linux-kernel - Re: [PATCH] sched/deadline: Add sched

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Date:	Mon, 27 Jan 2014 13:30:42 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	Henrik Austad <henrik@...tad.us>, Juri Lelli <juri.lelli@...il.com>
CC:	peterz@...radead.org, tglx@...utronix.de, mingo@...hat.com,
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	darren@...art.com, johan.eker@...csson.com, p.faure@...tech.ch,
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	rob@...dley.net
Subject: Re: [PATCH] sched/deadline: Add sched_dl documentation

Hi Henrik,

On 01/27/2014 12:53 PM, Henrik Austad wrote:
[...]
>> + In more details, the CBS algorithm assigns scheduling deadlines to
>> + tasks in the following way:
>> +
>> +  - Each SCHED_DEADLINE task is characterised by the "runtime",
>> +    "deadline", and "period" parameters;
>> +
>> +  - The state of the task is described by a "scheduling deadline", and
>> +    a "current runtime". These two parameters are initially set to 0;
>> +
>> +  - When a SCHED_DEADLINE task wakes up (becomes ready for execution),
>> +    the scheduler checks if
>> +
>> +                    current runtime                runtime
>> +         ---------------------------------- > ----------------
>> +         scheduling deadline - current time         period
>> +
>> +    then, if the scheduling deadline is smaller than the current time, or
>> +    this condition is verified, the scheduling deadline and the
>> +    current budget are re-initialised as
>
> Current runtime: time spent running _this_ period? or is _remaining_
> runtime this period? I get the feeling it's the latter.
>
> So, roughly, it is the ration
>
>        remaining_runtime / relative_time_to_deadline
>
> which needs to be greater than the assigned CPU bandwidth, and if so, the
> budget should be replensihed?
>
> Shouldn't there be something about not refilling the budget before a new
> period has started?
Uh... Maybe the description above can be improved :)
Do you think that using "remaining runtime" instead of "current runtime"
would help? If yes, I'll make this change.

Also, I see that some of your questions are answered by some items below...
Do you think that changing the order of the items in the presentation would
improve the readability? If you suggest a better ordering, I'll try to
rewrite the algorithm's description according to it.


			Thanks,
				Luca
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