linux-kernel - Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

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Date:	Mon, 23 Apr 2012 16:11:48 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Juri Lelli <juri.lelli@...il.com>
Cc:	tglx@...utronix.de, mingo@...hat.com, rostedt@...dmis.org,
	cfriesen@...tel.com, oleg@...hat.com, fweisbec@...il.com,
	darren@...art.com, johan.eker@...csson.com, p.faure@...tech.ch,
	linux-kernel@...r.kernel.org, claudio@...dence.eu.com,
	michael@...rulasolutions.com, fchecconi@...il.com,
	tommaso.cucinotta@...up.it, nicola.manica@...i.unitn.it,
	luca.abeni@...tn.it, dhaval.giani@...il.com, hgu1972@...il.com,
	paulmck@...ux.vnet.ibm.com, raistlin@...ux.it,
	insop.song@...csson.com, liming.wang@...driver.com
Subject: Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation.

On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote:
> +static int start_dl_timer(struct sched_dl_entity *dl_se)
> +{
> +       struct dl_rq *dl_rq = dl_rq_of_se(dl_se);
> +       struct rq *rq = rq_of_dl_rq(dl_rq);
> +       ktime_t now, act;
> +       ktime_t soft, hard;
> +       unsigned long range;
> +       s64 delta;
> +
> +       /*
> +        * We want the timer to fire at the deadline, but considering
> +        * that it is actually coming from rq->clock and not from
> +        * hrtimer's time base reading.
> +        */
> +       act = ns_to_ktime(dl_se->deadline);
> +       now = hrtimer_cb_get_time(&dl_se->dl_timer);
> +       delta = ktime_to_ns(now) - rq->clock;
> +       act = ktime_add_ns(act, delta);


Right, this all is very sad.. but I guess we'll have to like live with
it. The only other option is adding another timer base that tries to
keep itself in sync with rq->clock but that all sounds very painful
indeed.

Keeping up with rq->clock_task would be even more painful since it slows
the clock down in random fashion making the timer fire early.
Compensating that is going to be both fun and expensive.

> +       /*
> +        * If the expiry time already passed, e.g., because the value
> +        * chosen as the deadline is too small, don't even try to
> +        * start the timer in the past!
> +        */
> +       if (ktime_us_delta(act, now) < 0)
> +               return 0;
> +
> +       hrtimer_set_expires(&dl_se->dl_timer, act);
> +
> +       soft = hrtimer_get_softexpires(&dl_se->dl_timer);
> +       hard = hrtimer_get_expires(&dl_se->dl_timer);
> +       range = ktime_to_ns(ktime_sub(hard, soft));
> +       __hrtimer_start_range_ns(&dl_se->dl_timer, soft,
> +                                range, HRTIMER_MODE_ABS, 0);
> +
> +       return hrtimer_active(&dl_se->dl_timer);
> +} 

/me reminds himself to make __hrtimer_start_range_ns() return -ETIME.
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