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SubjectRe: [PATCH 02/20] blkio: Change CFQ to use CFS like queue time stamps
Vivek Goyal wrote:
...
>
> @@ -1245,10 +1429,10 @@ static int cfq_forced_dispatch(struct cfq_data *cfqd)
> struct cfq_queue *cfqq;
> int dispatched = 0;
>
> - while ((cfqq = cfq_rb_first(&cfqd->service_tree)) != NULL)
> + while ((cfqq = cfq_get_next_queue(cfqd)) != NULL)
> dispatched += __cfq_forced_dispatch_cfqq(cfqq);
>
> - cfq_slice_expired(cfqd, 0);
> + cfq_slice_expired(cfqd);
>
> BUG_ON(cfqd->busy_queues);
>
> @@ -1391,7 +1575,7 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
> cfqq->slice_dispatch >= cfq_prio_to_maxrq(cfqd, cfqq)) ||
> cfq_class_idle(cfqq))) {
> cfqq->slice_end = jiffies + 1;
> - cfq_slice_expired(cfqd, 0);
> + cfq_slice_expired(cfqd);

Hi Vivek,

I think here you should make sure that when updating cfqq->slice_end, cfqq->slice_end doesn't
equal to 0. Because if cfqq->slice_end == 0, cfq_slice_expired() just charge for 1 jiffy, but
if cfqq->slice_end is updated when it equals to 0(first request still in the air), at that time
cfqq->slice_start == 0, and slice_used is charged as "jiffies - cfqq->slice_start". Following
patch fixes this bug.

Signed-off-by: Gui Jianfeng <guijianfeng@cn.fujitsu.com>
---
block/cfq-iosched.c | 3 ++-
1 files changed, 2 insertions(+), 1 deletions(-)
diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index f23d713..12afc14 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1999,7 +1999,8 @@ static int cfq_dispatch_requests(struct request_queue *q, int force)
if (cfqd->busy_queues > 1 && ((!cfq_cfqq_sync(cfqq) &&
cfqq->slice_dispatch >= cfq_prio_to_maxrq(cfqd, cfqq)) ||
cfq_class_idle(cfqq))) {
- cfqq->slice_end = jiffies + 1;
+ if (cfqq->slice_end)
+ cfqq->slice_end = jiffies + 1;
cfq_slice_expired(cfqd);
}

--
1.5.4.rc3


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