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

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Message-ID: <YKz2bz6EiLwISOVV@google.com>
Date:   Tue, 25 May 2021 13:06:55 +0000
From:   Quentin Perret <qperret@...gle.com>
To:     Lukasz Luba <lukasz.luba@....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 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:

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?

Thanks,
Quentin