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

[Steffen Maier <maier@linux.ibm.com>]

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

Linux on IBM Z Development

https://www.ibm.com/privacy/us/en/
IBM Deutschland Research & Development GmbH
Vorsitzender des Aufsichtsrats: Matthias Hartmann
Geschaeftsfuehrung: Dirk Wittkopp
Sitz der Gesellschaft: Boeblingen
Registergericht: Amtsgericht Stuttgart, HRB 243294