Re: [PATCH] arm64: dts: qcom: sdm845: correct dynamic power coefficients

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On Fri, 6 Jan 2023 at 19:28, Bjorn Andersson <andersson@xxxxxxxxxx> wrote:
>
> On Fri, Jan 06, 2023 at 05:46:18PM +0100, Vincent Guittot wrote:
>
> Seems like using get_maintainer.pl would have saved you some trouble ;)

The worst is that I used it but only checked names and not emails
when I reused the list of the original patch :-(

>
> > While stressing EAS on my dragonboard RB3, I have noticed that LITTLE cores
> > where never selected as the most energy efficient CPU whatever the
> > utilization level of waking task.
> >
> > energy model framework uses its cost field to estimate the energy with
> > the formula:
> >
> >   nrg = cost of the selected OPP * utilization / CPU's max capacity
> >
> > which ends up selecting the CPU with lowest cost / max capacity ration
> > as long as the utilization fits in the OPP's capacity.
> >
> > If we compare the cost of a little OPP with similar capacity of a big OPP
> > like :
> >        OPP(kHz)   OPP capacity    cost     max capacity   cost/max capacity
> > LITTLE 1766400    407             351114   407            863
> > big    1056000    408             520267   1024           508
> >
> > This can be interpreted as the LITTLE core consumes 70% more than big core
> > for the same compute capacity.
> >
> > According to [1], LITTLE consumes 10% less than big core for Coremark
> > benchmark at those OPPs. If we consider that everything else stays
> > unchanged, the dynamic-power-coefficient of LITTLE core should be
> > only 53% of the current value: 290 * 53% = 154
> >
> > Set the dynamic-power-coefficient of CPU0-3 to 154 to fix the energy model.
> >
>
> This is sounds reasonable.
>
> But if the math was wrong for SDM845, I would assume that sm8150 and
> sm8250 are wrong as well, as that's what 0e0a8e35d725 is based on. And
> should I assume that patches for other platforms are off by 53% as well?

I don't think that we can assume that there is an error and in
particular the same 53% error for others.

>
> Can you help me understand how to arrive at this number? (Without
> considering everything else stays unchanged, if needed).

In order to do the full computation, we need the voltage of each OPP
which I don't have as they are provided by the LUT at boot IIUC.
Instead I have used the debugfs output of the energy model and
compared the value of (perf_state->cost/cpu_max_capacity) with the
energy and duration figures available in [1].

In the case of SDM845, it was pretty easy to compare the OPPs of big
and LITTLE because the duration and the perf result were the same for
2 OPPS so we should have :

(little OPP(1766400)->cost / little max capacity (407)) / (big
OPP(1056000)->cost / big max capacity(1024)) = little OPP(1766400)
energy / big OPP(1056000) energy

(little OPP(1766400)->cost / little max capacity (407)) / (big
OPP(1056000)->cost / big max capacity(1024)) = 0.90

but current output gives:

(little OPP(1766400)->cost / little max capacity (407)) / (big
OPP(1056000)->cost / big max capacity(1024)) = 1.70

As we consider everything else constant, it can be simplified by:

correct_little_dynamic-power-coefficient *  const_A  = 0.90

Whereas we currently have

current_little_dynamic-power-coefficient * const_A = 1.70

and we ends up with

correct_little_dynamic-power-coefficient  = 0.90 / 1.70 *
current_little_dynamic-power-coefficient = 154

That being said, it can be simpler as the energy model provide the power figures

little OPP(1766400)->power = 351114 uW
big OPP(1056000)->power = 195991 uW
ration = 1.79

[1] results gives
little OPP(1766400)->power = 193.281 mW
big OPP(1056000)->power = 216.405 mW
ratio = 0.89

The ratios are a bit different and give a
correct_little_dynamic-power-coefficient  = 144 which is different
than when using ->cost. This probably comes from rounding and which
figures have been used to compute the model.

If you have Voltage of OPP, the formula used in energy model is power
(uW) = dynamic-power-coefficient * uV^2 * Freq (Mhz) so you can
compute dynamic-power-coefficient for each OPPs. They should be close
and then you will have to decide which one is the "best"

I don't have access to sdm8150 or sdm8250 boards but you can use the
same process to check that the energy model is aligned with the
figures in [1]

[1] https://github.com/kdrag0n/freqbench/tree/master/results

Regards,
Vincent
>
> Regards,
> Bjorn
>
> > [1] https://github.com/kdrag0n/freqbench/tree/master/results/sdm845/main
> >
> > Fixes: 0e0a8e35d725 ("arm64: dts: qcom: sdm845: correct dynamic power coefficients")
> > Signed-off-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> > ---
> >  arch/arm64/boot/dts/qcom/sdm845.dtsi | 8 ++++----
> >  1 file changed, 4 insertions(+), 4 deletions(-)
> >
> > diff --git a/arch/arm64/boot/dts/qcom/sdm845.dtsi b/arch/arm64/boot/dts/qcom/sdm845.dtsi
> > index 65032b94b46d..869bdb9bce6e 100644
> > --- a/arch/arm64/boot/dts/qcom/sdm845.dtsi
> > +++ b/arch/arm64/boot/dts/qcom/sdm845.dtsi
> > @@ -198,7 +198,7 @@ CPU0: cpu@0 {
> >                       reg = <0x0 0x0>;
> >                       enable-method = "psci";
> >                       capacity-dmips-mhz = <611>;
> > -                     dynamic-power-coefficient = <290>;
> > +                     dynamic-power-coefficient = <154>;
> >                       qcom,freq-domain = <&cpufreq_hw 0>;
> >                       operating-points-v2 = <&cpu0_opp_table>;
> >                       interconnects = <&gladiator_noc MASTER_APPSS_PROC 3 &mem_noc SLAVE_EBI1 3>,
> > @@ -222,7 +222,7 @@ CPU1: cpu@100 {
> >                       reg = <0x0 0x100>;
> >                       enable-method = "psci";
> >                       capacity-dmips-mhz = <611>;
> > -                     dynamic-power-coefficient = <290>;
> > +                     dynamic-power-coefficient = <154>;
> >                       qcom,freq-domain = <&cpufreq_hw 0>;
> >                       operating-points-v2 = <&cpu0_opp_table>;
> >                       interconnects = <&gladiator_noc MASTER_APPSS_PROC 3 &mem_noc SLAVE_EBI1 3>,
> > @@ -243,7 +243,7 @@ CPU2: cpu@200 {
> >                       reg = <0x0 0x200>;
> >                       enable-method = "psci";
> >                       capacity-dmips-mhz = <611>;
> > -                     dynamic-power-coefficient = <290>;
> > +                     dynamic-power-coefficient = <154>;
> >                       qcom,freq-domain = <&cpufreq_hw 0>;
> >                       operating-points-v2 = <&cpu0_opp_table>;
> >                       interconnects = <&gladiator_noc MASTER_APPSS_PROC 3 &mem_noc SLAVE_EBI1 3>,
> > @@ -264,7 +264,7 @@ CPU3: cpu@300 {
> >                       reg = <0x0 0x300>;
> >                       enable-method = "psci";
> >                       capacity-dmips-mhz = <611>;
> > -                     dynamic-power-coefficient = <290>;
> > +                     dynamic-power-coefficient = <154>;
> >                       qcom,freq-domain = <&cpufreq_hw 0>;
> >                       operating-points-v2 = <&cpu0_opp_table>;
> >                       interconnects = <&gladiator_noc MASTER_APPSS_PROC 3 &mem_noc SLAVE_EBI1 3>,
> > --
> > 2.34.1
> >

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