linux-kernel - Re: CFQ: Preemption/timeout logic reversed?

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Date:	Mon, 20 Apr 2009 08:48:14 +0200
From:	Carl Henrik Lunde <chlunde@...g.uio.no>
To:	Jens Axboe <jens.axboe@...cle.com>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: CFQ: Preemption/timeout logic reversed?

On Mon, Apr 20, 2009 at 08:16, Jens Axboe <jens.axboe@...cle.com> wrote:
> 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.

OK, so maybe I'm tired (I am!), but I don't get it. :)

{
        if (... busy_rt_queues)
               cfq_slice_expire(timed_out=1)
}

cfq_slice_expire() {
{
        if (timed_out)
                slice_resid = slice_end - jiffies;
}

if preempted it would be 100 - 90 = +10?

>
>> 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.

Fair enough

>> 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 The comment above cfq_preempt_queue seems outdated too.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
>
>



-- 
mvh
Carl Henrik
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