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

[Luca Abeni]

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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