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

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Message-ID: <20090420082407.GV4593@kernel.dk>
Date:	Mon, 20 Apr 2009 10:24:07 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Carl Henrik Lunde <chlunde@...g.uio.no>
Cc:	linux-kernel@...r.kernel.org
Subject: Re: CFQ: Preemption/timeout logic reversed?

On Mon, Apr 20 2009, Carl Henrik Lunde wrote:
> 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?

Hrmpf, that is buggy, it should be jiffies - slice_end! Presumably that
has been buggy since the shift to rb service tree, since (IIRC) that was
when it was changed from "add to slice length" to "adjust in tree".

-- 
Jens Axboe

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