[PATCH V2 1/3] blk-mq: introduce bio retrieve mechanism

[Jianchao Wang]

Currently request requeue mechanism cannot work well with updating
nr_hw_queues. Because the requests are highly bound with specific
hw queue, requests on the dying hw queue have to be failed. And
this could be fatal for filesystem.

In addition, the request_queue need to be frozen and drained before
updating nr_hw_queues, if IO timeout, we have to depend on the LLDD
to do recovery. But the recovery path maybe sleeping to wait the
the request_queue to be drained. IO hang comes up.

To avoid the two case above, we introduce bio retrieve mechanism.
The bio retrieving will do following things:
 - flush requests on hctx->dispatch, sw queue or io scheduler queue
 - take the bios down from the requests and end the requests
 - requeue this bios and submit them through generic_make_request
   again later.

Then we could avoid to fail requests on dying hw queue and depend
on storage device to drain request_queue.

Signed-off-by: Jianchao Wang <jianchao.w.wang@xxxxxxxxxx>
---
 block/blk-core.c       |  2 ++
 block/blk-mq-sched.c   | 88 ++++++++++++++++++++++++++++++++++++++++++++++++++
 block/blk-mq.c         | 42 ++++++++++++++++++++++++
 include/linux/blk-mq.h |  4 +++
 include/linux/blkdev.h |  2 ++
 5 files changed, 138 insertions(+)

diff --git a/block/blk-core.c b/block/blk-core.c
index cdfabc5..f3c6fa8 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -807,6 +807,8 @@ void blk_cleanup_queue(struct request_queue *q)
 
 	/* @q won't process any more request, flush async actions */
 	del_timer_sync(&q->backing_dev_info->laptop_mode_wb_timer);
+	if (q->mq_ops)
+		cancel_delayed_work_sync(&q->bio_requeue_work);
 	blk_sync_queue(q);
 
 	/*
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 29bfe80..9d0b2a2 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -422,6 +422,94 @@ void blk_mq_sched_insert_requests(struct request_queue *q,
 	blk_mq_run_hw_queue(hctx, run_queue_async);
 }
 
+static void blk_mq_sched_retrieve_one_req(struct request *rq,
+		struct bio_list *list)
+{
+	struct bio *bio;
+
+	blk_steal_bios(list, rq);
+	blk_mq_end_request(rq, BLK_STS_OK);
+
+	bio_list_for_each(bio, list) {
+		/*
+		 * bio with BIO_QUEUE_ENTERED will enter queue with
+		 * blk_queue_enter_live.
+		 */
+		bio_clear_flag(bio, BIO_QUEUE_ENTERED);
+	}
+}
+
+static void __blk_mq_sched_retrieve_bios(struct blk_mq_hw_ctx *hctx)
+{
+	struct request_queue *q = hctx->queue;
+	struct bio_list bio_list;
+	LIST_HEAD(rq_list);
+	struct request *rq;
+
+	bio_list_init(&bio_list);
+
+	if (!list_empty_careful(&hctx->dispatch)) {
+		spin_lock(&hctx->lock);
+		if (!list_empty(&hctx->dispatch))
+			list_splice_tail_init(&hctx->dispatch, &rq_list);
+		spin_unlock(&hctx->lock);
+	}
+
+	if (!q->elevator)
+		blk_mq_flush_busy_ctxs(hctx, &rq_list);
+
+	while (!list_empty(&rq_list)) {
+		rq = list_first_entry(&rq_list, struct request, queuelist);
+		list_del_init(&rq->queuelist);
+		blk_mq_sched_retrieve_one_req(rq, &bio_list);
+	}
+
+	if (q->elevator) {
+		struct elevator_queue *e = hctx->queue->elevator;
+
+		while (e->type->ops.mq.has_work &&
+				e->type->ops.mq.has_work(hctx)) {
+			rq = e->type->ops.mq.dispatch_request(hctx);
+			if (!rq)
+				continue;
+
+			blk_mq_sched_retrieve_one_req(rq, &bio_list);
+		}
+	}
+	/* For the request with RQF_FLUSH_SEQ, blk_mq_end_request cannot end them
+	 * but just push the flush sm. So there could still be rqs in flush queue,
+	 * the caller will check q_usage_counter and come back again.
+	 */
+	blk_mq_requeue_bios(q, &bio_list, false);
+}
+
+/*
+ * When blk_mq_sched_retrieve_bios returns:
+ *  - All the rqs are ended, q_usage_counter is zero
+ *  - All the bios are queued to q->requeue_bios
+ */
+void blk_mq_sched_retrieve_bios(struct request_queue *q)
+{
+	struct blk_mq_hw_ctx *hctx;
+	int i;
+
+	BUG_ON(!atomic_read(&q->mq_freeze_depth) ||
+			!blk_queue_quiesced(q));
+
+	/*
+	 * Kick the requeue_work to flush the reqs in requeue_list
+	 */
+	blk_mq_kick_requeue_list(q);
+
+	while (!percpu_ref_is_zero(&q->q_usage_counter)) {
+		queue_for_each_hw_ctx(q, hctx, i)
+			__blk_mq_sched_retrieve_bios(hctx);
+	}
+
+	blk_mq_requeue_bios(q, NULL, true);
+}
+EXPORT_SYMBOL_GPL(blk_mq_sched_retrieve_bios);
+
 static void blk_mq_sched_free_tags(struct blk_mq_tag_set *set,
 				   struct blk_mq_hw_ctx *hctx,
 				   unsigned int hctx_idx)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index dcf10e3..f75598b 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -706,6 +706,46 @@ void blk_mq_requeue_request(struct request *rq, bool kick_requeue_list)
 }
 EXPORT_SYMBOL(blk_mq_requeue_request);
 
+static void blk_mq_bio_requeue_work(struct work_struct *work)
+{
+	struct request_queue *q =
+		container_of(work, struct request_queue, bio_requeue_work.work);
+	struct bio *bio;
+
+	/* Defects:
+	 *  - Bios from all cpus have to be issued on one.
+	 *  - The requeued older bios have to contend tags with following
+	 *    new bios.
+	 */
+	while (true) {
+		spin_lock_irq(&q->requeue_lock);
+		bio = bio_list_pop(&q->requeue_bios);
+		spin_unlock_irq(&q->requeue_lock);
+		if (!bio)
+			break;
+		/*
+		 * generic_make_request could invoke blk_queue_enter, then
+		 *  - sleep when queue is frozen
+		 *  - return with failing the bio when the queue is DYING
+		 */
+		generic_make_request(bio);
+	}
+}
+
+void blk_mq_requeue_bios(struct request_queue *q,
+		struct bio_list *bio_list, bool kick)
+{
+	if (bio_list) {
+		spin_lock_irq(&q->requeue_lock);
+		bio_list_merge(&q->requeue_bios, bio_list);
+		spin_unlock_irq(&q->requeue_lock);
+	}
+
+	if (kick)
+		kblockd_mod_delayed_work_on(WORK_CPU_UNBOUND, &q->bio_requeue_work, 0);
+}
+EXPORT_SYMBOL(blk_mq_requeue_bios);
+
 static void blk_mq_requeue_work(struct work_struct *work)
 {
 	struct request_queue *q =
@@ -2695,7 +2735,9 @@ struct request_queue *blk_mq_init_allocated_queue(struct blk_mq_tag_set *set,
 	q->sg_reserved_size = INT_MAX;
 
 	INIT_DELAYED_WORK(&q->requeue_work, blk_mq_requeue_work);
+	INIT_DELAYED_WORK(&q->bio_requeue_work, blk_mq_bio_requeue_work);
 	INIT_LIST_HEAD(&q->requeue_list);
+	bio_list_init(&q->requeue_bios);
 	spin_lock_init(&q->requeue_lock);
 
 	blk_queue_make_request(q, blk_mq_make_request);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 2286dc1..52187d4 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -260,6 +260,10 @@ void blk_mq_end_request(struct request *rq, blk_status_t error);
 void __blk_mq_end_request(struct request *rq, blk_status_t error);
 
 void blk_mq_requeue_request(struct request *rq, bool kick_requeue_list);
+void blk_mq_requeue_bios(struct request_queue *q,
+		struct bio_list *bio_list, bool kick);
+void blk_mq_sched_retrieve_bios(struct request_queue *q);
+
 void blk_mq_add_to_requeue_list(struct request *rq, bool at_head,
 				bool kick_requeue_list);
 void blk_mq_kick_requeue_list(struct request_queue *q);
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 6120756..0c83948 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -633,8 +633,10 @@ struct request_queue {
 	struct blk_flush_queue	*fq;
 
 	struct list_head	requeue_list;
+	struct bio_list 	requeue_bios;
 	spinlock_t		requeue_lock;
 	struct delayed_work	requeue_work;
+	struct delayed_work	bio_requeue_work;
 
 	struct mutex		sysfs_lock;
 
-- 
2.7.4