linux-kernel - Re: [PATCH 1/6] cpufreq: ondemand: Update sampling rate only for concerned policies

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Message-ID: <2813520.hGtHsUWkzA@vostro.rjw.lan>
Date:	Thu, 03 Dec 2015 03:16:48 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	linaro-kernel@...ts.linaro.org, linux-pm@...r.kernel.org,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/6] cpufreq: ondemand: Update sampling rate only for concerned policies

On Thursday, October 29, 2015 05:57:20 PM Viresh Kumar wrote:
> We are comparing policy->governor against cpufreq_gov_ondemand to make
> sure that we update sampling rate only for the concerned CPUs. But that
> isn't enough.
> 
> In case of governor_per_policy, there can be multiple instances of
> ondemand governor and we will always end up updating all of them with
> current code. What we rather need to do, is to compare dbs_data with
> poilcy->governor_data, which will match only for the policies governed
> by dbs_data.
> 
> This code is also racy as the governor might be getting stopped at that
> time and we may end up scheduling work for a policy, which we have just
> disabled.
> 
> Fix that by protecting the entire function with &od_dbs_cdata.mutex,
> which will prevent against races with policy START/STOP/etc.
> 
> After these locks are in place, we can safely get the policy via per-cpu
> dbs_info.
> 
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  drivers/cpufreq/cpufreq_ondemand.c | 40 ++++++++++++++++++++++++++++----------
>  1 file changed, 30 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
> index 03ac6ce54042..0800a937607b 100644
> --- a/drivers/cpufreq/cpufreq_ondemand.c
> +++ b/drivers/cpufreq/cpufreq_ondemand.c
> @@ -247,25 +247,43 @@ static void update_sampling_rate(struct dbs_data *dbs_data,
>  		unsigned int new_rate)
>  {
>  	struct od_dbs_tuners *od_tuners = dbs_data->tuners;
> +	struct od_cpu_dbs_info_s *dbs_info;
> +	struct cpu_dbs_info *cdbs;
> +	struct cpufreq_policy *policy;
> +	struct cpu_common_dbs_info *shared;
> +	unsigned long next_sampling, appointed_at;
>  	int cpu;
>  
>  	od_tuners->sampling_rate = new_rate = max(new_rate,
>  			dbs_data->min_sampling_rate);
>  
> +	/*
> +	 * Lock governor so that governor start/stop can't execute in parallel.
> +	 */
> +	mutex_lock(&od_dbs_cdata.mutex);
> +
>  	for_each_online_cpu(cpu) {
> -		struct cpufreq_policy *policy;
> -		struct od_cpu_dbs_info_s *dbs_info;
> -		unsigned long next_sampling, appointed_at;

Why are you moving the definitions of variables away from here?

If there's any technical reason, it should be mentioned in the changelog,
but if it's just aesthetics or something similar, it doesn't belong to this
patch.

> +		dbs_info = &per_cpu(od_cpu_dbs_info, cpu);
> +		cdbs = &dbs_info->cdbs;
> +		shared = cdbs->shared;
>  
> -		policy = cpufreq_cpu_get(cpu);
> -		if (!policy)
> +		/*
> +		 * A valid shared and shared->policy means governor hasn't
> +		 * stopped or exited yet.
> +		 */
> +		if (!shared || !shared->policy)
>  			continue;
> -		if (policy->governor != &cpufreq_gov_ondemand) {
> -			cpufreq_cpu_put(policy);
> +
> +		policy = shared->policy;
> +
> +		/*
> +		 * Update sampling rate for CPUs whose policy is governed by
> +		 * dbs_data. In case of governor_per_policy, only a single
> +		 * policy will be governed by dbs_data, otherwise there can be
> +		 * multiple policies that are governed by the same dbs_data.
> +		 */
> +		if (dbs_data != policy->governor_data)
>  			continue;
> -		}
> -		dbs_info = &per_cpu(od_cpu_dbs_info, cpu);
> -		cpufreq_cpu_put(policy);
>  
>  		if (!delayed_work_pending(&dbs_info->cdbs.dwork))
>  			continue;
> @@ -281,6 +299,8 @@ static void update_sampling_rate(struct dbs_data *dbs_data,
>  
>  		}
>  	}
> +
> +	mutex_unlock(&od_dbs_cdata.mutex);
>  }
>  
>  static ssize_t store_sampling_rate(struct dbs_data *dbs_data, const char *buf,
> 

Thanks,
Rafael

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