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

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Date:	Thu, 10 Dec 2015 14:27:39 +0100
From:	Luca Abeni <luca.abeni@...tn.it>
To:	Vincent Guittot <vincent.guittot@...aro.org>,
	Steve Muckle <steve.muckle@...aro.org>
Cc:	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,

first of all, thanks for adding me in the discussion.

On 12/09/2015 09:50 AM, Vincent Guittot wrote:
> adding Lucas
>
> On 9 December 2015 at 07:19, Steve Muckle <steve.muckle@...aro.org> wrote:
>> From: Vincent Guittot <vincent.guittot@...aro.org>
>>
>> Instead of monitoring the exec time of deadline tasks to evaluate the
>> CPU capacity consumed by deadline scheduler class, we can directly
>> calculate it thanks to the sum of utilization of deadline tasks on the
>> CPU.  We can remove deadline tasks from rt_avg metric and directly use
>> the average bandwidth of deadline scheduler in scale_rt_capacity.
>>
>> Based in part on a similar patch from Luca Abeni <luca.abeni@...tn.it>.
Just to check if my understanding of your patch is correct: what you do is
to track the total utilisation of the tasks that are assigned to a CPU/core,
independently from their state (active or inactive). The difference with my
patch is that I try to track the "active utilisation" (eliminating the utilisation
of the tasks that are blocked).

Is this understanding correct?
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).


[...]
>> +       /* 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?



				Luca
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