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Message-ID: <6488e9a1-19d4-5d6b-1446-684881618fce@arm.com>
Date: Tue, 17 Jul 2018 18:00:31 +0200
From: Dietmar Eggemann <dietmar.eggemann@....com>
To: Quentin Perret <quentin.perret@....com>
Cc: peterz@...radead.org, rjw@...ysocki.net,
linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
gregkh@...uxfoundation.org, mingo@...hat.com,
morten.rasmussen@....com, chris.redpath@....com,
patrick.bellasi@....com, valentin.schneider@....com,
vincent.guittot@...aro.org, thara.gopinath@...aro.org,
viresh.kumar@...aro.org, tkjos@...gle.com, joel@...lfernandes.org,
smuckle@...gle.com, adharmap@...cinc.com, skannan@...cinc.com,
pkondeti@...eaurora.org, juri.lelli@...hat.com,
edubezval@...il.com, srinivas.pandruvada@...ux.intel.com,
currojerez@...eup.net, javi.merino@...nel.org
Subject: Re: [RFC PATCH v4 03/12] PM: Introduce an Energy Model management
framework
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.
[...]
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