Re: [RFC PATCH 2/2] CPUfreq ondemand: handle QoS request on DVFS response latency

["Rafael J. Wysocki" <rjw@sisk.pl>]

On Saturday, February 25, 2012, Pavel Machek wrote:
> On Wed 2012-02-22 17:03:35, MyungJoo Ham wrote:
> > With QoS class, DVFS_RESPONSE_LATENCY, users (device drivers and
> > userspace processes) may express the desired maximum response latency
> > from DVFS mechanisms such as CPUfreq's ondemand governors. Based on such
> > QoS requests, the ondemand governor may flexibly adjust sampling rate
> > accordingly unless it goes below the min_sampling_rate.
> > 
> > The benefit of having DVFS_RESPONSE_LATENCY is to have faster response
> > from user inputs (mouse clicks, keyboard inputs, touchscreen touches,
> > and others) without increasing frequency unconditionally. Because some
> > input events may not require any performance increases, increasing the
> > frequency unconditionally for inputs may simply consume too much energy.
> > Adjusting sampling rate based on user inputs enabled to increase
> > frequency with less latency if it requires and not to increase frequency
> > if it does not require.
> > 
> > Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
> > Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
> > 
> > --
> > This patch depends on the patch
> > "PM / QoS: Introduce new classes: DMA-Throughput and DVFS-Latency".
> > and the patch
> > "CPUfreq ondemand: update sampling rate without waiting for next
> > sampling"
> > ---
> >  drivers/cpufreq/cpufreq_ondemand.c |  108 ++++++++++++++++++++++++++++++++----
> >  1 files changed, 96 insertions(+), 12 deletions(-)
> > 
> > diff --git a/drivers/cpufreq/cpufreq_ondemand.c b/drivers/cpufreq/cpufreq_ondemand.c
> > index 2d66649..b9188f1 100644
> > --- a/drivers/cpufreq/cpufreq_ondemand.c
> > +++ b/drivers/cpufreq/cpufreq_ondemand.c
> > @@ -22,6 +22,7 @@
> >  #include <linux/tick.h>
> >  #include <linux/ktime.h>
> >  #include <linux/sched.h>
> > +#include <linux/pm_qos.h>
> >  
> >  /*
> >   * dbs is used in this file as a shortform for demandbased switching
> > @@ -93,6 +94,7 @@ struct cpu_dbs_info_s {
> >  	 * when user is changing the governor or limits.
> >  	 */
> >  	struct mutex timer_mutex;
> > +	bool activated; /* dbs_timer_init is in effect */
> >  };
> >  static DEFINE_PER_CPU(struct cpu_dbs_info_s, od_cpu_dbs_info);
> >  
> > @@ -111,6 +113,8 @@ static struct dbs_tuners {
> >  	unsigned int sampling_down_factor;
> >  	unsigned int powersave_bias;
> >  	unsigned int io_is_busy;
> > +	struct notifier_block dvfs_lat_qos_db;
> > +	unsigned int dvfs_lat_qos_wants;
> >  } dbs_tuners_ins = {
> >  	.up_threshold = DEF_FREQUENCY_UP_THRESHOLD,
> >  	.sampling_down_factor = DEF_SAMPLING_DOWN_FACTOR,
> > @@ -164,6 +168,23 @@ static inline cputime64_t get_cpu_iowait_time(unsigned int cpu, cputime64_t *wal
> >  }
> >  
> >  /*
> > + * Find right sampling rate based on sampling_rate and
> > + * QoS requests on dvfs latency.
> > + */
> > +static unsigned int effective_sampling_rate(void)
> > +{
> > +	unsigned int effective;
> > +
> > +	if (dbs_tuners_ins.dvfs_lat_qos_wants)
> > +		effective = min(dbs_tuners_ins.dvfs_lat_qos_wants,
> > +				dbs_tuners_ins.sampling_rate);
> > +	else
> > +		effective = dbs_tuners_ins.sampling_rate;
> > +
> > +	return max(effective, min_sampling_rate);
> > +}
> > +
> > +/*
> >   * Find right freq to be set now with powersave_bias on.
> >   * Returns the freq_hi to be used right now and will set freq_hi_jiffies,
> >   * freq_lo, and freq_lo_jiffies in percpu area for averaging freqs.
> > @@ -207,7 +228,7 @@ static unsigned int powersave_bias_target(struct cpufreq_policy *policy,
> >  		dbs_info->freq_lo_jiffies = 0;
> >  		return freq_lo;
> >  	}
> > -	jiffies_total = usecs_to_jiffies(dbs_tuners_ins.sampling_rate);
> > +	jiffies_total = usecs_to_jiffies(effective_sampling_rate());
> >  	jiffies_hi = (freq_avg - freq_lo) * jiffies_total;
> >  	jiffies_hi += ((freq_hi - freq_lo) / 2);
> >  	jiffies_hi /= (freq_hi - freq_lo);
> > @@ -259,7 +280,8 @@ show_one(powersave_bias, powersave_bias);
> >  
> >  /**
> >   * update_sampling_rate - update sampling rate effective immediately if needed.
> > - * @new_rate: new sampling rate
> > + * @new_rate: new sampling rate. if it is 0, regard sampling rate is not
> > + *		changed and assume that qos request value is changed.
> >   *
> >   * If new rate is smaller than the old, simply updaing
> >   * dbs_tuners_int.sampling_rate might not be appropriate. For example,
> > @@ -273,9 +295,13 @@ show_one(powersave_bias, powersave_bias);
> >  static void update_sampling_rate(unsigned int new_rate)
> >  {
> >  	int cpu;
> > +	unsigned int effective;
> > +
> > +
> > +	if (new_rate)
> > +		dbs_tuners_ins.sampling_rate = max(new_rate, min_sampling_rate);
> >  
> > -	dbs_tuners_ins.sampling_rate = new_rate
> > -				     = max(new_rate, min_sampling_rate);
> > +	effective = effective_sampling_rate();
> >  
> >  	for_each_online_cpu(cpu) {
> >  		struct cpufreq_policy *policy;
> > @@ -283,21 +309,31 @@ static void update_sampling_rate(unsigned int new_rate)
> >  		struct timer_list *timer;
> >  		unsigned long appointed_at;
> >  
> > +		/*
> > +		 * mutex_destory(&dbs_info->timer_mutex) should not happen
> > +		 * in this context.
> > +		 */
> > +		mutex_lock(&dbs_mutex);
> > +
> >  		policy = cpufreq_cpu_get(cpu);
> >  		if (!policy)
> > -			continue;
> > +			goto next;
> >  		dbs_info = &per_cpu(od_cpu_dbs_info, policy->cpu);
> >  		cpufreq_cpu_put(policy);
> >  
> > +		/* timer_mutex destroyed or will be destoyed soon */
> > +		if (!dbs_info->activated)
> > +			goto next;
> > +
> >  		mutex_lock(&dbs_info->timer_mutex);
> >  
> >  		if (!delayed_work_pending(&dbs_info->work))
> > -			goto next;
> > +			goto next_timer_mutex;
> >  
> >  		timer = &dbs_info->work.timer;
> >  		appointed_at = timer->expires;
> >  
> > -		if (time_before(jiffies + usecs_to_jiffies(new_rate),
> > +		if (time_before(jiffies + usecs_to_jiffies(effective),
> >  				appointed_at)) {
> >  
> >  			mutex_unlock(&dbs_info->timer_mutex);
> > @@ -305,12 +341,15 @@ static void update_sampling_rate(unsigned int new_rate)
> >  			mutex_lock(&dbs_info->timer_mutex);
> >  
> >  			schedule_delayed_work_on(dbs_info->cpu, &dbs_info->work,
> > -						 usecs_to_jiffies(new_rate));
> > +						 usecs_to_jiffies(effective));
> >  
> >  		}
> > -next:
> > +next_timer_mutex:
> >  		mutex_unlock(&dbs_info->timer_mutex);
> > +next:
> > +		mutex_unlock(&dbs_mutex);
> >  	}
> > +
> >  }
> 
> I don't think gotos are helpful here. Can you use normal program
> structure or move it to subroutine...?

I agree with Pavel that gotos don't make that code particularly clear.

Thanks,
Rafael