Date:	Mon, 30 Nov 2015 13:05:03 +0100
From:	Lucas Stach <l.stach@...gutronix.de>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Rafael Wysocki <rjw@...ysocki.net>, linaro-kernel@...ts.linaro.org,
	linux-pm@...r.kernel.org,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] cpufreq: governor: replace per-cpu delayed work
 with timers

Am Donnerstag, den 29.10.2015, 17:57 +0530 schrieb Viresh Kumar:
> cpufreq governors evaluate load at sampling rate and based on that they
> update frequency for a group of CPUs belonging to the same cpufreq
> policy.
> 
> This is required to be done in a single thread for all policy->cpus, but
> because we don't want to wakeup idle CPUs to do just that, we use
> deferrable work for this. If we would have used a single delayed
> deferrable work for the entire policy, there were chances that the CPU
> required to run the handler can be in idle and we might end up not
> changing the frequency for the entire group with load variations.
> 
> And so we were forced to keep per-cpu works, and only the one that
> expires first need to do the real work and others are rescheduled for
> next sampling time.
> 
> We have been using the more complex solution until now, where we used a
> delayed deferrable work for this, which is a combination of a timer and
> a work.
> 
> This could be made lightweight by keeping per-cpu deferred timers with a
> single work item, which is scheduled by the first timer that expires.
> 
> This patch does just that and here are important changes:
> - The timer handler will run in irq context and so we need to use a
>   spin_lock instead of the timer_mutex. And so a separate timer_lock is
>   created. This also makes the use of the mutex and lock quite clear, as
>   we know what exactly they are protecting.
> - A new field 'skip_work' is added to track when the timer handlers can
>   queue a work. More comments present in code.
> 

I don't want to block this patch on that, but maybe as a thought for
further consideration: Wouldn't it make sense to use a single unbound
deferrable work item for this? There was some work to make this possible
already: "timer: make deferrable cpu unbound timers really not bound to
a cpu"

Regards,
Lucas

> Suggested-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> ---
>  drivers/cpufreq/cpufreq_governor.c | 139 +++++++++++++++++++++----------------
>  drivers/cpufreq/cpufreq_governor.h |  20 ++++--
>  drivers/cpufreq/cpufreq_ondemand.c |   8 +--
>  3 files changed, 98 insertions(+), 69 deletions(-)
> 
> diff --git a/drivers/cpufreq/cpufreq_governor.c b/drivers/cpufreq/cpufreq_governor.c
> index 999e1f6addf9..a3f9bc9b98e9 100644
> --- a/drivers/cpufreq/cpufreq_governor.c
> +++ b/drivers/cpufreq/cpufreq_governor.c
> @@ -158,47 +158,52 @@ void dbs_check_cpu(struct dbs_data *dbs_data, int cpu)
>  }
>  EXPORT_SYMBOL_GPL(dbs_check_cpu);
>  
> -static inline void __gov_queue_work(int cpu, struct dbs_data *dbs_data,
> -		unsigned int delay)
> +void gov_add_timers(struct cpufreq_policy *policy, unsigned int delay)
>  {
> -	struct cpu_dbs_info *cdbs = dbs_data->cdata->get_cpu_cdbs(cpu);
> -
> -	mod_delayed_work_on(cpu, system_wq, &cdbs->dwork, delay);
> -}
> -
> -void gov_queue_work(struct dbs_data *dbs_data, struct cpufreq_policy *policy,
> -		unsigned int delay, bool all_cpus)
> -{
> -	int i;
> +	struct dbs_data *dbs_data = policy->governor_data;
> +	struct cpu_dbs_info *cdbs;
> +	int cpu;
>  
> -	if (!all_cpus) {
> -		/*
> -		 * Use raw_smp_processor_id() to avoid preemptible warnings.
> -		 * We know that this is only called with all_cpus == false from
> -		 * works that have been queued with *_work_on() functions and
> -		 * those works are canceled during CPU_DOWN_PREPARE so they
> -		 * can't possibly run on any other CPU.
> -		 */
> -		__gov_queue_work(raw_smp_processor_id(), dbs_data, delay);
> -	} else {
> -		for_each_cpu(i, policy->cpus)
> -			__gov_queue_work(i, dbs_data, delay);
> +	for_each_cpu(cpu, policy->cpus) {
> +		cdbs = dbs_data->cdata->get_cpu_cdbs(cpu);
> +		cdbs->timer.expires = jiffies + delay;
> +		add_timer_on(&cdbs->timer, cpu);
>  	}
>  }
> -EXPORT_SYMBOL_GPL(gov_queue_work);
> +EXPORT_SYMBOL_GPL(gov_add_timers);
>  
> -static inline void gov_cancel_work(struct dbs_data *dbs_data,
> -		struct cpufreq_policy *policy)
> +static inline void gov_cancel_timers(struct cpufreq_policy *policy)
>  {
> +	struct dbs_data *dbs_data = policy->governor_data;
>  	struct cpu_dbs_info *cdbs;
>  	int i;
>  
>  	for_each_cpu(i, policy->cpus) {
>  		cdbs = dbs_data->cdata->get_cpu_cdbs(i);
> -		cancel_delayed_work_sync(&cdbs->dwork);
> +		del_timer_sync(&cdbs->timer);
>  	}
>  }
>  
> +void gov_cancel_work(struct cpu_common_dbs_info *shared)
> +{
> +	unsigned long flags;
> +
> +	/*
> +	 * No work will be queued from timer handlers after skip_work is
> +	 * updated. And so we can safely cancel the work first and then the
> +	 * timers.
> +	 */
> +	spin_lock_irqsave(&shared->timer_lock, flags);
> +	shared->skip_work++;
> +	spin_unlock_irqrestore(&shared->timer_lock, flags);
> +
> +	cancel_work_sync(&shared->work);
> +
> +	gov_cancel_timers(shared->policy);
> +
> +	shared->skip_work = 0;
> +}
> +
>  /* Will return if we need to evaluate cpu load again or not */
>  static bool need_load_eval(struct cpu_common_dbs_info *shared,
>  			   unsigned int sampling_rate)
> @@ -217,29 +222,21 @@ static bool need_load_eval(struct cpu_common_dbs_info *shared,
>  	return true;
>  }
>  
> -static void dbs_timer(struct work_struct *work)
> +static void dbs_work_handler(struct work_struct *work)
>  {
> -	struct cpu_dbs_info *cdbs = container_of(work, struct cpu_dbs_info,
> -						 dwork.work);
> -	struct cpu_common_dbs_info *shared = cdbs->shared;
> +	struct cpu_common_dbs_info *shared = container_of(work, struct
> +					cpu_common_dbs_info, work);
>  	struct cpufreq_policy *policy;
>  	struct dbs_data *dbs_data;
>  	unsigned int sampling_rate, delay;
> -	bool modify_all = true;
> -
> -	mutex_lock(&shared->timer_mutex);
> +	bool eval_load;
>  
>  	policy = shared->policy;
> -
> -	/*
> -	 * Governor might already be disabled and there is no point continuing
> -	 * with the work-handler.
> -	 */
> -	if (!policy)
> -		goto unlock;
> -
>  	dbs_data = policy->governor_data;
>  
> +	/* Kill all timers */
> +	gov_cancel_timers(policy);
> +
>  	if (dbs_data->cdata->governor == GOV_CONSERVATIVE) {
>  		struct cs_dbs_tuners *cs_tuners = dbs_data->tuners;
>  
> @@ -250,14 +247,44 @@ static void dbs_timer(struct work_struct *work)
>  		sampling_rate = od_tuners->sampling_rate;
>  	}
>  
> -	if (!need_load_eval(cdbs->shared, sampling_rate))
> -		modify_all = false;
> +	eval_load = need_load_eval(shared, sampling_rate);
>  
> -	delay = dbs_data->cdata->gov_dbs_timer(policy, modify_all);
> -	gov_queue_work(dbs_data, policy, delay, modify_all);
> +	/*
> +	 * Make sure cpufreq_governor_limits() isn't evaluating load in
> +	 * parallel.
> +	 */
> +	mutex_lock(&shared->timer_mutex);
> +	delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
> +	mutex_unlock(&shared->timer_mutex);
> +
> +	shared->skip_work--;
> +	gov_add_timers(policy, delay);
> +}
> +
> +static void dbs_timer_handler(unsigned long data)
> +{
> +	struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
> +	struct cpu_common_dbs_info *shared = cdbs->shared;
> +	struct cpufreq_policy *policy;
> +	unsigned long flags;
> +
> +	spin_lock_irqsave(&shared->timer_lock, flags);
> +	policy = shared->policy;
> +
> +	/*
> +	 * Timer handler isn't allowed to queue work at the moment, because:
> +	 * - Another timer handler has done that
> +	 * - We are stopping the governor
> +	 * - Or we are updating the sampling rate of ondemand governor
> +	 */
> +	if (shared->skip_work)
> +		goto unlock;
> +
> +	shared->skip_work++;
> +	queue_work(system_wq, &shared->work);
>  
>  unlock:
> -	mutex_unlock(&shared->timer_mutex);
> +	spin_unlock_irqrestore(&shared->timer_lock, flags);
>  }
>  
>  static void set_sampling_rate(struct dbs_data *dbs_data,
> @@ -288,6 +315,8 @@ static int alloc_common_dbs_info(struct cpufreq_policy *policy,
>  		cdata->get_cpu_cdbs(j)->shared = shared;
>  
>  	mutex_init(&shared->timer_mutex);
> +	spin_lock_init(&shared->timer_lock);
> +	INIT_WORK(&shared->work, dbs_work_handler);
>  	return 0;
>  }
>  
> @@ -452,7 +481,9 @@ static int cpufreq_governor_start(struct cpufreq_policy *policy,
>  		if (ignore_nice)
>  			j_cdbs->prev_cpu_nice = kcpustat_cpu(j).cpustat[CPUTIME_NICE];
>  
> -		INIT_DEFERRABLE_WORK(&j_cdbs->dwork, dbs_timer);
> +		__setup_timer(&j_cdbs->timer, dbs_timer_handler,
> +			      (unsigned long)j_cdbs,
> +			      TIMER_DEFERRABLE | TIMER_IRQSAFE);
>  	}
>  
>  	if (cdata->governor == GOV_CONSERVATIVE) {
> @@ -470,8 +501,7 @@ static int cpufreq_governor_start(struct cpufreq_policy *policy,
>  		od_ops->powersave_bias_init_cpu(cpu);
>  	}
>  
> -	gov_queue_work(dbs_data, policy, delay_for_sampling_rate(sampling_rate),
> -		       true);
> +	gov_add_timers(policy, delay_for_sampling_rate(sampling_rate));
>  	return 0;
>  }
>  
> @@ -485,16 +515,9 @@ static int cpufreq_governor_stop(struct cpufreq_policy *policy,
>  	if (!shared || !shared->policy)
>  		return -EBUSY;
>  
> -	/*
> -	 * Work-handler must see this updated, as it should not proceed any
> -	 * further after governor is disabled. And so timer_mutex is taken while
> -	 * updating this value.
> -	 */
> -	mutex_lock(&shared->timer_mutex);
> +	gov_cancel_work(shared);
>  	shared->policy = NULL;
> -	mutex_unlock(&shared->timer_mutex);
>  
> -	gov_cancel_work(dbs_data, policy);
>  	return 0;
>  }
>  
> diff --git a/drivers/cpufreq/cpufreq_governor.h b/drivers/cpufreq/cpufreq_governor.h
> index 0c7589016b6c..76742902491e 100644
> --- a/drivers/cpufreq/cpufreq_governor.h
> +++ b/drivers/cpufreq/cpufreq_governor.h
> @@ -132,12 +132,20 @@ static void *get_cpu_dbs_info_s(int cpu)				\
>  struct cpu_common_dbs_info {
>  	struct cpufreq_policy *policy;
>  	/*
> -	 * percpu mutex that serializes governor limit change with dbs_timer
> -	 * invocation. We do not want dbs_timer to run when user is changing
> -	 * the governor or limits.
> +	 * Per policy mutex that serializes load evaluation from limit-change
> +	 * and work-handler.
>  	 */
>  	struct mutex timer_mutex;
> +
> +	/*
> +	 * Per policy lock that serializes access to queuing work from timer
> +	 * handlers.
> +	 */
> +	spinlock_t timer_lock;
> +
>  	ktime_t time_stamp;
> +	unsigned int skip_work;
> +	struct work_struct work;
>  };
>  
>  /* Per cpu structures */
> @@ -152,7 +160,7 @@ struct cpu_dbs_info {
>  	 * wake-up from idle.
>  	 */
>  	unsigned int prev_load;
> -	struct delayed_work dwork;
> +	struct timer_list timer;
>  	struct cpu_common_dbs_info *shared;
>  };
>  
> @@ -268,11 +276,11 @@ static ssize_t show_sampling_rate_min_gov_pol				\
>  
>  extern struct mutex cpufreq_governor_lock;
>  
> +void gov_add_timers(struct cpufreq_policy *policy, unsigned int delay);
> +void gov_cancel_work(struct cpu_common_dbs_info *shared);
>  void dbs_check_cpu(struct dbs_data *dbs_data, int cpu);
>  int cpufreq_governor_dbs(struct cpufreq_policy *policy,
>  		struct common_dbs_data *cdata, unsigned int event);
> -void gov_queue_work(struct dbs_data *dbs_data, struct cpufreq_policy *policy,
> -		unsigned int delay, bool all_cpus);
>  void od_register_powersave_bias_handler(unsigned int (*f)
>  		(struct cpufreq_policy *, unsigned int, unsigned int),
>  		unsigned int powersave_bias);
> diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
> index 0848c8ac6847..9e0293c23285 100644
> --- a/drivers/cpufreq/cpufreq_ondemand.c
> +++ b/drivers/cpufreq/cpufreq_ondemand.c
> @@ -285,13 +285,11 @@ static void update_sampling_rate(struct dbs_data *dbs_data,
>  			continue;
>  
>  		next_sampling = jiffies + usecs_to_jiffies(new_rate);
> -		appointed_at = dbs_info->cdbs.dwork.timer.expires;
> +		appointed_at = dbs_info->cdbs.timer.expires;
>  
>  		if (time_before(next_sampling, appointed_at)) {
> -			cancel_delayed_work_sync(&dbs_info->cdbs.dwork);
> -
> -			gov_queue_work(dbs_data, policy,
> -				       usecs_to_jiffies(new_rate), true);
> +			gov_cancel_work(shared);
> +			gov_add_timers(policy, usecs_to_jiffies(new_rate));
>  
>  		}
>  	}

-- 
Pengutronix e.K.             | Lucas Stach                 |
Industrial Linux Solutions   | http://www.pengutronix.de/  |

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