linux-kernel - Re: [PATCH v9 09/15] sched: Introduce sched_energy

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Date:   Wed, 21 Nov 2018 15:14:46 +0000
From:   Quentin Perret <quentin.perret@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     rjw@...ysocki.net, linux-kernel@...r.kernel.org,
        linux-pm@...r.kernel.org, gregkh@...uxfoundation.org,
        mingo@...hat.com, dietmar.eggemann@....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 v9 09/15] sched: Introduce sched_energy_present static key

Hi Peter,

On Wednesday 21 Nov 2018 at 14:08:22 (+0100), Peter Zijlstra wrote:
> On Mon, Nov 19, 2018 at 02:18:51PM +0000, Quentin Perret wrote:
> 
> > +static void sched_energy_start(int ndoms_new, cpumask_var_t doms_new[])
> > +{
> > +	/*
> > +	 * The conditions for EAS to start are checked during the creation of
> > +	 * root domains. If one of them meets all conditions, it will have a
> > +	 * non-null list of performance domains.
> > +	 */
> > +	while (ndoms_new) {
> > +		if (cpu_rq(cpumask_first(doms_new[ndoms_new - 1]))->rd->pd)
> > +			goto enable;
> > +		ndoms_new--;
> > +	}
> 
> That seems quite ugly; can't you simply return a bool from
> build_perf_domains() ?
> 
> Something like the below, but less fugly ;-)
> 
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -299,7 +299,7 @@ static void destroy_perf_domain_rcu(stru
>  #define EM_MAX_COMPLEXITY 2048
>  
>  extern struct cpufreq_governor schedutil_gov;
> -static void build_perf_domains(const struct cpumask *cpu_map)
> +static bool build_perf_domains(const struct cpumask *cpu_map)
>  {
>  	int i, nr_pd = 0, nr_cs = 0, nr_cpus = cpumask_weight(cpu_map);
>  	struct perf_domain *pd = NULL, *tmp;
> @@ -365,7 +365,7 @@ static void build_perf_domains(const str
>  	if (tmp)
>  		call_rcu(&tmp->rcu, destroy_perf_domain_rcu);
>  
> -	return;
> +	return !!pd;
>  
>  free:
>  	free_pd(pd);
> @@ -373,6 +373,8 @@ static void build_perf_domains(const str
>  	rcu_assign_pointer(rd->pd, NULL);
>  	if (tmp)
>  		call_rcu(&tmp->rcu, destroy_perf_domain_rcu);
> +
> +	return false;
>  }
>  #else
>  static void free_pd(struct perf_domain *pd) { }
> @@ -2173,6 +2175,9 @@ void partition_sched_domains(int ndoms_n
>  	}
>  
>  #if defined(CONFIG_ENERGY_MODEL) && defined(CONFIG_CPU_FREQ_GOV_SCHEDUTIL)
> +	{
> +	bool has_eas = false;

That one wants to be declared '__maybe_unused' at the top of
partition_sched_domains() perhaps ? Just to avoid the { } section.
Hopefully the compiler will be smart enough to not use stack space for
it if not needed.

> +
>  	/* Build perf. domains: */
>  	for (i = 0; i < ndoms_new; i++) {
>  		for (j = 0; j < n && !sched_energy_update; j++) {
> @@ -2181,10 +2186,13 @@ void partition_sched_domains(int ndoms_n
>  				goto match3;

And we want to do 'has_eas = true' just before 'goto match3' here.
Otherwise we'll end up disabling EAS if we hit 'goto match3' for all
root domains.

>  		}
>  		/* No match - add perf. domains for a new rd */
> -		build_perf_domains(doms_new[i]);
> +		has_eas |= build_perf_domains(doms_new[i]);
>  match3:
>  		;
>  	}
> +
> +	sched_energy_set(has_eas);
> +	}
>  #endif
>  
>  	/* Remember the new sched domains: */

Other than that I guess that should work OK :-)

Thanks,
Quentin