linux-kernel - Re: [PATCH v6 07/14] sched/topology: Introduce sched_energy

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Message-ID: <f206f192-1bbf-f868-c8c6-8215334639db@arm.com>
Date:   Wed, 5 Sep 2018 23:06:38 -0700
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Quentin Perret <quentin.perret@....com>, peterz@...radead.org,
        rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org
Cc:     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@...eaurora.org,
        skannan@...eaurora.org, pkondeti@...eaurora.org,
        juri.lelli@...hat.com, edubezval@...il.com,
        srinivas.pandruvada@...ux.intel.com, currojerez@...eup.net,
        javi.merino@...nel.org
Subject: Re: [PATCH v6 07/14] sched/topology: Introduce sched_energy_present
 static key

On 08/20/2018 02:44 AM, Quentin Perret wrote:
> In order to ensure a minimal performance impact on non-energy-aware
> systems, introduce a static_key guarding the access to Energy-Aware
> Scheduling (EAS) code.
> 
> The static key is set iff all the following conditions are met for at
> least one root domain:
>    1. all online CPUs of the root domain are covered by the Energy
>       Model (EM);
>    2. the complexity of the root domain's EM is low enough to keep
>       scheduling overheads low;
>    3. the root domain has an asymmetric CPU capacity topology (detected
>       by looking for the SD_ASYM_CPUCAPACITY flag in the sched_domain
>       hierarchy).

This is pretty much the list (+ is schedutil running) of conditions to 
set rd->pd != NULL in build_perf_domains().

So when testing 'static_branch_unlikely(&sched_energy_present) && 
rcu_dereference(rd->pd)' don't you test two times the same thing?

Also, if let's say somebody wants to run another EM user (e.g. a thermal 
governor, like IPA) but not EAS on a asymmetric CPU capacity system. 
This can't be achieved with the current static branch approach

So what about using a (disabled by default ?) sched_feature + rd->pd != 
NULL instead?

[...]