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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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