On Wed, Jun 20, 2018 at 10:55:22AM +0800, 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@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;
I guess this should be '= false'.
> }
> mutex_unlock(&set->tag_list_lock);
> - synchronize_rcu();
> + if (shared)
> + synchronize_rcu();
> INIT_LIST_HEAD(&q->tag_set_list);
> }
>
With the '= false' change I tested this and it resolves the issue for me.
Tested-by: Andrew Jones <drjones@xxxxxxxxxx>
Thanks,
drew
