SCSI probing may synchronously create and destroy a lot of request_queues
for non-existent devices. Any synchronize_rcu() in queue creation or
destroy path may introduce long latency during booting, see detailed
description in comment of blk_register_queue().
This patch removes two synchronize_rcu() inside blk_cleanup_queue()
for this case:
1) commit c2856ae2f315d75(blk-mq: quiesce queue before freeing queue)
need synchronize_rcu() for implementing blk_mq_quiesce_queue(), but
when queue isn't initialized, it isn't necessary to do that since
only pass-through requests are involved, no original issue in
scsi_execute() at all.
2) when only one request queue is attached to tags, no necessary to
call synchronize_rcu() too.
Without this patch, it may take more 20+ seconds for virtio-scsi to
complete disk probe. With this patch, the time becomes less than 100ms.
Reported-by: Andrew Jones <drjones@xxxxxxxxxx>
Cc: Andrew Jones <drjones@xxxxxxxxxx>
Cc: linux-scsi@xxxxxxxxxxxxxxx
Cc: "Martin K. Petersen" <martin.petersen@xxxxxxxxxx>
Cc: Christoph Hellwig <hch@xxxxxx>
Signed-off-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
block/blk-core.c | 8 ++++++--
block/blk-mq.c | 5 ++++-
2 files changed, 10 insertions(+), 3 deletions(-)
diff --git a/block/blk-core.c b/block/blk-core.c
index cf0ee764b908..f0129e20b773 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -766,9 +766,13 @@ void blk_cleanup_queue(struct request_queue *q)
* make sure all in-progress dispatch are completed because
* blk_freeze_queue() can only complete all requests, and
* dispatch may still be in-progress since we dispatch requests
- * from more than one contexts
+ * from more than one contexts.
+ *
+ * No need to quiesce queue if it isn't initialized yet since
+ * blk_freeze_queue() should be enough for cases of passthrough
+ * request.
*/
- if (q->mq_ops)
+ if (q->mq_ops && blk_queue_init_done(q))
blk_mq_quiesce_queue(q);
/* for synchronous bio-based driver finish in-flight integrity i/o */
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 70c65bb6c013..63680b243466 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2351,6 +2351,7 @@ static void blk_mq_update_tag_set_depth(struct blk_mq_tag_set *set,
static void blk_mq_del_queue_tag_set(struct request_queue *q)
{
struct blk_mq_tag_set *set = q->tag_set;
+ bool shared = true;
mutex_lock(&set->tag_list_lock);
list_del_rcu(&q->tag_set_list);
@@ -2359,9 +2360,11 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
set->flags &= ~BLK_MQ_F_TAG_SHARED;
/* update existing queue */
blk_mq_update_tag_set_depth(set, false);
+ shared = true;
}
mutex_unlock(&set->tag_list_lock);
- synchronize_rcu();
+ if (shared)
+ synchronize_rcu();
INIT_LIST_HEAD(&q->tag_set_list);
}
--
2.9.5
