linux-kernel - Re: Question about SCHED_DEADLINE and sched

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Message-ID: <55CB0D9E.6020700@riesch.at>
Date:	Wed, 12 Aug 2015 11:10:54 +0200
From:	Michael Riesch <michael@...sch.at>
To:	Juri Lelli <juri.lelli@....com>
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 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.

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