[PATCH linux-2.6.12-rc6-mm1] blk: cfq_find_next_crq fix

[PATCH linux-2.6.12-rc6-mm1] blk: cfq_find_next_crq fix

[Tejun Heo]

 Hello, Jens.
 Hello, Andrew.

 In cfq_find_next_crq(), cfq tries to find the next request by
choosing one of two requests before and after the current one.
Currently, when choosing the next request, if there's no next request,
the next candidate is NULL, resulting in selection of the previous
request.  This results in weird scheduling.  Once we reach the end, we
always seek backward.

 The correct behavior is using the first request as the next
candidate.  cfq_choose_req() already has logics for handling wrapped
requests.

 Signed-off-by: Tejun Heo <htejun@gmail.com>

Index: blk-fixes/drivers/block/cfq-iosched.c
===================================================================
--- blk-fixes.orig/drivers/block/cfq-iosched.c	2005-06-15 22:44:55.000000000 +0900
+++ blk-fixes/drivers/block/cfq-iosched.c	2005-06-15 22:45:21.000000000 +0900
@@ -375,9 +375,10 @@ cfq_find_next_crq(struct cfq_data *cfqd,
 	struct cfq_rq *crq_next = NULL, *crq_prev = NULL;
 	struct rb_node *rbnext, *rbprev;
 
+	rbnext = NULL;
 	if (ON_RB(&last->rb_node))
 		rbnext = rb_next(&last->rb_node);
-	else {
+	if (!rbnext) {
 		rbnext = rb_first(&cfqq->sort_list);
 		if (rbnext == &last->rb_node)
 			rbnext = NULL;

Re: [PATCH linux-2.6.12-rc6-mm1] blk: cfq_find_next_crq fix

[Jens Axboe]

On Thu, Jun 16 2005, Tejun Heo wrote:
>  Hello, Jens.
>  Hello, Andrew.
> 
>  In cfq_find_next_crq(), cfq tries to find the next request by
> choosing one of two requests before and after the current one.
> Currently, when choosing the next request, if there's no next request,
> the next candidate is NULL, resulting in selection of the previous
> request.  This results in weird scheduling.  Once we reach the end, we
> always seek backward.
> 
>  The correct behavior is using the first request as the next
> candidate.  cfq_choose_req() already has logics for handling wrapped
> requests.

Good spotting, applied. I'll set up an iosched git tree as well, with a
cfq-ts branch.

-- 
Jens Axboe