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

[Ming Lei <ming.lei@redhat.com>]

On Tue, Jul 28, 2020 at 09:18:59AM +0200, Christoph Hellwig wrote:
> I like the tagset based interface.  But the idea of doing a per-hctx
> allocation and wait doesn't seem very scalable.
> 
> Paul, do you have any good idea for an interface that waits on
> multiple srcu heads?  As far as I can tell we could just have a single
> global completion and counter, and each call_srcu would just just
> decrement it and then the final one would do the wakeup.  It would just
> be great to figure out a way to keep the struct rcu_synchronize and
> counter on stack to avoid an allocation.
> 
> But if we can't do with an on-stack object I'd much rather just embedd
> the rcu_head in the hw_ctx.

I think we can do that, please see the following patch which is against Sagi's V5:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c3856377b961..fc46e77460f1 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -27,6 +27,7 @@
 #include <linux/crash_dump.h>
 #include <linux/prefetch.h>
 #include <linux/blk-crypto.h>
+#include <linux/rcupdate_wait.h>
 
 #include <trace/events/block.h>
 
@@ -209,6 +210,50 @@ void blk_mq_quiesce_queue_nowait(struct request_queue *q)
 }
 EXPORT_SYMBOL_GPL(blk_mq_quiesce_queue_nowait);
 
+struct blk_mq_srcu_sync {
+	struct rcu_synchronize srcu_sync;
+	atomic_t count;
+};
+
+static void blk_mq_srcu_sync_init(struct blk_mq_srcu_sync *sync, int count)
+{
+	init_completion(&sync->srcu_sync.completion);
+	init_rcu_head(&sync->srcu_sync.head);
+
+	atomic_set(&sync->count, count);
+}
+
+static void blk_mq_srcu_sync_wait(struct blk_mq_srcu_sync *sync)
+{
+	wait_for_completion(&sync->srcu_sync.completion);
+	destroy_rcu_head_on_stack(&sync->srcu_sync.head);
+}
+
+static void blk_mq_wakeme_after_rcu(struct rcu_head *head)
+{
+	struct blk_mq_srcu_sync *sync;
+
+	sync = container_of(head, struct blk_mq_srcu_sync, srcu_sync.head);
+
+	if (atomic_dec_and_test(&sync->count))
+		complete(&sync->srcu_sync.completion);
+}
+
+static void blk_mq_quiesce_blocking_queue_async(struct request_queue *q,
+		struct blk_mq_srcu_sync *sync)
+{
+	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));
+		call_srcu(hctx->srcu, &sync->srcu_sync.head,
+				blk_mq_wakeme_after_rcu);
+	}
+}
+
 /**
  * blk_mq_quiesce_queue() - wait until all ongoing dispatches have finished
  * @q: request queue.
@@ -2880,6 +2925,45 @@ 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) {
+		struct blk_mq_srcu_sync sync;
+		int count = 0;
+
+		list_for_each_entry(q, &set->tag_list, tag_set_list)
+			count++;
+
+		blk_mq_srcu_sync_init(&sync, count);
+
+		list_for_each_entry(q, &set->tag_list, tag_set_list)
+			blk_mq_quiesce_blocking_queue_async(q, &sync);
+
+		blk_mq_srcu_sync_wait(&sync);
+
+	} 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..d5e0974a1dcc 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -532,6 +532,8 @@ 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 blk_mq_tag_set *set);
+void blk_mq_unquiesce_tagset(struct blk_mq_tag_set *set);
 
 unsigned int blk_mq_rq_cpu(struct request *rq);
 


-- 
Ming