linux-kernel - Re: [PATCH 1/3] Added runqueue clock normalized with cpufreq

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Date:	Fri, 17 Dec 2010 20:27:51 +0100
From:	Harald Gustafsson <hgu1972@...il.com>
To:	Dario Faggioli <raistlin@...ux.it>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Harald Gustafsson <harald.gustafsson@...csson.com>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Claudio Scordino <claudio@...dence.eu.com>,
	Michael Trimarchi <trimarchi@...is.sssup.it>,
	Fabio Checconi <fabio@...dalf.sssup.it>,
	Tommaso Cucinotta <cucinotta@...up.it>,
	Juri Lelli <juri.lelli@...il.com>
Subject: Re: [PATCH 1/3] Added runqueue clock normalized with cpufreq

2010/12/17 Dario Faggioli <raistlin@...ux.it>:
> On Fri, 2010-12-17 at 15:29 +0100, Peter Zijlstra wrote:
>> Solving the CPUfreq problem involves writing a SCHED_DEADLINE aware
>> CPUfreq governor. The governor must know about the constraints placed on
>> the system by the task-set. You simply cannot lower the frequency when
>> your system is at u=1.
>>
> We already did the very same thing (for another EU Project called
> FRESCOR), although it was done in an userspace sort of daemon. It was
> also able to consider other "high level" parameters like some estimation
> of the QoS of each application and of the global QoS of the system.
>
> However, converting the basic mechanism into a CPUfreq governor should
> be easily doable... The only problem is finding the time for that! ;-P

I'm a bit choked before the holidays, but I can fix this in the
beginning of next year.
At the same time as I do a new version of the current patches that takes
in Peter's comments.

>> The simple solution would be to slow down the runtime accounting of
>> SCHED_DEADLINE tasks by freq/max_freq. So instead of having:
>>
>>   dl_se->runtime -= delta;
>>
>> you do something like:
>>
>>   dl_se->runtime -= (freq * delta) / max_freq;
>>
>> Which auto-magically grows the actual bandwidth, and since the deadlines
>> are wall-time already it all works out nicely. It also keeps the
>> overhead inside SCHED_DEADLINE.
>>
> And, at least for the meantime, this seems a very very nice solution.
> The only thing I don't like is that division which would end up in being
> performed at each tick/update_curr_dl(), but we can try to find out a
> way to mitigate this, what do you think Harald?

Yes, I will do something like this instead, need to make sure that
everything is consider first though.
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