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SubjectRe: [PATCH 02/20] blkio: Change CFQ to use CFS like queue time stamps
On Wed, Nov 11, 2009 at 08:48:09AM +0800, Gui Jianfeng wrote:
> 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.
>

Hi Gui,

This can happen only once during a one wrap around cycle of jiffies. That
too depends in case we are hitting jiffies+1 as 0 or not.

So I would not worry much about it right now.

In fact, not updating slice_end, will make idle or async queue slice last
much longer than it should have.

Thanks
Vivek


> 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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