2005-11-10 14:00:51

by Tejun Heo

Subject: [PATCH] cfq-iosched: fix slice_left calculation

When cfq slice expires, remainder of slice is calculated and stored in
cfqq->slice_left. Current code calculates the opposite of remainder -
how many jiffies the cfqq has used past slice end. This patch fixes
the bug.

Signed-off-by: Tejun Heo <[email protected]>

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -861,8 +861,8 @@ __cfq_slice_expired(struct cfq_data *cfq
* store what was left of this slice, if the queue idled out
* or was preempted
*/
- if (time_after(now, cfqq->slice_end))
- cfqq->slice_left = now - cfqq->slice_end;
+ if (time_after(cfqq->slice_end, now))
+ cfqq->slice_left = cfqq->slice_end - now;
else
cfqq->slice_left = 0;

2005-11-10 17:09:50

by Jens Axboe

Subject: Re: [PATCH] cfq-iosched: fix slice_left calculation

On Thu, Nov 10 2005, Tejun Heo wrote:
> When cfq slice expires, remainder of slice is calculated and stored in
> cfqq->slice_left. Current code calculates the opposite of remainder -
> how many jiffies the cfqq has used past slice end. This patch fixes
> the bug.
>
> Signed-off-by: Tejun Heo <[email protected]>
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -861,8 +861,8 @@ __cfq_slice_expired(struct cfq_data *cfq
> * store what was left of this slice, if the queue idled out
> * or was preempted
> */
> - if (time_after(now, cfqq->slice_end))
> - cfqq->slice_left = now - cfqq->slice_end;
> + if (time_after(cfqq->slice_end, now))
> + cfqq->slice_left = cfqq->slice_end - now;
> else
> cfqq->slice_left = 0;

That looks more correct, good spotting. Applied.

--
Jens Axboe