Re: CFQ: Preemption/timeout logic reversed?

From: Jens Axboe
Date: Mon Apr 20 2009 - 02:16:19 EST


On Mon, Apr 20 2009, Carl Henrik Lunde wrote:
> Hi!
>
> It seems the preemption "bonus" logic in CFQ is reversed, a preempted
> process is given an additional delay in start time instead of a bonus.
> This seems unfair. I'm not sure if it's a good idea to let

Hmm? ->slice_resid is a long, so if we preempt the process 10 jiffies
before it was supposed to end, the resid will be -10. So it'll not
increase the rb_key, it'll decrease it.

> slice_resid grow without limit as shown below, but isn't this more
> like the way it was intended to work? Or did I misunderstand
> something?

->slice_resid is reset when it gets repositioned in the rb tree. The
intent was not to increase the slice length, but instead allow it sooner
service again.

> PS! The comment above cfq_preempt_queue seems outdated too.

Yep, the slice length comment is out dated indeed.

> Code not tested, just showing what I mean:
>
> diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
> index 664ebfd..ea18d45 100644
> --- a/block/cfq-iosched.c
> +++ b/block/cfq-iosched.c
> @@ -292,7 +292,8 @@ cfq_prio_to_slice(struct cfq_data *cfqd, struct
> cfq_queue *cfqq)
> static inline void
> cfq_set_prio_slice(struct cfq_data *cfqd, struct cfq_queue *cfqq)
> {
> - cfqq->slice_end = cfq_prio_to_slice(cfqd, cfqq) + jiffies;
> + cfqq->slice_end = cfq_prio_to_slice(cfqd, cfqq) +
> cfqq->slice_resid + jiffies;

So if ->slice_resid is negative because we preempted this queue, it'll
now get a shorter slice. That's not very nice :-)

--
Jens Axboe

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