Re: [RFC PATCH 1/4] scsi: Allow drivers to set BLK_MQ_F_BLOCKING

[Bart Van Assche <bvanassche@acm.org>]

On 3/7/22 16:39, Mike Christie wrote:
> The software iscsi driver's queuecommand can block and taking the extra
> hop from kblockd to its workqueue results in a performance hit. Allowing
> it to set BLK_MQ_F_BLOCKING and transmit from that context directly
> results in a 20-30% improvement in IOPs for workloads like: [...]

That's impressive!

> @@ -2952,8 +2954,8 @@ scsi_host_block(struct Scsi_Host *shost)
>   	}
>   
>   	/*
> -	 * SCSI never enables blk-mq's BLK_MQ_F_BLOCKING flag so
> -	 * calling synchronize_rcu() once is enough.
> +	 * Drivers that use this helper enable blk-mq's BLK_MQ_F_BLOCKING flag
> +	 * so calling synchronize_rcu() once is enough.
>   	 */
>   	WARN_ON_ONCE(shost->tag_set.flags & BLK_MQ_F_BLOCKING);

s/enable/do not set/ ?

> diff --git a/include/scsi/scsi_host.h b/include/scsi/scsi_host.h
> index 72e1a347baa6..0d106dc9309d 100644
> --- a/include/scsi/scsi_host.h
> +++ b/include/scsi/scsi_host.h
> @@ -75,6 +75,10 @@ struct scsi_host_template {
>   	 */
>   	int (* queuecommand)(struct Scsi_Host *, struct scsi_cmnd *);
>   
> +	/*
> +	 * Set To true if the queuecommand function can block.
> +	 */
> +	bool queuecommand_blocks;
>   	/*
>   	 * The commit_rqs function is used to trigger a hardware
>   	 * doorbell after some requests have been queued with

I'm not sure what the best name is for this new flag. Some function 
names refer to sleeping (e.g. might_sleep()) while the flag 
BLK_MQ_F_BLOCKING has "blocking" in its name. Although I do not have a 
strong opinion about this: has it been considered to use a name like 
queuecommand_sleeps or queuecommand_may_sleep instead of 
queuecommand_blocks?

Thanks,

Bart.