Re: [PATCH v2 1/1] PM: EM: Increase energy calculation precision

[Lukasz Luba]

Hi Peter, Vincent,

Gentle ping.


On 7/20/21 10:41 AM, Lukasz Luba wrote:
> The Energy Model (EM) provides useful information about device power in
> each performance state to other subsystems like: Energy Aware Scheduler
> (EAS). The energy calculation in EAS does arithmetic operation based on
> the EM em_cpu_energy(). Current implementation of that function uses
> em_perf_state::cost as a pre-computed cost coefficient equal to:
> cost = power * max_frequency / frequency.
> The 'power' is expressed in milli-Watts (or in abstract scale).
>
> There are corner cases when the EAS energy calculation for two Performance
> Domains (PDs) return the same value. The EAS compares these values to
> choose smaller one. It might happen that this values are equal due to
> rounding error. In such scenario, we need better resolution, e.g. 1000
> times better. To provide this possibility increase the resolution in the
> em_perf_state::cost for 64-bit architectures. The costs for increasing
> resolution in 32-bit architectures are pretty high (64-bit division) and
> the returns do not justify the increased costs.
>
> This patch allows to avoid the rounding to milli-Watt errors, which might
> occur in EAS energy estimation for each Performance Domains (PD). The
> rounding error is common for small tasks which have small utilization
> value.
>
> There are two places in the code where it makes a difference:
> 1. In the find_energy_efficient_cpu() where we are searching for
> best_delta. We might suffer there when two PDs return the same result,
> like in the example below.
>
> Scenario:
> Low utilized system e.g. ~200 sum_util for PD0 and ~220 for PD1. There
> are quite a few small tasks ~10-15 util. These tasks would suffer for
> the rounding error. Such system utilization has been seen while playing
> some simple games. In such condition our partner reported 5..10mA less
> battery drain.
>
> Some details:
> We have two Perf Domains (PDs): PD0 (big) and PD1 (little)
> Let's compare w/o patch set ('old') and w/ patch set ('new')
> We are comparing energy w/ task and w/o task placed in the PDs
>
> a) 'old' w/o patch set, PD0
> task_util = 13
> cost = 480
> sum_util_w/o_task = 215
> sum_util_w_task = 228
> scale_cpu = 1024
> energy_w/o_task = 480 * 215 / 1024 = 100.78 => 100
> energy_w_task = 480 * 228 / 1024 = 106.87 => 106
> energy_diff = 106 - 100 = 6
> (this is equal to 'old' PD1's energy_diff in 'c)')
>
> b) 'new' w/ patch set, PD0
> task_util = 13
> cost = 480 * 1000 = 480000
> sum_util_w/o_task = 215
> sum_util_w_task = 228
> energy_w/o_task = 480000 * 215 / 1024 = 100781
> energy_w_task = 480000 * 228 / 1024  = 106875
> energy_diff = 106875 - 100781 = 6094
> (this is not equal to 'new' PD1's energy_diff in 'd)')
>
> c) 'old' w/o patch set, PD1
> task_util = 13
> cost = 160
> sum_util_w/o_task = 283
> sum_util_w_task = 293
> scale_cpu = 355
> energy_w/o_task = 160 * 283 / 355 = 127.55 => 127
> energy_w_task = 160 * 296 / 355 = 133.41 => 133
> energy_diff = 133 - 127 = 6
> (this is equal to 'old' PD0's energy_diff in 'a)')
>
> d) 'new' w/ patch set, PD1
> task_util = 13
> cost = 160 * 1000 = 160000
> sum_util_w/o_task = 283
> sum_util_w_task = 293
> scale_cpu = 355
> energy_w/o_task = 160000 * 283 / 355 = 127549
> energy_w_task = 160000 * 296 / 355 =   133408
> energy_diff = 133408 - 127549 = 5859
> (this is not equal to 'new' PD0's energy_diff in 'b)')
>
> 2. Difference in the the last find_energy_efficient_cpu(): margin filter.
> With this patch the margin comparison also has better resolution,
> so it's possible to have better task placement thanks to that.
>
> Fixes: 27871f7a8a341ef ("PM: Introduce an Energy Model management framework")
> Reported-by: CCJ Yeh <CCj.Yeh@xxxxxxxxxxxx>
> Signed-off-by: Lukasz Luba <lukasz.luba@xxxxxxx>
> ---
>   include/linux/energy_model.h | 16 ++++++++++++++++
>   kernel/power/energy_model.c  |  3 ++-
>   2 files changed, 18 insertions(+), 1 deletion(-)
>
> diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
> index 3f221dbf5f95..1834752c5617 100644
> --- a/include/linux/energy_model.h
> +++ b/include/linux/energy_model.h
> @@ -53,6 +53,22 @@ struct em_perf_domain {
>   #ifdef CONFIG_ENERGY_MODEL
>   #define EM_MAX_POWER 0xFFFF
>   
> +/*
> + * Increase resolution of energy estimation calculations for 64-bit
> + * architectures. The extra resolution improves decision made by EAS for the
> + * task placement when two Performance Domains might provide similar energy
> + * estimation values (w/o better resolution the values could be equal).
> + *
> + * We increase resolution only if we have enough bits to allow this increased
> + * resolution (i.e. 64-bit). The costs for increasing resolution when 32-bit
> + * are pretty high and the returns do not justify the increased costs.
> + */
> +#ifdef CONFIG_64BIT
> +#define em_scale_power(p) ((p) * 1000)
> +#else
> +#define em_scale_power(p) (p)
> +#endif
> +
>   struct em_data_callback {
>   	/**
>   	 * active_power() - Provide power at the next performance state of
> diff --git a/kernel/power/energy_model.c b/kernel/power/energy_model.c
> index 0f4530b3a8cd..bf312c04c514 100644
> --- a/kernel/power/energy_model.c
> +++ b/kernel/power/energy_model.c
> @@ -170,7 +170,8 @@ static int em_create_perf_table(struct device *dev, struct em_perf_domain *pd,
>   	/* Compute the cost of each performance state. */
>   	fmax = (u64) table[nr_states - 1].frequency;
>   	for (i = 0; i < nr_states; i++) {
> -		table[i].cost = div64_u64(fmax * table[i].power,
> +		unsigned long power_res = em_scale_power(table[i].power);
> +		table[i].cost = div64_u64(fmax * power_res,
>   					  table[i].frequency);
>   	}
>   

Do you have any comments to this patch?
It's EM-only change now, no other files touched.
If it's OK, I will ask Rafael to take it.

Regards,
Lukasz