, especially
> for SCSI, where the actual driver can't even get at the current flag.
> 

BLK_MQ_F_NO_MANAGED_IRQ is just for avoiding deadlock in draining IO
during cpu hotplug. So far, only stacking blk-mq drivers need this flag.

That is why the flag is named as NO_MANAGED_IRQ, or we may add flag of
BLK_MQ_F_STACKING for this purpose too.

thanks,
Ming

Re: [PATCH V11 07/12] blk-mq: stop to handle IO and drain IO before hctx becomes inactive

[Ming Lei]

On Wed, May 13, 2020 at 01:59:32PM +0200, Christoph Hellwig wrote:
> > +	if (unlikely(direct_issue && rq->mq_ctx->cpu != raw_smp_processor_id()))
> > +		smp_mb();
> > +	else
> > +		barrier();
> 
> Independ of the fact that I still think this is horrible and hacky,

Why?

I believe it has been documented clearly. Or can you share better idea?

> I also still don't see what the barrier() is trying to help with.

It is a compiler barrier, the two OPs are still to be ordered in case
of single CPU.


Thanks,
Ming

Re: [PATCH V11 11/12] blk-mq: re-submit IO in case that hctx is inactive

[Ming Lei]

On Wed, May 13, 2020 at 02:21:47PM +0200, Christoph Hellwig wrote:
> Use of the BLK_MQ_REQ_FORCE is pretty bogus here..
> 
> > +	if (rq->rq_flags & RQF_PREEMPT)
> > +		flags |= BLK_MQ_REQ_PREEMPT;
> > +	if (reserved)
> > +		flags |= BLK_MQ_REQ_RESERVED;
> > +	/*
> > +	 * Queue freezing might be in-progress, and wait freeze can't be
> > +	 * done now because we have request not completed yet, so mark this
> > +	 * allocation as BLK_MQ_REQ_FORCE for avoiding this allocation &
> > +	 * freeze hung forever.
> > +	 */
> > +	flags |= BLK_MQ_REQ_FORCE;
> > +
> > +	/* avoid allocation failure by clearing NOWAIT */
> > +	nrq = blk_get_request(rq->q, rq->cmd_flags & ~REQ_NOWAIT, flags);
> > +	if (!nrq)
> > +		return;
> 
> blk_get_request returns an ERR_PTR.
> 
> But I'd rather avoid the magic new BLK_MQ_REQ_FORCE hack when we can
> just open code it and document what is going on:

BLK_MQ_REQ_FORCE is actually not a hack, there are other use cases
which need that too, see commit log of patch 10/12.

> 
> static struct blk_mq_tags *blk_mq_rq_tags(struct request *rq)
> {
> 	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> 
> 	if (rq->q->elevator)
> 		return hctx->sched_tags;
> 	return hctx->tags;
> }
> 
> static void blk_mq_resubmit_rq(struct request *rq)
> {
> 	struct blk_mq_alloc_data alloc_data = {
> 		.cmd_flags	= rq->cmd_flags & ~REQ_NOWAIT;
> 	};
> 	struct request *nrq;
> 
> 	if (rq->rq_flags & RQF_PREEMPT)
> 		alloc_data.flags |= BLK_MQ_REQ_PREEMPT;
> 	if (blk_mq_tag_is_reserved(blk_mq_rq_tags(rq), rq->internal_tag))
> 		alloc_data.flags |= BLK_MQ_REQ_RESERVED;
> 
> 	/*
> 	 * We must still be able to finish a resubmission due to a hotplug
> 	 * even even if a queue freeze is in progress.
> 	 */
> 	percpu_ref_get(&q->q_usage_counter);
> 	nrq = blk_mq_get_request(rq->q, NULL, &alloc_data);
> 	blk_queue_exit(q);

This way works too.

> 
> 	if (!nrq)
> 		return; // XXX: warn?

It isn't possible because we clears NO_WAIT flag.


Thanks, 
Ming

Re: [PATCH V11 11/12] blk-mq: re-submit IO in case that hctx is inactive

[Ming Lei]

On Wed, May 13, 2020 at 08:03:13AM -0700, Bart Van Assche wrote:
> On 2020-05-13 05:21, Christoph Hellwig wrote:
> > Use of the BLK_MQ_REQ_FORCE is pretty bogus here..
> > 
> >> +	if (rq->rq_flags & RQF_PREEMPT)
> >> +		flags |= BLK_MQ_REQ_PREEMPT;
> >> +	if (reserved)
> >> +		flags |= BLK_MQ_REQ_RESERVED;
> >> +	/*
> >> +	 * Queue freezing might be in-progress, and wait freeze can't be
> >> +	 * done now because we have request not completed yet, so mark this
> >> +	 * allocation as BLK_MQ_REQ_FORCE for avoiding this allocation &
> >> +	 * freeze hung forever.
> >> +	 */
> >> +	flags |= BLK_MQ_REQ_FORCE;
> >> +
> >> +	/* avoid allocation failure by clearing NOWAIT */
> >> +	nrq = blk_get_request(rq->q, rq->cmd_flags & ~REQ_NOWAIT, flags);
> >> +	if (!nrq)
> >> +		return;
> > 
> > blk_get_request returns an ERR_PTR.
> > 
> > But I'd rather avoid the magic new BLK_MQ_REQ_FORCE hack when we can
> > just open code it and document what is going on:
> > 
> > static struct blk_mq_tags *blk_mq_rq_tags(struct request *rq)
> > {
> > 	struct blk_mq_hw_ctx *hctx = rq->mq_hctx;
> > 
> > 	if (rq->q->elevator)
> > 		return hctx->sched_tags;
> > 	return hctx->tags;
> > }
> > 
> > static void blk_mq_resubmit_rq(struct request *rq)
> > {
> > 	struct blk_mq_alloc_data alloc_data = {
> > 		.cmd_flags	= rq->cmd_flags & ~REQ_NOWAIT;
> > 	};
> > 	struct request *nrq;
> > 
> > 	if (rq->rq_flags & RQF_PREEMPT)
> > 		alloc_data.flags |= BLK_MQ_REQ_PREEMPT;
> > 	if (blk_mq_tag_is_reserved(blk_mq_rq_tags(rq), rq->internal_tag))
> > 		alloc_data.flags |= BLK_MQ_REQ_RESERVED;
> > 
> > 	/*
> > 	 * We must still be able to finish a resubmission due to a hotplug
> > 	 * even even if a queue freeze is in progress.
> > 	 */
> > 	percpu_ref_get(&q->q_usage_counter);
> > 	nrq = blk_mq_get_request(rq->q, NULL, &alloc_data);
> > 	blk_queue_exit(q);
> > 
> > 	if (!nrq)
> > 		return; // XXX: warn?
> > 	if (nrq->q->mq_ops->initialize_rq_fn)
> > 		rq->mq_ops->initialize_rq_fn(nrq);
> > 
> > 	blk_rq_copy_request(nrq, rq);
> > 	...
> 
> I don't like this because the above code allows allocation of requests
> and tags while a request queue is frozen. I'm concerned that this will
> break code that assumes that no tags are allocated while a request queue
> is frozen. If a request queue has a single hardware queue with 64 tags,

The above code path will never be called for single hw queue.

> if the above code allocates tag 40 and if blk_mq_tag_update_depth()
> reduces the queue depth to 32, will nrq become a dangling pointer?

allocation for nrq is just like other normal allocation, and if
it doesn't work with blk_mq_tag_update_depth(), it must be a more
generic issue instead of relating with this specific use case.

The only difference is that 'nrq' will be allocated from a new active
hctx, so the two requests can co-exist and we needn't to worry deadlock.


thanks,
Ming

Re: [PATCH V11 07/12] blk-mq: stop to handle IO and drain IO before hctx becomes inactive

[Bart Van Assche]

On 2020-05-13 04:59, Christoph Hellwig wrote:
>> +	if (unlikely(direct_issue && rq->mq_ctx->cpu != raw_smp_processor_id()))
>> +		smp_mb();
>> +	else
>> +		barrier();
> 
> Independ of the fact that I still think this is horrible and hacky,
> I also still don't see what the barrier() is trying to help with.

I'm also less than enthusiast about this code.

Bart.

Re: [PATCH V11 07/12] blk-mq: stop to handle IO and drain IO before hctx becomes inactive

[Ming Lei]

On Wed, May 13, 2020 at 01:59:32PM +0200, Christoph Hellwig wrote:
> > +	if (unlikely(direct_issue && rq->mq_ctx->cpu != raw_smp_processor_id()))
> > +		smp_mb();
> > +	else
> > +		barrier();
> 
> Independ of the fact that I still think this is horrible and hacky,
> I also still don't see what the barrier() is trying to help with.

It is correct and clean to only add smp_mb() for ordering setting
rq->tag/assigning req and checking INACTIVE, and just a normal usage
of memory barrier.

barrier() is used in most likely case because smp_mb() is only required
when direct io issue process is migrated to other CPU, that seldom
happens, so just an optimization.

So I do not agree it is horrible and hacky.

Thanks,
Ming