Re: [PATCH 05/14] scsi: push host_lock down into scsi_{host,target}_queue_ready

[Hannes Reinecke]

On 06/25/2014 06:51 PM, Christoph Hellwig wrote:
> Prepare for not taking a host-wide lock in the dispatch path by pushing
> the lock down into the places that actually need it.  Note that this
> patch is just a preparation step, as it will actually increase lock
> roundtrips and thus decrease performance on its own.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
>   drivers/scsi/scsi_lib.c |   75 ++++++++++++++++++++++++-----------------------
>   1 file changed, 39 insertions(+), 36 deletions(-)
>
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 6989b6f..18e6449 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -1300,18 +1300,18 @@ static inline int scsi_dev_queue_ready(struct request_queue *q,
>   /*
>    * scsi_target_queue_ready: checks if there we can send commands to target
>    * @sdev: scsi device on starget to check.
> - *
> - * Called with the host lock held.
>    */
>   static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
>   					   struct scsi_device *sdev)
>   {
>   	struct scsi_target *starget = scsi_target(sdev);
> +	int ret = 0;
>
> +	spin_lock_irq(shost->host_lock);
>   	if (starget->single_lun) {
>   		if (starget->starget_sdev_user &&
>   		    starget->starget_sdev_user != sdev)
> -			return 0;
> +			goto out;
>   		starget->starget_sdev_user = sdev;
>   	}
>
> @@ -1319,57 +1319,66 @@ static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
>   		/*
>   		 * unblock after target_blocked iterates to zero
>   		 */
> -		if (--starget->target_blocked == 0) {
> -			SCSI_LOG_MLQUEUE(3, starget_printk(KERN_INFO, starget,
> -					 "unblocking target at zero depth\n"));
> -		} else
> -			return 0;
> +		if (--starget->target_blocked != 0)
> +			goto out;
> +
> +		SCSI_LOG_MLQUEUE(3, starget_printk(KERN_INFO, starget,
> +				 "unblocking target at zero depth\n"));
>   	}
>
>   	if (scsi_target_is_busy(starget)) {
>   		list_move_tail(&sdev->starved_entry, &shost->starved_list);
> -		return 0;
> +		goto out;
>   	}
>
> -	return 1;
> +	scsi_target(sdev)->target_busy++;
> +	ret = 1;
> +out:
> +	spin_unlock_irq(shost->host_lock);
> +	return ret;
>   }
>
>   /*
>    * scsi_host_queue_ready: if we can send requests to shost, return 1 else
>    * return 0. We must end up running the queue again whenever 0 is
>    * returned, else IO can hang.
> - *
> - * Called with host_lock held.
>    */
>   static inline int scsi_host_queue_ready(struct request_queue *q,
>   				   struct Scsi_Host *shost,
>   				   struct scsi_device *sdev)
>   {
> +	int ret = 0;
> +
> +	spin_lock_irq(shost->host_lock);
> +
>   	if (scsi_host_in_recovery(shost))
> -		return 0;
> +		goto out;
>   	if (shost->host_busy == 0 && shost->host_blocked) {
>   		/*
>   		 * unblock after host_blocked iterates to zero
>   		 */
> -		if (--shost->host_blocked == 0) {
> -			SCSI_LOG_MLQUEUE(3,
> -				shost_printk(KERN_INFO, shost,
> -					     "unblocking host at zero depth\n"));
> -		} else {
> -			return 0;
> -		}
> +		if (--shost->host_blocked != 0)
> +			goto out;
> +
> +		SCSI_LOG_MLQUEUE(3,
> +			shost_printk(KERN_INFO, shost,
> +				     "unblocking host at zero depth\n"));
>   	}
>   	if (scsi_host_is_busy(shost)) {
>   		if (list_empty(&sdev->starved_entry))
>   			list_add_tail(&sdev->starved_entry, &shost->starved_list);
> -		return 0;
> +		goto out;
>   	}
>
>   	/* We're OK to process the command, so we can't be starved */
>   	if (!list_empty(&sdev->starved_entry))
>   		list_del_init(&sdev->starved_entry);
>
> -	return 1;
> +	shost->host_busy++;
> +	ret = 1;
> +out:
> +	spin_unlock_irq(shost->host_lock);
> +	return ret;
>   }
>
>   /*
> @@ -1550,7 +1559,7 @@ static void scsi_request_fn(struct request_queue *q)
>   			blk_start_request(req);
>   		sdev->device_busy++;
>
> -		spin_unlock(q->queue_lock);
> +		spin_unlock_irq(q->queue_lock);
>   		cmd = req->special;
>   		if (unlikely(cmd == NULL)) {
>   			printk(KERN_CRIT "impossible request in %s.\n"
> @@ -1560,7 +1569,6 @@ static void scsi_request_fn(struct request_queue *q)
>   			blk_dump_rq_flags(req, "foo");
>   			BUG();
>   		}
> -		spin_lock(shost->host_lock);
>
>   		/*
>   		 * We hit this when the driver is using a host wide
> @@ -1571,9 +1579,11 @@ static void scsi_request_fn(struct request_queue *q)
>   		 * a run when a tag is freed.
>   		 */
>   		if (blk_queue_tagged(q) && !blk_rq_tagged(req)) {
> +			spin_lock_irq(shost->host_lock);
>   			if (list_empty(&sdev->starved_entry))
>   				list_add_tail(&sdev->starved_entry,
>   					      &shost->starved_list);
> +			spin_unlock_irq(shost->host_lock);
>   			goto not_ready;
>   		}
>
> @@ -1581,16 +1591,7 @@ static void scsi_request_fn(struct request_queue *q)
>   			goto not_ready;
>
>   		if (!scsi_host_queue_ready(q, shost, sdev))
> -			goto not_ready;
> -
> -		scsi_target(sdev)->target_busy++;
> -		shost->host_busy++;
> -
> -		/*
> -		 * XXX(hch): This is rather suboptimal, scsi_dispatch_cmd will
> -		 *		take the lock again.
> -		 */
> -		spin_unlock_irq(shost->host_lock);
> +			goto host_not_ready;
>
>   		/*
>   		 * Finally, initialize any error handling parameters, and set up
> @@ -1613,9 +1614,11 @@ static void scsi_request_fn(struct request_queue *q)
>
>   	return;
>
> - not_ready:
> + host_not_ready:
> +	spin_lock_irq(shost->host_lock);
> +	scsi_target(sdev)->target_busy--;
>   	spin_unlock_irq(shost->host_lock);
> -
> + not_ready:
>   	/*
>   	 * lock q, handle tag, requeue req, and decrement device_busy. We
>   	 * must return with queue_lock held.
Reviewed-by: Hannes Reinecke <hare@xxxxxxx>

Cheers,

Hannes
--
Dr. Hannes Reinecke		      zSeries & Storage
hare@xxxxxxx			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)
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