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To: Morten Rasmussen <morten.rasmussen@arm.com>, peterz@infradead.org,
        mingo@redhat.com
From: Michael Turquette <mturquette@baylibre.com>
In-Reply-To: <1436293469-25707-40-git-send-email-morten.rasmussen@arm.com>
Cc: vincent.guittot@linaro.org, daniel.lezcano@linaro.org,
        "Dietmar Eggemann" <Dietmar.Eggemann@arm.com>, yuyang.du@intel.com,
        rjw@rjwysocki.net, "Juri Lelli" <Juri.Lelli@arm.com>,
        sgurrappadi@nvidia.com, pang.xunlei@zte.com.cn,
        linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
        "Juri Lelli" <juri.lelli@arm.com>
References: <1436293469-25707-1-git-send-email-morten.rasmussen@arm.com>
 <1436293469-25707-40-git-send-email-morten.rasmussen@arm.com>
Message-ID: <20150708151956.9112.52403@quantum>
User-Agent: alot/0.3.5
Subject: Re: [RFCv5 PATCH 39/46] sched/cpufreq_sched: use static key for cpu
 frequency selection
Date: Wed, 08 Jul 2015 08:19:56 -0700
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Quoting Morten Rasmussen (2015-07-07 11:24:22)
> From: Juri Lelli <juri.lelli@arm.com>
> 
> Introduce a static key to only affect scheduler hot paths when sched
> governor is enabled.
> 
> cc: Ingo Molnar <mingo@redhat.com>
> cc: Peter Zijlstra <peterz@infradead.org>
> 
> Signed-off-by: Juri Lelli <juri.lelli@arm.com>
> ---
>  kernel/sched/cpufreq_sched.c | 14 ++++++++++++++
>  kernel/sched/fair.c          |  1 +
>  kernel/sched/sched.h         |  6 ++++++
>  3 files changed, 21 insertions(+)
> 
> diff --git a/kernel/sched/cpufreq_sched.c b/kernel/sched/cpufreq_sched.c
> index 5020f24..2968f3a 100644
> --- a/kernel/sched/cpufreq_sched.c
> +++ b/kernel/sched/cpufreq_sched.c
> @@ -203,6 +203,18 @@ void cpufreq_sched_set_cap(int cpu, unsigned long capacity)
>         return;
>  }
>  
> +static inline void set_sched_energy_freq(void)
> +{
> +       if (!sched_energy_freq())
> +               static_key_slow_inc(&__sched_energy_freq);
> +}
> +
> +static inline void clear_sched_energy_freq(void)
> +{
> +       if (sched_energy_freq())
> +               static_key_slow_dec(&__sched_energy_freq);
> +}
> +
>  static int cpufreq_sched_start(struct cpufreq_policy *policy)
>  {
>         struct gov_data *gd;
> @@ -243,6 +255,7 @@ static int cpufreq_sched_start(struct cpufreq_policy *policy)
>  
>         policy->governor_data = gd;
>         gd->policy = policy;
> +       set_sched_energy_freq();
>         return 0;
>  
>  err:
> @@ -254,6 +267,7 @@ static int cpufreq_sched_stop(struct cpufreq_policy *policy)
>  {
>         struct gov_data *gd = policy->governor_data;
>  
> +       clear_sched_energy_freq();

<paranoia>

These controls are exposed to userspace via cpufreq sysfs knobs. Should
we use a struct static_key_deferred and static_key_slow_dec_deferred()
instead? This helps avoid a possible attack vector for slowing down the
system.

</paranoia>

I don't really know what a sane default rate limit would be in that case
though.  Otherwise feel free to add:

Reviewed-by: Michael Turquette <mturquette@baylibre.com>

Regards,
Mike

>         if (cpufreq_driver_might_sleep()) {
>                 kthread_stop(gd->task);
>         }
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index f7cb6c9..d395bc9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -4282,6 +4282,7 @@ static inline void hrtick_update(struct rq *rq)
>  #endif
>  
>  static bool cpu_overutilized(int cpu);
> +struct static_key __sched_energy_freq __read_mostly = STATIC_KEY_INIT_FALSE;
>  
>  /*
>   * The enqueue_task method is called before nr_running is
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 30aa0c4..b5e27d9 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1476,6 +1476,12 @@ unsigned long arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
>  }
>  #endif
>  
> +extern struct static_key __sched_energy_freq;
> +static inline bool sched_energy_freq(void)
> +{
> +       return static_key_false(&__sched_energy_freq);
> +}
> +
>  #ifdef CONFIG_CPU_FREQ_GOV_SCHED
>  void cpufreq_sched_set_cap(int cpu, unsigned long util);
>  #else
> -- 
> 1.9.1
> 
> --
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