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SubjectRe: [PATCH 06/14] scsi: convert target_busy to an atomic_t
On 06/25/2014 06:51 PM, Christoph Hellwig wrote:
> Avoid taking the host-wide host_lock to check the per-target queue limit.
> Instead we do an atomic_inc_return early on to grab our slot in the queue,
> and if nessecary decrement it after finishing all checks.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> drivers/scsi/scsi_lib.c | 52 ++++++++++++++++++++++++++------------------
> include/scsi/scsi_device.h | 4 ++--
> 2 files changed, 33 insertions(+), 23 deletions(-)
>
> diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> index 18e6449..5e269d6 100644
> --- a/drivers/scsi/scsi_lib.c
> +++ b/drivers/scsi/scsi_lib.c
> @@ -294,7 +294,7 @@ void scsi_device_unbusy(struct scsi_device *sdev)
>
> spin_lock_irqsave(shost->host_lock, flags);
> shost->host_busy--;
> - starget->target_busy--;
> + atomic_dec(&starget->target_busy);
> if (unlikely(scsi_host_in_recovery(shost) &&
> (shost->host_failed || shost->host_eh_scheduled)))
> scsi_eh_wakeup(shost);
> @@ -361,7 +361,7 @@ static inline int scsi_device_is_busy(struct scsi_device *sdev)
> static inline int scsi_target_is_busy(struct scsi_target *starget)
> {
> return ((starget->can_queue > 0 &&
> - starget->target_busy >= starget->can_queue) ||
> + atomic_read(&starget->target_busy) >= starget->can_queue) ||
> starget->target_blocked);
> }
>
> @@ -1305,37 +1305,49 @@ 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;
> + unsigned int busy;
>
> - spin_lock_irq(shost->host_lock);
> if (starget->single_lun) {
> + spin_lock_irq(shost->host_lock);
> if (starget->starget_sdev_user &&
> - starget->starget_sdev_user != sdev)
> - goto out;
> + starget->starget_sdev_user != sdev) {
> + spin_unlock_irq(shost->host_lock);
> + return 0;
> + }
> starget->starget_sdev_user = sdev;
> + spin_unlock_irq(shost->host_lock);
> }
>
> - if (starget->target_busy == 0 && starget->target_blocked) {
> + busy = atomic_inc_return(&starget->target_busy) - 1;
> + if (busy == 0 && starget->target_blocked) {
> /*
> * unblock after target_blocked iterates to zero
> */
> - if (--starget->target_blocked != 0)
> - goto out;
> + spin_lock_irq(shost->host_lock);
> + if (--starget->target_blocked != 0) {
> + spin_unlock_irq(shost->host_lock);
> + goto out_dec;
> + }
> + spin_unlock_irq(shost->host_lock);
>
> 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);
> - goto out;
> - }
> + if (starget->can_queue > 0 && busy >= starget->can_queue)
> + goto starved;
> + if (starget->target_blocked)
> + goto starved;
>
> - scsi_target(sdev)->target_busy++;
> - ret = 1;
> -out:
> + return 1;
> +
> +starved:
> + spin_lock_irq(shost->host_lock);
> + list_move_tail(&sdev->starved_entry, &shost->starved_list);
> spin_unlock_irq(shost->host_lock);
> - return ret;
> +out_dec:
> + atomic_dec(&starget->target_busy);
> + return 0;
> }
>
> /*
> @@ -1445,7 +1457,7 @@ static void scsi_kill_request(struct request *req, struct request_queue *q)
> spin_unlock(sdev->request_queue->queue_lock);
> spin_lock(shost->host_lock);
> shost->host_busy++;
> - starget->target_busy++;
> + atomic_inc(&starget->target_busy);
> spin_unlock(shost->host_lock);
> spin_lock(sdev->request_queue->queue_lock);
>
> @@ -1615,9 +1627,7 @@ static void scsi_request_fn(struct request_queue *q)
> return;
>
> host_not_ready:
> - spin_lock_irq(shost->host_lock);
> - scsi_target(sdev)->target_busy--;
> - spin_unlock_irq(shost->host_lock);
> + atomic_dec(&scsi_target(sdev)->target_busy);
> not_ready:
> /*
> * lock q, handle tag, requeue req, and decrement device_busy. We
> diff --git a/include/scsi/scsi_device.h b/include/scsi/scsi_device.h
> index 816e8a2..446f741 100644
> --- a/include/scsi/scsi_device.h
> +++ b/include/scsi/scsi_device.h
> @@ -290,8 +290,8 @@ struct scsi_target {
> unsigned int expecting_lun_change:1; /* A device has reported
> * a 3F/0E UA, other devices on
> * the same target will also. */
> - /* commands actually active on LLD. protected by host lock. */
> - unsigned int target_busy;
> + /* commands actually active on LLD. */
> + atomic_t target_busy;
> /*
> * LLDs should set this in the slave_alloc host template callout.
> * If set to zero then there is not limit.
>
Reviewed-by: Hannes Reinecke <hare@suse.de>

Cheers,

Hannes
--
Dr. Hannes Reinecke zSeries & Storage
hare@suse.de +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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