Re: [RFCv6 PATCH 09/10] sched: deadline: use deadline bandwidth in scale_rt_capacity

From: Luca Abeni
Date: Thu Dec 10 2015 - 08:27:50 EST

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@xxxxxxxxxx> wrote:
From: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

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@xxxxxxxx>.
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?

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