Messages in this thread | | | Date | Mon, 20 Apr 2009 10:24:07 +0200 | From | Jens Axboe <> | Subject | Re: CFQ: Preemption/timeout logic reversed? |
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On Mon, Apr 20 2009, Carl Henrik Lunde wrote: > On Mon, Apr 20, 2009 at 08:16, Jens Axboe <jens.axboe@oracle.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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