Re: [PATCH 5/5] powercap/drivers/dtpm: Scale the power with the load

Lukasz Luba
2021-03-09T10:02:26Z

Hi Daniel,

I've started reviewing the series, please find some comments below.

On 3/1/21 9:21 PM, Daniel Lezcano wrote:
> Currently the power consumption is based on the current OPP power
> assuming the entire performance domain is fully loaded.
>
> That gives very gross power estimation and we can do much better by
> using the load to scale the power consumption.
>
> Use the utilization to normalize and scale the power usage over the
> max possible power.
>
> Tested on a rock960 with 2 big CPUS, the power consumption estimation
> conforms with the expected one.
>
> Before this change:
>
> ~$ ~/dhrystone -t 1 -l 10000&
> ~$ cat /sys/devices/virtual/powercap/dtpm/dtpm:0/dtpm:0:1/constraint_0_max_power_uw
> 2260000
>
> After this change:
>
> ~$ ~/dhrystone -t 1 -l 10000&
> ~$ cat /sys/devices/virtual/powercap/dtpm/dtpm:0/dtpm:0:1/constraint_0_max_power_uw
> 1130000
>
> ~$ ~/dhrystone -t 2 -l 10000&
> ~$ cat /sys/devices/virtual/powercap/dtpm/dtpm:0/dtpm:0:1/constraint_0_max_power_uw
> 2260000
>
> Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
> ---
> drivers/powercap/dtpm_cpu.c | 21 +++++++++++++++++----
> 1 file changed, 17 insertions(+), 4 deletions(-)
>
> diff --git a/drivers/powercap/dtpm_cpu.c b/drivers/powercap/dtpm_cpu.c
> index e728ebd6d0ca..8379b96468ef 100644
> --- a/drivers/powercap/dtpm_cpu.c
> +++ b/drivers/powercap/dtpm_cpu.c
> @@ -68,27 +68,40 @@ static u64 set_pd_power_limit(struct dtpm *dtpm, u64 power_limit)
> return power_limit;
> }
>
> +static u64 scale_pd_power_uw(struct cpumask *cpus, u64 power)

renamed 'cpus' into 'pd_mask', see below

> +{
> + unsigned long max, util;
> + int cpu, load = 0;

IMHO 'int load' looks odd when used with 'util' and 'max'.
I would put in the line above to have them all the same type and
renamed to 'sum_util'.

> +
> + for_each_cpu(cpu, cpus) {

I would avoid the temporary CPU mask in the get_pd_power_uw()
with this modified loop:

for_each_cpu_and(cpu, pd_mask, cpu_online_mask) {

> + max = arch_scale_cpu_capacity(cpu);
> + util = sched_cpu_util(cpu, max);
> + load += ((util * 100) / max);

Below you can find 3 optimizations. Since we are not in the hot
path here, it's up to if you would like to use all/some of them
or just ignore.

1st optimization.
If we use 'load += (util << 10) / max' in the loop, then
we could avoid div by 100 and use a right shift:
(power * load) >> 10

2nd optimization.
Since we use EM CPU mask, which span all CPUs with the same
arch_scale_cpu_capacity(), you can avoid N divs inside the loop
and do it once, below the loop.

3rd optimization.
If we just simply add all 'util' into 'sum_util' (no mul or div in
the loop), then we might just have simple macro

#define CALC_POWER_USAGE(power, sum_util, max) \
(((power * (sum_util << 10)) / max) >> 10)

> + }
> +
> + return (power * load) / 100;
> +}
> +
> static u64 get_pd_power_uw(struct dtpm *dtpm)
> {
> struct dtpm_cpu *dtpm_cpu = to_dtpm_cpu(dtpm);
> struct em_perf_domain *pd;
> struct cpumask cpus;

Since we don't need the 'nr_cpus' we also don't need the
cpumask which occupy stack; Maybe use
struct cpumask *pd_mask;

then

> unsigned long freq;
> - int i, nr_cpus;
> + int i;
>
> pd = em_cpu_get(dtpm_cpu->cpu);
> freq = cpufreq_quick_get(dtpm_cpu->cpu);
>
> cpumask_and(&cpus, cpu_online_mask, to_cpumask(pd->cpus));

remove ^^^^^ and set
pd_mask = em_span_cpus(pd);

> - nr_cpus = cpumask_weight(&cpus);
>
> for (i = 0; i < pd->nr_perf_states; i++) {
>
> if (pd->table[i].frequency < freq)
> continue;
>
> - return pd->table[i].power *
> - MICROWATT_PER_MILLIWATT * nr_cpus;
> + return scale_pd_power_uw(&cpus, pd->table[i].power *
> + MICROWATT_PER_MILLIWATT);

Instead of '&cpus' I would put 'pd_mask' and that should do the job.

> }
>
> return 0;
>

Apart from that, the design idea with util looks good.

Regards,
Lukasz