linux-kernel - Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt

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Date:	Fri, 11 Dec 2015 08:48:34 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	Vincent Guittot <vincent.guittot@...aro.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Juri Lelli <Juri.Lelli@....com>,
	Patrick Bellasi <patrick.bellasi@....com>,
	Michael Turquette <mturquette@...libre.com>
Subject: Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in
 scale_rt_capacity

Hi Vincent,

On 12/10/2015 05:11 PM, Vincent Guittot wrote:
[...]
>> If yes, I think your approach is safe (and easier to implement - modulo a
>> small
>> issue when a task terminates of switches to other scheduling policies; I
>> think
>> there already are some "XXX" comments in the current code). However, it
>> allows to
>> save less energy (or reclaim less CPU time). For example, if I create a
>> SCHED_DEADLINE
>> task with runtime 90ms and period 100ms it will not allow to scale the CPU
>> frequency
>> even if it never executes (because is always blocked).
>
> Yes, i agree. If the task behavior is not aligned with its deadline
> parameters, we will over provisioned CPU capacity to the deadline
> scheduler.
> This metric is not used to select the OPP but to provisioned some CPU
> capacity to the deadline scheduler and to inform the CFS scheduler of
> the remaining capacity.
Ah, sorry; I missed this point.


>>>> +       /* This is the "average utilization" for this runqueue */
>>>> +       s64 avg_bw;
>>>>    };
>>
>> Small nit: why "average" utilization? I think a better name would be
>> "runqueue utilization"
>> or "local utilization", or something similar... If I understand correctly
>> (sorry if I
>> missed something), this is not an average, but the sum of the utilisations
>> of the tasks
>> on this runqueue... No?
>
> I have used "average" because it doesn't reflect the active/actual
> utilization of the run-queue but the forecasted average bandwidth of
> the CPU that will be used by deadline task.
Well, this is just nitpicking, so feel free to ignore (I just mentioned
this point because I was initially confused by the "average" name). But I
think this is "maximum", or "worst-case", not "average", because (as far
as I can understand) this field indicates that SCHED_DEADLINE tasks will
not be able to consume more than this fraction of CPU (if they try to
consume more, the scheduler throttles them).

> I'm open to change the name if another one makes more sense
In real-time literature this is often called simply "utilization" (or "worst-case
utilization" by someone): when a task can have a variable execution time, its
utilization is defined as WCET (maximum execution time) / period.



				Luca
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