blk-mq: drain I/O when all CPUs in a hctx are offline

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 a
cpuhp state handled after the CPU is down, so there isn't any chance to
quiesce the hctx before shutting down the CPU.

Add new CPUHP_AP_BLK_MQ_ONLINE state to stop allocating from blk-mq hctxs
where the last CPU goes away, and wait for completion of in-flight
requests.  This guarantees that there is no inflight I/O before shutting
down the managed IRQ.

Add a BLK_MQ_F_STACKING and set it for dm-rq and loop, so we don't need
to wait for completion of in-flight requests from these drivers to avoid
a potential dead-lock. It is safe to do this for stacking drivers as those
do not use interrupts at all and their I/O completions are triggered by
underlying devices I/O completion.

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

[hch: different retry mechanism, merged two patches, minor cleanups]

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Daniel Wagner <dwagner@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 110 insertions, 2 deletions

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 560ef5df8993..9a36ac1c1fa1 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -375,14 +375,30 @@ static struct request *__blk_mq_alloc_request(struct blk_mq_alloc_data *data)
 			e->type->ops.limit_depth(data->cmd_flags, data);
 	}
 
+retry:
 	data->ctx = blk_mq_get_ctx(q);
 	data->hctx = blk_mq_map_queue(q, data->cmd_flags, data->ctx);
 	if (!(data->flags & BLK_MQ_REQ_INTERNAL))
 		blk_mq_tag_busy(data->hctx);
 
+	/*
+	 * Waiting allocations only fail because of an inactive hctx.  In that
+	 * case just retry the hctx assignment and tag allocation as CPU hotplug
+	 * should have migrated us to an online CPU by now.
+	 */
 	tag = blk_mq_get_tag(data);
-	if (tag == BLK_MQ_NO_TAG)
-		return NULL;
+	if (tag == BLK_MQ_NO_TAG) {
+		if (data->flags & BLK_MQ_REQ_NOWAIT)
+			return NULL;
+
+		/*
+		 * Give up the CPU and sleep for a random short time to ensure
+		 * that thread using a realtime scheduling class are migrated
+		 * off the the CPU, and thus off the hctx that is going away.
+		 */
+		msleep(3);
+		goto retry;
+	}
 	return blk_mq_rq_ctx_init(data, tag, alloc_time_ns);
 }
 
@@ -2335,6 +2351,86 @@ fail:
 	return -ENOMEM;
 }
 
+struct rq_iter_data {
+	struct blk_mq_hw_ctx *hctx;
+	bool has_rq;
+};
+
+static bool blk_mq_has_request(struct request *rq, void *data, bool reserved)
+{
+	struct rq_iter_data *iter_data = data;
+
+	if (rq->mq_hctx != iter_data->hctx)
+		return true;
+	iter_data->has_rq = true;
+	return false;
+}
+
+static bool blk_mq_hctx_has_requests(struct blk_mq_hw_ctx *hctx)
+{
+	struct blk_mq_tags *tags = hctx->sched_tags ?
+			hctx->sched_tags : hctx->tags;
+	struct rq_iter_data data = {
+		.hctx	= hctx,
+	};
+
+	blk_mq_all_tag_iter(tags, blk_mq_has_request, &data);
+	return data.has_rq;
+}
+
+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) ||
+	    !blk_mq_last_cpu_in_hctx(cpu, hctx))
+		return 0;
+
+	/*
+	 * Prevent new request from being allocated on the current hctx.
+	 *
+	 * The smp_mb__after_atomic() Pairs with the implied barrier in
+	 * test_and_set_bit_lock in sbitmap_get().  Ensures the inactive flag is
+	 * seen once we return from the tag allocator.
+	 */
+	set_bit(BLK_MQ_S_INACTIVE, &hctx->state);
+	smp_mb__after_atomic();
+
+	/*
+	 * Try to grab a reference to the queue and wait for any outstanding
+	 * requests.  If we could not grab a reference the queue has been
+	 * frozen and there are no requests.
+	 */
+	if (percpu_ref_tryget(&hctx->queue->q_usage_counter)) {
+		while (blk_mq_hctx_has_requests(hctx))
+			msleep(5);
+		percpu_ref_put(&hctx->queue->q_usage_counter);
+	}
+
+	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;
+}
+
 /*
  * 'cpu' is going away. splice any existing rq_list entries from this
  * software queue to the hw queue dispatch list, and ensure that it
@@ -2348,6 +2444,9 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu, struct hlist_node *node)
 	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;
 
@@ -2371,6 +2470,9 @@ 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_STACKING))
+		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);
 }
@@ -2430,6 +2532,9 @@ static int blk_mq_init_hctx(struct request_queue *q,
 {
 	hctx->queue_num = hctx_idx;
 
+	if (!(hctx->flags & BLK_MQ_F_STACKING))
+		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];
@@ -3684,6 +3789,9 @@ 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);