linux-kernel - [PATCH 10/14] scsi: only maintain target_blocked if the driver has a target queue limit

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Date:	Wed, 25 Jun 2014 18:51:57 +0200
From:	Christoph Hellwig <hch@....de>
To:	James Bottomley <James.Bottomley@...senPartnership.com>
Cc:	Jens Axboe <axboe@...nel.dk>,
	Bart Van Assche <bvanassche@...ionio.com>,
	Robert Elliott <Elliott@...com>, linux-scsi@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH 10/14] scsi: only maintain target_blocked if the driver has a target queue limit

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@....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 a39d5ba..a64b9d3 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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