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

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Message-ID: <4F956A95.5030304@gmail.com>
Date:	Mon, 23 Apr 2012 16:43:33 +0200
From:	Juri Lelli <juri.lelli@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
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 04/23/2012 01:55 PM, Peter Zijlstra wrote:
> 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.
>

Ok, I'll fix the comment, extend it and add T.'s references in the
Documentation.

>> + */
>> +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.
>

Yes, could happen :-\.

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

Do we need to do something about that? If we cannot go for bigger space
probably limit dl_deadline (or warn the user)..

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