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Subject[PATCH 10/14] scsi: only maintain target_blocked if the driver has a target queue limit
Date
This saves us an atomic operation for each I/O submission and completion
for the usual case where the driver doesn't set a per-target can_queue
value. Only a few iscsi hardware offload drivers set the per-target
can_queue value at the moment.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
drivers/scsi/scsi_lib.c | 17 ++++++++++++-----
1 file changed, 12 insertions(+), 5 deletions(-)

diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
index 0e33dee..763b3c9 100644
--- a/drivers/scsi/scsi_lib.c
+++ b/drivers/scsi/scsi_lib.c
@@ -295,7 +295,8 @@ void scsi_device_unbusy(struct scsi_device *sdev)
unsigned long flags;

atomic_dec(&shost->host_busy);
- atomic_dec(&starget->target_busy);
+ if (starget->can_queue > 0)
+ atomic_dec(&starget->target_busy);

if (unlikely(scsi_host_in_recovery(shost) &&
(shost->host_failed || shost->host_eh_scheduled))) {
@@ -1335,6 +1336,9 @@ static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
spin_unlock_irq(shost->host_lock);
}

+ if (starget->can_queue <= 0)
+ return 1;
+
busy = atomic_inc_return(&starget->target_busy) - 1;
if (busy == 0 && atomic_read(&starget->target_blocked) > 0) {
if (atomic_dec_return(&starget->target_blocked) > 0)
@@ -1344,7 +1348,7 @@ static inline int scsi_target_queue_ready(struct Scsi_Host *shost,
"unblocking target at zero depth\n"));
}

- if (starget->can_queue > 0 && busy >= starget->can_queue)
+ if (busy >= starget->can_queue)
goto starved;
if (atomic_read(&starget->target_blocked) > 0)
goto starved;
@@ -1356,7 +1360,8 @@ starved:
list_move_tail(&sdev->starved_entry, &shost->starved_list);
spin_unlock_irq(shost->host_lock);
out_dec:
- atomic_dec(&starget->target_busy);
+ if (starget->can_queue > 0)
+ atomic_dec(&starget->target_busy);
return 0;
}

@@ -1473,7 +1478,8 @@ static void scsi_kill_request(struct request *req, struct request_queue *q)
*/
atomic_inc(&sdev->device_busy);
atomic_inc(&shost->host_busy);
- atomic_inc(&starget->target_busy);
+ if (starget->can_queue > 0)
+ atomic_inc(&starget->target_busy);

blk_complete_request(req);
}
@@ -1642,7 +1648,8 @@ static void scsi_request_fn(struct request_queue *q)
return;

host_not_ready:
- atomic_dec(&scsi_target(sdev)->target_busy);
+ if (&scsi_target(sdev)->can_queue > 0)
+ atomic_dec(&scsi_target(sdev)->target_busy);
not_ready:
/*
* lock q, handle tag, requeue req, and decrement device_busy. We
--
1.7.10.4


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