linux-kernel - Re: [PATCH 08/16] sched: add period support for -deadline tasks.

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Message-ID: <4F85FE07.2060605@gmail.com>
Date:	Wed, 11 Apr 2012 23:56:23 +0200
From:	Juri Lelli <juri.lelli@...il.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com,
	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 08/16] sched: add period support for -deadline tasks.

On 04/11/2012 10:32 PM, Steven Rostedt wrote:
> On Fri, 2012-04-06 at 09:14 +0200, Juri Lelli wrote:
>
>> @@ -293,7 +293,11 @@ static void replenish_dl_entity(struct sched_dl_entity *dl_se)
>>    * assigned (function returns true if it can).
>>    *
>>    * For this to hold, we must check if:
>> - *   runtime / (deadline - t)<  dl_runtime / dl_deadline .
>> + *   runtime / (deadline - t)<  dl_runtime / dl_period .
>> + *
>> + * Notice that the bandwidth check is done against the period. For
>> + * task with deadline equal to period this is the same of using
>> + * dl_deadline instead of dl_period in the equation above.
>
> First, it seems that the function returns true if:
>
> 	dl_runtime / dl_period<  runtime / (deadline - t)
>

Right, the comment is wrong! Just reverse the inequality as you did.
  
>
> I'm a little confused by this. We are comparing the ratio of runtime
> left and deadline left, to the ratio of total runtime to period.
>
> I'm actually confused by this premise anyway. What's the purpose of
> comparing the ratio? If runtime<  (deadline - t) wouldn't it not be able
> to complete anyway? Or are we thinking that the runtime will be
> interrupted proportionally by other tasks?
>

We are actually applying one of the CBS rules. We want to be able to
"slow down" a deadline task if it is going to exceed its reserved
bandwidth. We are in fact checking here that the bandwidth this task
will consume from t to its deadline is no more than its reserved one.

>
>
>>    */
>>   static bool dl_entity_overflow(struct sched_dl_entity *dl_se, u64 t)
>>   {
>> @@ -312,7 +316,7 @@ static bool dl_entity_overflow(struct sched_dl_entity *dl_se, u64 t)
>>   	 * 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;
>> +	left = dl_se->dl_period * dl_se->runtime;
>>   	right = (dl_se->deadline - t) * dl_se->dl_runtime;
>>
>>   	return dl_time_before(right, left);
>
>

Thanks,

- Juri
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