linux-kernel - Re: Question about SCHED_DEADLINE and sched

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Message-ID: <55CB2575.4060201@arm.com>
Date:	Wed, 12 Aug 2015 11:52:37 +0100
From:	Juri Lelli <juri.lelli@....com>
To:	Michael Riesch <michael@...sch.at>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"juri.lelli@...il.com" <juri.lelli@...il.com>
Subject: Re: Question about SCHED_DEADLINE and sched_yield() usage

Hi,

On 12/08/15 10:10, Michael Riesch wrote:
> Hi Juri,
> 
> On 08/11/2015 01:55 PM, Juri Lelli wrote:
>> As you are running a 3.14 kernel, you probably missed this fix
>> 5bfd126e80dc "sched/deadline: Fix sched_yield() behavior". Can
>> you please check?
> 
> I stumbled over this commit but somehow managed to ignore it. Anyway, I
> upgraded to 4.1, now the application shows the expected behavior.
> 
>>> As far as I understand, I have to call sched_yield() if the the
>>> execution time of one loop iteration is either not constant or unknown
>>> (both cases being very likely), because if I do not, a new loop
>>> iteration could be started if the time budget is not empty.
>>>
>>
>> It depends. The sched_yield() semantic for SCHED_DEADLINE might
>> be used to implement some sort of reclaiming mechanism (not
>> there yet) where you inform the scheduler that you are not going
>> to use the remaining runtime in this period; and the scheduler
>> could recycle this spare runtime for other tasks that are running
>> short of it.
>>
>> However, I'd say that in your case you can also live without it.
>> SCHED_DEADLINE can handle sporadic tasks, it depends on how you
>> implement your userspace loop I guess. If you just check the active
>> flag, and this flag is always set, you are right that you may
>> end up executing back to back, though; in which case it seems that yield
>> semantic could do the trick.
> 
> Since samples are generated and the resulting curve looks like it was
> sampled with a constant frequency, I think that sched_yield() is to be
> used in this context. Before I used sched_yield(), I had to use some
> sleep statements, which made the sample frequency not deterministic and
> filled the CPU up. Now it seems to work pretty well.
> 

Great!

Is there any way you can share your application sources? Maybe
it is already on Github or something? I'd be really curious to
have a look at it :).

> Congrats on the deadline scheduler - it is a great way to introduce some
> real-time capability - and thank you for your help.

Sure, no problem. Thanks a lot for asking your question on the
list!

Best,

- Juri

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