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

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

[Ming Lei]

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;
 	}
 	mutex_unlock(&set->tag_list_lock);
-	synchronize_rcu();
+	if (shared)
+		synchronize_rcu();
 	INIT_LIST_HEAD(&q->tag_set_list);
 }
 
-- 
2.9.5

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

[Andrew Jones]

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.

Tested-by: Andrew Jones <drjones@redhat.com>

Thanks,
drew

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

[Jens Axboe]

On 6/19/18 8:55 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.

Looks reasonable to me. But this is something that we've been breaking
multiple times over the years, any chance you could add a blktests
test for it?

-- 
Jens Axboe

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

[Jens Axboe]

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);
+
 	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

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

[Ming Lei]

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

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

[Ming Lei]

On Fri, Jun 22, 2018 at 08:47:35AM -0600, Jens Axboe wrote:
> On 6/19/18 8:55 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.
> 
> Looks reasonable to me. But this is something that we've been breaking
> multiple times over the years, any chance you could add a blktests
> test for it?

Looks a good idea, I guess it can be triggered on scsi_debug too, will cook
a patch later.

thanks,
Ming

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

[Jens Axboe]

On 6/22/18 3:31 PM, Ming Lei wrote:
>> 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;

That'd work too, yeah.

-- 
Jens Axboe