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

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Date:	Mon, 23 Apr 2012 13:55:13 +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:
> +/*
> + * Here we check if --at time t-- an entity (which is probably being
> + * [re]activated or, in general, enqueued) can use its remaining runtime
> + * and its current deadline _without_ exceeding the bandwidth it is
> + * assigned (function returns true if it can).
> + *
> + * For this to hold, we must check if:
> + *   runtime / (deadline - t) < dl_runtime / dl_deadline .

It might be good to put a few words in as to why that is.. I know I
always forget (but know where to find it by now), also might be good to
refer those papers Tommaso listed when Steven asked this a while back.

> + */
> +static bool dl_entity_overflow(struct sched_dl_entity *dl_se, u64 t)
> +{
> +       u64 left, right;
> +
> +       /*
> +        * left and right are the two sides of the equation above,
> +        * after a bit of shuffling to use multiplications instead
> +        * of divisions.
> +        *
> +        * Note that none of the time values involved in the two
> +        * multiplications are absolute: dl_deadline and dl_runtime
> +        * are the relative deadline and the maximum runtime of each
> +        * instance, runtime is the runtime left for the last instance
> +        * and (deadline - t), since t is rq->clock, is the time left
> +        * to the (absolute) deadline. Therefore, overflowing the u64
> +        * type is very unlikely to occur in both cases.
> +        */
> +       left = dl_se->dl_deadline * dl_se->runtime;
> +       right = (dl_se->deadline - t) * dl_se->dl_runtime;


>From what I can see there are no constraints on the values in
__setparam_dl() so the above left term can be constructed to be an
overflow.

Ideally we'd use u128 here, but I don't think people will let us :/

> +       return dl_time_before(right, left);
> +} 
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