linux-kernel - [PATCH] cciss: Optimize scan

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Date:	Wed, 22 Apr 2015 11:43:40 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Jens Axboe <axboe@...nel.dk>
Cc:	Mike Miller <mike.miller@...com>, Don Brace <don.brace@...s.com>,
	iss_storagedev@...com, storagedev@...s.com,
	linux-kernel@...r.kernel.org, dave@...olabs.net
Subject: [PATCH] cciss: Optimize scan_thread

Two rather small optimizations found while going through driver
code:

1) Use the cheaper alternative to set_current_state() as we are
sure the task will block right afterward.

2) Checks for list_empty without the scan_mutex. The list_empty
function is very much designed to work without locks, obviously
as long as the head (scan_q) is reliable. In this case if another
thread is doing add_to_scan_list(), we still buckle in the outer
loop, so it will be caught upon the next iteration -- and if
kthread_should_stop() hits, it does not matter _anyway_ as we'd
still need to abort the function regardless of the status of
the scan_q.

Compile tested only.

Signed-off-by: Davidlohr Bueso <dbueso@...e.de>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Mike Miller <mike.miller@...com>
Cc: Don Brace <don.brace@...s.com>
---
 drivers/block/cciss.c | 9 ++-------
 1 file changed, 2 insertions(+), 7 deletions(-)

diff --git a/drivers/block/cciss.c b/drivers/block/cciss.c
index ff20f19..7dd3750 100644
--- a/drivers/block/cciss.c
+++ b/drivers/block/cciss.c
@@ -3727,18 +3727,13 @@ static int scan_thread(void *data)
 	struct ctlr_info *h;

 	while (1) {
-		set_current_state(TASK_INTERRUPTIBLE);
+		__set_current_state(TASK_INTERRUPTIBLE);
 		schedule();
 		if (kthread_should_stop())
 			break;

-		while (1) {
+		while (!list_empty(&scan_q)) {
 			mutex_lock(&scan_mutex);
-			if (list_empty(&scan_q)) {
-				mutex_unlock(&scan_mutex);
-				break;
-			}
-
 			h = list_entry(scan_q.next,
 				       struct ctlr_info,
 				       scan_list);
-- 
2.1.4

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