[PATCH] scsi: avoid to run synchronize_rcu for each device in scsi_host_block

[PATCH] scsi: avoid to run synchronize_rcu for each device in scsi_host_block

[Ming Lei]

scsi_host_block() calls scsi_internal_device_block() for each
scsi_device, and scsi_internal_device_block() calls
blk_mq_quiesce_queue() for each LUN. However synchronize_rcu is
run from blk_mq_quiesce_queue().

This way may become unnecessary slow in case of lots of LUNs.

So use scsi_internal_device_block() to implement scsi_host_block(),
and just run once synchronize_rcu() because scsi never supports
tagset of BLK_MQ_F_BLOCKING.

Cc: Bart Van Assche <bvanassche@acm.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dexuan Cui <decui@microsoft.com>
Cc: Hannes Reinecke <hare@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/scsi/scsi_lib.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 47835c4b4ee0..089ac92ac6c3 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -2841,11 +2841,22 @@ scsi_host_block(struct Scsi_Host *shost)
 	struct scsi_device *sdev;
 	int ret = 0;
 
+	/*
+	 * Call scsi_internal_device_block_nowait then we can avoid to
+	 * run synchronize_rcu() one time for every LUN.
+	 */
 	shost_for_each_device(sdev, shost) {
-		ret = scsi_internal_device_block(sdev);
+		mutex_lock(&sdev->state_mutex);
+		ret = scsi_internal_device_block_nowait(sdev);
+		mutex_unlock(&sdev->state_mutex);
 		if (ret)
 			break;
 	}
+
+	WARN_ON_ONCE(shost->tag_set.flags & BLK_MQ_F_BLOCKING);
+
+	if (!ret)
+		synchronize_rcu();
 	return ret;
 }
 EXPORT_SYMBOL_GPL(scsi_host_block);
-- 
2.25.2

Re: [PATCH] scsi: avoid to run synchronize_rcu for each device in scsi_host_block

[Bart Van Assche]

On 4/22/20 3:14 AM, Ming Lei wrote:
> So use scsi_internal_device_block() to implement scsi_host_block(),
> and just run once synchronize_rcu() because scsi never supports
> tagset of BLK_MQ_F_BLOCKING.

Please add a reference to BLK_MQ_F_BLOCKING as a comment in the modified 
code.

Thanks,

Bart.

Re: [PATCH] scsi: avoid to run synchronize_rcu for each device in scsi_host_block

[Steffen Maier]

On 4/22/20 12:14 PM, Ming Lei wrote:
> scsi_host_block() calls scsi_internal_device_block() for each
> scsi_device, and scsi_internal_device_block() calls
> blk_mq_quiesce_queue() for each LUN. However synchronize_rcu is
> run from blk_mq_quiesce_queue().
> 
> This way may become unnecessary slow in case of lots of LUNs.
> 
> So use scsi_internal_device_block() to implement scsi_host_block(),

scsi_internal_device_block()
=?=>
scsi_internal_device_block_nowait()

> and just run once synchronize_rcu() because scsi never supports
> tagset of BLK_MQ_F_BLOCKING.
> 
> Cc: Bart Van Assche <bvanassche@acm.org>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Dexuan Cui <decui@microsoft.com>
> Cc: Hannes Reinecke <hare@suse.de>
> Signed-off-by: Ming Lei <ming.lei@redhat.com>
> ---
>   drivers/scsi/scsi_lib.c | 13 ++++++++++++-
>   1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 47835c4b4ee0..089ac92ac6c3 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -2841,11 +2841,22 @@ scsi_host_block(struct Scsi_Host *shost)
>   	struct scsi_device *sdev;
>   	int ret = 0;
>   
> +	/*
> +	 * Call scsi_internal_device_block_nowait then we can avoid to
> +	 * run synchronize_rcu() one time for every LUN.
> +	 */
>   	shost_for_each_device(sdev, shost) {
> -		ret = scsi_internal_device_block(sdev);
> +		mutex_lock(&sdev->state_mutex);
> +		ret = scsi_internal_device_block_nowait(sdev);
> +		mutex_unlock(&sdev->state_mutex);
>   		if (ret)
>   			break;
>   	}
> +
> +	WARN_ON_ONCE(shost->tag_set.flags & BLK_MQ_F_BLOCKING);
> +
> +	if (!ret)
> +		synchronize_rcu();
>   	return ret;
>   }
>   EXPORT_SYMBOL_GPL(scsi_host_block);
> 


-- 
Mit freundlichen Gruessen / Kind regards
Steffen Maier

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