Re: [RFC PATCH v4 03/12] PM: Introduce an Energy Model management framework

From: Dietmar Eggemann
Date: Tue Jul 17 2018 - 12:00:42 EST

On 07/17/2018 04:19 PM, Quentin Perret wrote:
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:

[...]

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

Yeah, just realized that I used the wrong eas_v4 branch. With the one you mentioned here I still get ~0.2-0.3us diff with the non-optimized approach but at least values in the same ballpark as yours (performance governor to keep s^2 low):

v4:

Function Hit Time Avg s^2
A53 - cpu [0,3-5]
compute_energy 233 1455.140 us 6.245 us 0.022 us
...
A57 - cpu [1-2]
compute_energy 130 602.980 us 4.638 us 0.043 us

v4 + '(naive) calculating capacity on the fly':

Function Hit Time Avg s^2
A53 - cpu [0,3-5]
compute_energy 531 3460.200 us 6.516 us 0.044 us
A57 - cpu [1-2]
compute_energy 141 700.220 us 4.966 us 0.106 us

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 ?

I assume with you 'unsigned long cost' optimization I will get the same test result than you so I guess that's the optimization which assures that we don't have to pay the simplification of the EM with scheduler runtime.

[...]

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 :-)

Agreed.

[...]