[PATCH v5 1/2] blk-mq: add tagset quiesce interface

[Sagi Grimberg]

drivers that have shared tagsets may need to quiesce potentially a lot
of request queues that all share a single tagset (e.g. nvme). Add an interface
to quiesce all the queues on a given tagset. This interface is useful because
it can speedup the quiesce by doing it in parallel.

For tagsets that have BLK_MQ_F_BLOCKING set, we use call_srcu to all hctxs
in parallel such that all of them wait for the same rcu elapsed period with
a per-hctx heap allocated rcu_synchronize. for tagsets that don't have
BLK_MQ_F_BLOCKING set, we simply call a single synchronize_rcu as this is
sufficient.

Signed-off-by: Sagi Grimberg <sagi at grimberg.me>
---
 block/blk-mq.c         | 66 ++++++++++++++++++++++++++++++++++++++++++
 include/linux/blk-mq.h |  4 +++
 2 files changed, 70 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index abcf590f6238..c37e37354330 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -209,6 +209,42 @@ void blk_mq_quiesce_queue_nowait(struct request_queue *q)
 }
 EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue_nowait);
 
+static void blk_mq_quiesce_blocking_queue_async(struct request_queue *q)
+{
+	struct blk_mq_hw_ctx *hctx;
+	unsigned int i;
+
+	blk_mq_quiesce_queue_nowait(q);
+
+	queue_for_each_hw_ctx(q, hctx, i) {
+		WARN_ON_ONCE(!(hctx->flags & BLK_MQ_F_BLOCKING));
+		hctx->rcu_sync = kmalloc(sizeof(*hctx->rcu_sync), GFP_KERNEL);
+		if (!hctx->rcu_sync)
+			continue;
+
+		init_completion(&hctx->rcu_sync->completion);
+		init_rcu_head(&hctx->rcu_sync->head);
+		call_srcu(hctx->srcu, &hctx->rcu_sync->head,
+				wakeme_after_rcu);
+	}
+}
+
+static void blk_mq_quiesce_blocking_queue_async_wait(struct request_queue *q)
+{
+	struct blk_mq_hw_ctx *hctx;
+	unsigned int i;
+
+	queue_for_each_hw_ctx(q, hctx, i) {
+		WARN_ON_ONCE(!(hctx->flags & BLK_MQ_F_BLOCKING));
+		if (!hctx->rcu_sync) {
+			synchronize_srcu(hctx->srcu);
+			continue;
+		}
+		wait_for_completion(&hctx->rcu_sync->completion);
+		destroy_rcu_head(&hctx->rcu_sync->head);
+	}
+}
+
 /**
  * blk_mq_quiesce_queue() - wait until all ongoing dispatches have finished
  * @q: request queue.
@@ -2884,6 +2920,36 @@ static void queue_set_hctx_shared(struct request_queue *q, bool shared)
 	}
 }
 
+void blk_mq_quiesce_tagset(struct blk_mq_tag_set *set)
+{
+	struct request_queue *q;
+
+	mutex_lock(&set->tag_list_lock);
+	if (set->flags & BLK_MQ_F_BLOCKING) {
+		list_for_each_entry(q, &set->tag_list, tag_set_list)
+			blk_mq_quiesce_blocking_queue_async(q);
+		list_for_each_entry(q, &set->tag_list, tag_set_list)
+			blk_mq_quiesce_blocking_queue_async_wait(q);
+	} else {
+		list_for_each_entry(q, &set->tag_list, tag_set_list)
+			blk_mq_quiesce_queue_nowait(q);
+		synchronize_rcu();
+	}
+	mutex_unlock(&set->tag_list_lock);
+}
+EXPORT_SYMBOL_GPL(blk_mq_quiesce_tagset);
+
+void blk_mq_unquiesce_tagset(struct blk_mq_tag_set *set)
+{
+	struct request_queue *q;
+
+	mutex_lock(&set->tag_list_lock);
+	list_for_each_entry(q, &set->tag_list, tag_set_list)
+		blk_mq_unquiesce_queue(q);
+	mutex_unlock(&set->tag_list_lock);
+}
+EXPORT_SYMBOL_GPL(blk_mq_unquiesce_tagset);
+
 static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set,
 					bool shared)
 {
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 23230c1d031e..a85f2dedc947 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -5,6 +5,7 @@
 #include <linux/blkdev.h>
 #include <linux/sbitmap.h>
 #include <linux/srcu.h>
+#include <linux/rcupdate_wait.h>
 
 struct blk_mq_tags;
 struct blk_flush_queue;
@@ -170,6 +171,7 @@ struct blk_mq_hw_ctx {
 	 */
 	struct list_head	hctx_list;
 
+	struct rcu_synchronize	*rcu_sync;
 	/**
 	 * @srcu: Sleepable RCU. Use as lock when type of the hardware queue is
 	 * blocking (BLK_MQ_F_BLOCKING). Must be the last member - see also
@@ -532,6 +534,8 @@ int blk_mq_map_queues(struct blk_mq_queue_map *qmap);
 void blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, int nr_hw_queues);
 
 void blk_mq_quiesce_queue_nowait(struct request_queue *q);
+void blk_mq_quiesce_tagset(struct request_queue *q);
+void blk_mq_unquiesce_tagset(struct request_queue *q);
 
 unsigned int blk_mq_rq_cpu(struct request *rq);
 
-- 
2.25.1