Re: [PATCH V2] blk-mq: avoid to synchronize rcu inside blk_cleanup_queue()

[Jens Axboe <axboe@kernel.dk>]

On 6/22/18 4:12 PM, Ming Lei wrote:
> 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@redhat.com>
> Cc: Andrew Jones <drjones@redhat.com>
> Cc: linux-scsi@vger.kernel.org
> Cc: "Martin K. Petersen" <martin.petersen@oracle.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>  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..8a6771ac0adb 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;
>  
>  	mutex_lock(&set->tag_list_lock);
>  	list_del_rcu(&q->tag_set_list);
> @@ -2360,8 +2361,10 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
>  		/* update existing queue */
>  		blk_mq_update_tag_set_depth(set, false);
>  	}
> +	shared = set->flags & BLK_MQ_F_TAG_SHARED;
>  	mutex_unlock(&set->tag_list_lock);
> -	synchronize_rcu();
> +	if (shared)
> +		synchronize_rcu();

Shouldn't this be set if it _was_ shared as well, not just if it's
still shared?

-- 
Jens Axboe