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Date:   Tue, 2 Oct 2018 14:34:16 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Quentin Perret <quentin.perret@arm.com>
Cc:     rjw@rjwysocki.net, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, gregkh@linuxfoundation.org,
        mingo@redhat.com, dietmar.eggemann@arm.com,
        morten.rasmussen@arm.com, chris.redpath@arm.com,
        patrick.bellasi@arm.com, valentin.schneider@arm.com,
        vincent.guittot@linaro.org, thara.gopinath@linaro.org,
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        skannan@codeaurora.org, pkondeti@codeaurora.org,
        juri.lelli@redhat.com, edubezval@gmail.com,
        srinivas.pandruvada@linux.intel.com, currojerez@riseup.net,
        javi.merino@kernel.org
Subject: Re: [PATCH v7 05/14] sched: Introduce a sched_feat for Energy Aware
 Scheduling
Message-ID: <20181002123416.GA3439@hirez.programming.kicks-ass.net>
References: <20180912091309.7551-1-quentin.perret@arm.com>
 <20180912091309.7551-6-quentin.perret@arm.com>
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On Wed, Sep 12, 2018 at 10:13:00AM +0100, Quentin Perret wrote:
> In order to make sure Energy Aware Scheduling (EAS) doesn't hurt
> systems not using it, add a new sched_feat, called ENERGY_AWARE,
> guarding the access to EAS code paths.
> 
> Signed-off-by: Quentin Perret <quentin.perret@arm.com>
> ---
>  kernel/sched/features.h | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/features.h b/kernel/sched/features.h
> index 85ae8488039c..26ff6752e492 100644
> --- a/kernel/sched/features.h
> +++ b/kernel/sched/features.h
> @@ -90,3 +90,10 @@ SCHED_FEAT(WA_BIAS, true)
>   * UtilEstimation. Use estimated CPU utilization.
>   */
>  SCHED_FEAT(UTIL_EST, true)
> +
> +/*
> + * Energy-Aware Scheduling. Whether or not tasks will be placed into an
> + * energy-aware fashion depends on this feature being enabled and on the
> + * root domain having an Energy Model attached.
> + */
> +SCHED_FEAT(ENERGY_AWARE, false)

So these are debug knobs.. I would expect there to be a regular
static_key that is controlled by the presence of EM data.

Remember, if you compile with SCHED_DEBUG=n, this becomes a compile time
constant: false, and the compiler gets to do lots of DCE.