CFQ v2 high cpu load fix(?)

[Ronny V. Vindenes]

CFQ v2 is much better in a lot of cases, but certain situations trigger
a cpu load so high it starves the rest of the system thus completely
ruining the interactive experience. While casually looking at the
problem, I stumbled upon a patch by Arjan van de Ven sent to lkml on
sept. 1 (Subject: block fixes). Part of it is already included in the
CFQ v2 patches and after applying the rest[1] I'm no longer able to
trigger the problem.

[1] Patch attached against 2.6.9-rc4-ck1 but applies to rc4-mm1 with
some minor fuzz.

-- 
Ronny V. Vindenes <s864@xxxxxxxxx>
--- patches/linux-2.6.9-rc4-ck1/drivers/block/ll_rw_blk.c	2004-10-12 12:25:09.798003278 +0200
+++ linux-2.6.9-rc4-ck1/drivers/block/ll_rw_blk.c	2004-10-12 12:25:42.959479479 +0200
@@ -100,7 +100,7 @@
 		nr = q->nr_requests;
 	q->nr_congestion_on = nr;
 
-	nr = q->nr_requests - (q->nr_requests / 8) - 1;
+	nr = q->nr_requests - (q->nr_requests / 8) - (q->nr_requests/16)- 1;
 	if (nr < 1)
 		nr = 1;
 	q->nr_congestion_off = nr;
@@ -1758,8 +1758,10 @@
 {
 	DEFINE_WAIT(wait);
 	struct request *rq;
+	struct io_context *ioc;
 
 	generic_unplug_device(q);
+	ioc = get_io_context(GFP_NOIO);
 	do {
 		struct request_list *rl = &q->rq;
 
@@ -1769,7 +1771,6 @@
 		rq = get_request(q, rw, GFP_NOIO);
 
 		if (!rq) {
-			struct io_context *ioc;
 
 			io_schedule();
 
@@ -1779,12 +1780,11 @@
 			 * up to a big batch of them for a small period time.
 			 * See ioc_batching, ioc_set_batching
 			 */
-			ioc = get_io_context(GFP_NOIO);
 			ioc_set_batching(q, ioc);
-			put_io_context(ioc);
 		}
 		finish_wait(&rl->wait[rw], &wait);
 	} while (!rq);
+	put_io_context(ioc);
 
 	return rq;
 }