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

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

On Fri, Jun 22, 2018 at 08:58:39AM -0600, Jens Axboe wrote:
> On 6/22/18 5:42 AM, Andrew Jones wrote:
> > 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@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..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.
> 
> That logic still doesn't look correct to me. Does the below work?
> 
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index afd2596ea3d3..222d4fc0e524 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -762,9 +762,13 @@ 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 8e57b84e50e9..18ad2b95ff63 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -2351,8 +2351,12 @@ 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);
> +
> +	shared = !list_is_singular(&set->tag_list);

Or:
	shared = set->flags & BLK_MQ_F_TAG_SHARED;

> +
>  	list_del_rcu(&q->tag_set_list);
>  	if (list_is_singular(&set->tag_list)) {
>  		/* just transitioned to unshared */
> @@ -2361,7 +2365,8 @@ static void blk_mq_del_queue_tag_set(struct request_queue *q)
>  		blk_mq_update_tag_set_depth(set, false);
>  	}
>  	mutex_unlock(&set->tag_list_lock);
> -	synchronize_rcu();
> +	if (shared)
> +		synchronize_rcu();
>  	INIT_LIST_HEAD(&q->tag_set_list);
>  }
>  
> 
> -- 
> Jens Axboe
> 

Will prepare a V2 soon.

Thanks,
Ming