linux-kernel - Re: [PATCH v2 3/3] PM / EM: Skip inefficient OPPs

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Message-ID: <d21e69fd-9da5-3e5d-662c-c753b581cab0@arm.com>
Date:   Tue, 25 May 2021 14:34:12 +0100
From:   Lukasz Luba <lukasz.luba@....com>
To:     Quentin Perret <qperret@...gle.com>
Cc:     Vincent Donnefort <vincent.donnefort@....com>,
        peterz@...radead.org, rjw@...ysocki.net, viresh.kumar@...aro.org,
        vincent.guittot@...aro.org, linux-kernel@...r.kernel.org,
        ionela.voinescu@....com, dietmar.eggemann@....com
Subject: Re: [PATCH v2 3/3] PM / EM: Skip inefficient OPPs

Hi Quentin,

On 5/25/21 2:06 PM, Quentin Perret wrote:
> Hi Lukasz,
> 
> On Tuesday 25 May 2021 at 12:03:14 (+0100), Lukasz Luba wrote:
>> That's a few more instructions to parse the 'flags' filed. I'm not sure
>> if that brings speed improvements vs. if we not parse and have bool
>> filed with a simple looping. The out-of-order core might even suffer
>> from this parsing and loop index manipulations...
> 
> I'm not sure what you mean about parsing here? I'm basically suggesting
> to do something along the lines of:

I thought Vincent was going to re-use the 'flags' for it and keep it for
other purpose as well - which would require to parse/map-to-feature.
That's why I commented the patch earlier, pointing out that we shouldn't
prepare the code for future unknown EM_PERF_STATE_*. We can always
modify it when we need to add another feature later.

> 
> diff --git a/include/linux/energy_model.h b/include/linux/energy_model.h
> index daaeccfb9d6e..f02de32d2325 100644
> --- a/include/linux/energy_model.h
> +++ b/include/linux/energy_model.h
> @@ -128,13 +128,11 @@ struct em_perf_state *em_pd_get_efficient_state(struct em_perf_domain *pd,
> 
>          for (i = 0; i < pd->nr_perf_states; i++) {
>                  ps = &pd->table[i];
> -               if (ps->flags & EM_PERF_STATE_INEFFICIENT)
> -                       continue;
>                  if (ps->frequency >= freq)
>                          break;
>          }
> 
> -       return ps;
> +       return &pd->table[ps->next_efficient_idx];
>   }
> 
> What would be wrong with that?

Until we measure it, I don't know TBH. It looks OK for the first glance.
I like it also because it's self-contained, doesn't require parsing,
doesn't bring any 'generic' variable.

Regards,
Lukasz

> 
> Thanks,
> Quentin
>