Re: [PATCH] sched/idle: Prevent stopping the tick when there is no cpuidle driver

[Pierre Gondois]

Hello Anna-Maria,

On 1/12/24 11:56, Anna-Maria Behnsen wrote:
> Pierre Gondois <pierre.gondois@xxxxxxx> writes:
>
> > Hello Anna-Maria,
> >
> > On 1/9/24 17:24, Anna-Maria Behnsen wrote:
> > > When there is no cpuidle driver, there is no instance which could bring
> > > the CPU into a deeper C state. But at the moment the code does
> > > unconditionally try to stop the tick. So the aim of the patch is to
> > > remove this unconditional stop of the tick.
> >
> > I agree that the absence of cpuidle driver prevents from reaching deep
> > idle states. FWIU, there is however still benefits in stopping the tick
> > on such platform.
>
> What's the benefit?

I did the following test:
- on an arm64 Juno-r2 platform (2 big A-72 and 4 little A-53 CPUs)
- booting with 'cpuidle.off=1'
- using the energy counters of the platforms
  (the counters measure energy for the whole cluster of big/little CPUs)
- letting the platform idling during 10s

Without patch:
|       |     big-CPUs | little-CPUs |
|:------|-------------:|------------:|
| count | 10           | 10          |
| mean  |  0.353266    |  0.33399    |
| std   |  0.000254574 |  0.00206803 |
| min   |  0.352991    |  0.332145   |
| 25%   |  0.353039    |  0.332506   |
| 50%   |  0.353267    |  0.333089   |
| 75%   |  0.353412    |  0.335231   |
| max   |  0.353737    |  0.337964   |

With patch:
|       |     big-CPUs |  little-CPUs |
|:------|-------------:|-------------:|
| count | 10           | 10           |
| mean  |  0.375086    |  0.352451    |
| std   |  0.000299919 |  0.000752727 |
| min   |  0.374527    |  0.351743    |
| 25%   |  0.374872    |  0.35181     |
| 50%   |  0.37512     |  0.352063    |
| 75%   |  0.375335    |  0.353256    |
| max   |  0.375485    |  0.353461    |

So the energy consumption would be up:
- ~6% for the big CPUs
- ~10% for the litte CPUs

Regards,
Pierre


> Thanks,
>
>          Anna-Maria