[PATCH] blk: cache queue_congestion_on/off_threshold values

From: "Chen, Kenneth W" <kenneth.w.chen@intel.com>

It's kind of redundant that queue_congestion_on/off_threshold gets
calculated on every I/O and they produce the same number over and over
again unless q->nr_requests gets changed (which is probably a very rare
event).  We can cache those values in the request_queue structure.

drivers/block/ll_rw_blk.c

@@ -70,14 +70,7 @@ EXPORT_SYMBOL(blk_max_pfn);
  */
 static inline int queue_congestion_on_threshold(struct request_queue *q)
 {
-	int ret;
-
-	ret = q->nr_requests - (q->nr_requests / 8) + 1;
-
-	if (ret > q->nr_requests)
-		ret = q->nr_requests;
-
-	return ret;
+	return q->nr_congestion_on;
 }
 
 /*
@@ -85,14 +78,22 @@ static inline int queue_congestion_on_threshold(struct request_queue *q)
  */
 static inline int queue_congestion_off_threshold(struct request_queue *q)
 {
-	int ret;
+	return q->nr_congestion_off;
+}
 
-	ret = q->nr_requests - (q->nr_requests / 8) - 1;
+static void blk_queue_congestion_threshold(struct request_queue *q)
+{
+	int nr;
 
-	if (ret < 1)
-		ret = 1;
+	nr = q->nr_requests - (q->nr_requests / 8) + 1;
+	if (nr > q->nr_requests)
+		nr = q->nr_requests;
+	q->nr_congestion_on = nr;
 
-	return ret;
+	nr = q->nr_requests - (q->nr_requests / 8) - 1;
+	if (nr < 1)
+		nr = 1;
+	q->nr_congestion_off = nr;
 }
 
 /*
@@ -229,6 +230,7 @@ void blk_queue_make_request(request_queue_t * q, make_request_fn * mfn)
 	blk_queue_max_sectors(q, MAX_SECTORS);
 	blk_queue_hardsect_size(q, 512);
 	blk_queue_dma_alignment(q, 511);
+	blk_queue_congestion_threshold(q);
 
 	q->unplug_thresh = 4;		/* hmm */
 	q->unplug_delay = (3 * HZ) / 1000;	/* 3 milliseconds */
@@ -2946,6 +2948,7 @@ queue_requests_store(struct request_queue *q, const char *page, size_t count)
 	int ret = queue_var_store(&q->nr_requests, page, count);
 	if (q->nr_requests < BLKDEV_MIN_RQ)
 		q->nr_requests = BLKDEV_MIN_RQ;
+	blk_queue_congestion_threshold(q);
 
 	if (rl->count[READ] >= queue_congestion_on_threshold(q))
 		set_queue_congested(q, READ);

include/linux/blkdev.h

@@ -334,6 +334,8 @@ struct request_queue
 	 * queue settings
 	 */
 	unsigned long		nr_requests;	/* Max # of requests */
+	unsigned int		nr_congestion_on;
+	unsigned int		nr_congestion_off;
 
 	unsigned short		max_sectors;
 	unsigned short		max_phys_segments;