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SubjectRe: [RFC PATCH v4 03/12] PM: Introduce an Energy Model management framework
Hi Dietmar,

On Tuesday 17 Jul 2018 at 10:57:13 (+0200), Dietmar Eggemann wrote:
> On 07/16/2018 12:29 PM, Quentin Perret wrote:
> I see an impact of 'calculating capacity on the fly' in
> compute_energy()->em_fd_energy(). Running the first energy test case (# task
> equal 10) on the Juno r0 board with function profiling gives me:
>
> v4:
>
> Function Hit Time Avg s^2
> A53 - cpu [0,3-5]
> compute_energy 14620 30790.86 us 2.106 us 8.421 us
> compute_energy 682 1512.960 us 2.218 us 0.154 us
> compute_energy 1207 2627.820 us 2.177 us 0.172 us
> compute_energy 93 206.720 us 2.222 us 0.151 us
> A57 - cpu [1-2]
> compute_energy 153 160.100 us 1.046 us 0.190 us
> compute_energy 136 130.760 us 0.961 us 0.077 us
>
>
> v4 + 'calculating capacity on the fly':
>
> Function Hit Time Avg s^2
> A53 - cpu [0,3-5]
> compute_energy 11623 26941.12 us 2.317 us 12.203 us
> compute_energy 5062 11771.48 us 2.325 us 0.819 us
> compute_energy 4391 10396.78 us 2.367 us 1.753 us
> compute_energy 2234 5265.640 us 2.357 us 0.955 us
> A57 - cpu [1-2]
> compute_energy 59 66.020 us 1.118 us 0.112 us
> compute_energy 229 234.880 us 1.025 us 0.135 us
>
> The code is not optimized, I just replaced cs->capacity with
> arch_scale_cpu_capacity(NULL, cpu) (max_cap) and 'max_cap * cs->frequency /
> max_freq' respectively.
> There are 3 compute_energy() calls per wake-up on a system with 2 frequency
> domains.

First, thank you very much for looking into this :-)

So, I guess you see this overhead because of the extra division involved
by computing 'cap = max_cap * cs->frequency / max_freq'. However, I
think there is an opportunity to optimize things a bit and avoid that
overhead entirely. My suggestion is to remove the 'capacity' field from
the em_cap_state struct and to add a 'cost' parameter instead:

struct em_cap_state {
unsigned long frequency;
unsigned long power;
unsigned long cost;
};

I define the 'cost' of a capacity state as:

cost = power * max_freq / freq;

Since 'power', 'max_freq' and 'freq' do not change at run-time (as opposed
to 'capacity'), this coefficient is static and computed when the table is
first created. Then, based on this, you can implement em_fd_energy() as
follows:

static inline unsigned long em_fd_energy(struct em_freq_domain *fd,
unsigned long max_util, unsigned long sum_util)
{
unsigned long freq, scale_cpu;
struct em_cap_state *cs;
int i, cpu;

/* Map the utilization value to a frequency */
cpu = cpumask_first(to_cpumask(fd->cpus));
scale_cpu = arch_scale_cpu_capacity(NULL, cpu);
cs = &fd->table[fd->nr_cap_states - 1];
freq = map_util_freq(max_util, cs->frequency, scale_cpu);

/* Find the lowest capacity state above this frequency */
for (i = 0; i < fd->nr_cap_states; i++) {
cs = &fd->table[i];
if (cs->frequency >= freq)
break;
}

/*
* The capacity of a CPU at a specific performance state is defined as:
*
* cap = freq * scale_cpu / max_freq
*
* The energy consumed by this CPU can be estimated as:
*
* nrg = power * util / cap
*
* because (util / cap) represents the percentage of busy time of the
* CPU. Based on those definitions, we have:
*
* nrg = power * util * max_freq / (scale_cpu * freq)
*
* which can be re-arranged as a product of two terms:
*
* nrg = (power * max_freq / freq) * (util / scale_cpu)
*
* The first term is static, and is stored in the em_cap_state struct
* as 'cost'. The parameters of the second term change at run-time.
*/
return cs->cost * sum_util / scale_cpu;
}

With the above implementation, there is no additional division in
em_fd_energy() compared to v4, so I would expect to see no significant
difference in computation time.

I tried to reproduce your test case and I get the following numbers on
my Juno r0 (using the performance governor):

v4:
***
Function Hit Time Avg s^2
A53 - cpu [0,3-5]
compute_energy 1796 12685.66 us 7.063 us 0.039 us
compute_energy 4214 28060.02 us 6.658 us 0.919 us
compute_energy 2743 20167.86 us 7.352 us 0.067 us
compute_energy 13958 97122.68 us 6.958 us 9.255 us
A57 - cpu [1-2]
compute_energy 86 448.800 us 5.218 us 0.106 us
compute_energy 163 847.600 us 5.200 us 0.128 us


'v5' (with 'cost'):
*******************
Function Hit Time Avg s^2
A53 - cpu [0,3-5]
compute_energy 1695 11153.54 us 6.580 us 0.022 us
compute_energy 16823 113709.5 us 6.759 us 27.109 us
compute_energy 677 4490.060 us 6.632 us 0.028 us
compute_energy 1959 13595.66 us 6.940 us 0.029 us
A57 - cpu [1-2]
compute_energy 211 1089.860 us 5.165 us 0.122 us
compute_energy 83 420.860 us 5.070 us 0.075 us


So I don't observe any obvious regression with my optimization applied.
The v4 branch I used is the one mentioned in the cover letter:
http://www.linux-arm.org/git?p=linux-qp.git;a=shortlog;h=refs/heads/upstream/eas_v4

And I just pushed the WiP branch I used to compare against:
http://www.linux-arm.org/git?p=linux-qp.git;a=shortlog;h=refs/heads/upstream/eas_v5-WiP-compute_energy_profiling

Is this also fixing the regression on your side ?

>
> > The second option simplifies the code of the EM framework significantly
> > (no more em_rescale_cpu_capacity()) and shouldn't introduce massive
> > overheads on the scheduler side (the energy calculation already
> > requires one multiplication and one division, so nothing new on that
> > side). At the same time, that would make it a whole lot easier to
> > interface the EM framework with IPA without having to deal with RCU all
> > over the place.
>
> IMO, em_rescale_cpu_capacity() is just the capacity related example what the
> EM needs if its values can be changed at runtime. There might be other use
> cases in the future like changing power values depending on temperature.
> So maybe it's a good idea to not have this 'EM values can change at runtime'
> feature in the first version of the EM and emphasize on simplicity of the
> code instead (if we can eliminate the extra runtime overhead).

I agree that it would be nice to keep it simple in the beginning. If
there is strong and demonstrated use-case for updating the EM at
run-time later, then we can re-introduce the RCU protection. But until
then, we can avoid the complex implementation at no obvious cost (given
my results above) so that sounds like a good trade-off to me :-)

Thanks,
Quentin

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