linux-kernel - Re: [PATCH] cpufreq: Record stats when fast switching is enabled

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Date:   Thu, 31 Jan 2019 14:00:04 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Matthias Kaehlcke <mka@...omium.org>
Cc:     "Rafael J . Wysocki" <rjw@...ysocki.net>, linux-pm@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Douglas Anderson <dianders@...omium.org>
Subject: Re: [PATCH] cpufreq: Record stats when fast switching is enabled

On 30-01-19, 17:51, Matthias Kaehlcke wrote:
> When fast switching is enabled currently no cpufreq stats are
> recorded and the corresponding sysfs attributes appear empty (see
> also commit 1aefc75b2449 ("cpufreq: stats: Make the stats code
> non-modular")).
> 
> Record the stats after a successful fast switch and re-enable access
> through sysfs when fast switching is enabled. Since
> cpufreq_stats_update() can now be called in interrupt context (during
> a fast switch) disable local IRQs while holding the stats spinlock.
> 
> Signed-off-by: Matthias Kaehlcke <mka@...omium.org>
> ---
> The change is so simple that I wonder if I'm missing some important
> reason why the stats can't/shouldn't be updated during/after a fast
> switch ...
> 
> I would expect that holding the stats spinlock briefly in
> cpufreq_stats_update() shouldn't be a problem. In theory it would
> also be an option to have a per stats lock, though it seems overkill
> from my (possibly ignorant) point of view.
> ---
>  drivers/cpufreq/cpufreq.c       |  8 +++++++-
>  drivers/cpufreq/cpufreq_stats.c | 11 +++--------
>  2 files changed, 10 insertions(+), 9 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq.c b/drivers/cpufreq/cpufreq.c
> index e35a886e00bcf..63aadb0bbddfe 100644
> --- a/drivers/cpufreq/cpufreq.c
> +++ b/drivers/cpufreq/cpufreq.c
> @@ -1857,9 +1857,15 @@ EXPORT_SYMBOL(cpufreq_unregister_notifier);
>  unsigned int cpufreq_driver_fast_switch(struct cpufreq_policy *policy,
>  					unsigned int target_freq)
>  {
> +	unsigned int freq;
> +
>  	target_freq = clamp_val(target_freq, policy->min, policy->max);
>  
> -	return cpufreq_driver->fast_switch(policy, target_freq);
> +	freq = cpufreq_driver->fast_switch(policy, target_freq);
> +	if (freq)
> +		cpufreq_stats_record_transition(policy, freq);
> +
> +	return freq;
>  }
>  EXPORT_SYMBOL_GPL(cpufreq_driver_fast_switch);
>  
> diff --git a/drivers/cpufreq/cpufreq_stats.c b/drivers/cpufreq/cpufreq_stats.c
> index 1572129844a5b..21b919bfaeccf 100644
> --- a/drivers/cpufreq/cpufreq_stats.c
> +++ b/drivers/cpufreq/cpufreq_stats.c
> @@ -30,11 +30,12 @@ struct cpufreq_stats {
>  static void cpufreq_stats_update(struct cpufreq_stats *stats)
>  {
>  	unsigned long long cur_time = get_jiffies_64();
> +	unsigned long flags;
>  
> -	spin_lock(&cpufreq_stats_lock);
> +	spin_lock_irqsave(&cpufreq_stats_lock, flags);
>  	stats->time_in_state[stats->last_index] += cur_time - stats->last_time;
>  	stats->last_time = cur_time;
> -	spin_unlock(&cpufreq_stats_lock);
> +	spin_unlock_irqrestore(&cpufreq_stats_lock, flags);
>  }

The only problem that I can think of (or recall) is that this routine
also gets called when time_in_state sysfs file is read and that can
end up taking lock which the scheduler's hotpath will wait for.

-- 
viresh