linux-kernel - Re: [PATCH v3] sched: cpufreq: Rename map_util_perf to sugov_apply_dvfs

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Message-ID: <20240901173552.h2pkra5fhkidzxd4@airbuntu>
Date: Sun, 1 Sep 2024 18:35:52 +0100
From: Qais Yousef <qyousef@...alina.io>
To: Christian Loehle <christian.loehle@....com>
Cc: Ingo Molnar <mingo@...nel.org>, Peter Zijlstra <peterz@...radead.org>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org
Subject: Re: [PATCH v3] sched: cpufreq: Rename map_util_perf to
 sugov_apply_dvfs_headroom

On 08/16/24 16:44, Christian Loehle wrote:
> On 8/9/24 00:44, Qais Yousef wrote:
> > We are providing headroom for the utilization to grow until the next
> > decision point to pick the next frequency. Give the function a better
> > name and give it some documentation. It is not really mapping anything.
> > 
> > Also move it to cpufreq_schedutil.c. This function relies on updating
> > util signal appropriately to give a headroom to grow. This is tied to
> > schedutil and scheduler and not something that can be shared with other
> > governors.
> > 
> > Acked-by: Viresh Kumar <viresh.kumar@...aro.org>
> > Acked-by: Rafael J. Wysocki <rafael@...nel.org>
> > Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>
> > Signed-off-by: Qais Yousef <qyousef@...alina.io>
> > ---
> > 
> > Changes in v3:
> > 
> > 	1. Add Reviewed-by from Vincent
> > 
> > Changes in v2:
> > 
> > 	1. Add Acked-by from Viresh and Raphael (Thanks!)
> > 	2. Move the function to cpufreq_schedutil.c instead of sched.h
> > 	3. Name space the function with sugov_ to indicate it is special to
> > 	   this governor only and not generic.
> > 
> >  include/linux/sched/cpufreq.h    |  5 -----
> >  kernel/sched/cpufreq_schedutil.c | 20 +++++++++++++++++++-
> >  2 files changed, 19 insertions(+), 6 deletions(-)
> > 
> > diff --git a/include/linux/sched/cpufreq.h b/include/linux/sched/cpufreq.h
> > index bdd31ab93bc5..d01755d3142f 100644
> > --- a/include/linux/sched/cpufreq.h
> > +++ b/include/linux/sched/cpufreq.h
> > @@ -28,11 +28,6 @@ static inline unsigned long map_util_freq(unsigned long util,
> >  {
> >  	return freq * util / cap;
> >  }
> > -
> > -static inline unsigned long map_util_perf(unsigned long util)
> > -{
> > -	return util + (util >> 2);
> > -}
> >  #endif /* CONFIG_CPU_FREQ */
> >  
> >  #endif /* _LINUX_SCHED_CPUFREQ_H */
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index eece6244f9d2..575df3599813 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -178,12 +178,30 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> >  	return cpufreq_driver_resolve_freq(policy, freq);
> >  }
> >  
> > +/*
> > + * DVFS decision are made at discrete points. If CPU stays busy, the util will
> > + * continue to grow, which means it could need to run at a higher frequency
> > + * before the next decision point was reached. IOW, we can't follow the util as
> > + * it grows immediately, but there's a delay before we issue a request to go to
> > + * higher frequency. The headroom caters for this delay so the system continues
> > + * to run at adequate performance point.
> > + *
> > + * This function provides enough headroom to provide adequate performance
> > + * assuming the CPU continues to be busy.
> > + *
> > + * At the moment it is a constant multiplication with 1.25.
> > + */
> > +static inline unsigned long sugov_apply_dvfs_headroom(unsigned long util)
> > +{
> > +	return util + (util >> 2);
> > +}
> > +
> >  unsigned long sugov_effective_cpu_perf(int cpu, unsigned long actual,
> >  				 unsigned long min,
> >  				 unsigned long max)
> >  {
> >  	/* Add dvfs headroom to actual utilization */
> > -	actual = map_util_perf(actual);
> > +	actual = sugov_apply_dvfs_headroom(actual);
> 
> Maybe you can even get rid of the comment above now.
> sugov_apply_dvfs_headroom(actual) is pretty self-explanatory.

It was actually not clear to folks based on previous discussion. And I already
doing many changes on how dvfs headroom is done on another series. So I think
it is worth it.

> 
> Anyway
> Reviewed-by: Christian Loehle <christian.loehle@....com>

Thanks for having a look!

> 
> >  	/* Actually we don't need to target the max performance */
> >  	if (actual < max)
> >  		max = actual;
>