linux-kernel - Re: [PATCH 2/2] cpufreq: schedutil: consolidate capacity margin calculation

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Date:   Thu, 28 Sep 2017 14:33:35 +0800
From:   Leo Yan <leo.yan@...aro.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Chris Redpath <Chris.Redpath@....com>,
        Joel Fernandes <joelaf@...gle.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 2/2] cpufreq: schedutil: consolidate capacity margin
 calculation

On Mon, Sep 25, 2017 at 02:55:07PM +0100, Patrick Bellasi wrote:
> On 23-Sep 07:04, Leo Yan wrote:
> > Scheduler CFS class has variable 'capacity_margin' to calculate the
> > capacity margin, and schedutil governor also needs to compensate the
> > same margin for frequency tipping point. Below are formulas used in
> > CFS class and schedutil governor separately:
> > 
> > CFS:       U` = U * capacity_margin / 1024 = U * 1.25
> > Schedutil: U` = U + U >> 2 = U + U * 0.25  = U * 1.25
> > 
> > This patch consolidates the capacity margin calculation so let
> > schedutil to use same formula with CFS class. As result this can avoid
> > the mismatch issue between schedutil and CFS class after change
> > 'capacity_margin' to other values.
> > 
> > Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> > Cc: Morten Rasmussen <morten.rasmussen@....com>
> > Cc: Chris Redpath <Chris.Redpath@....com>
> > Cc: Joel Fernandes <joelaf@...gle.com>
> > Cc: Vincent Guittot <vincent.guittot@...aro.org>
> > Signed-off-by: Leo Yan <leo.yan@...aro.org>
> > ---
> >  kernel/sched/cpufreq_schedutil.c | 5 +++--
> >  kernel/sched/sched.h             | 1 +
> >  2 files changed, 4 insertions(+), 2 deletions(-)
> > 
> > diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> > index 9209d83..067abbe 100644
> > --- a/kernel/sched/cpufreq_schedutil.c
> > +++ b/kernel/sched/cpufreq_schedutil.c
> > @@ -155,7 +155,8 @@ static void sugov_update_commit(struct sugov_policy *sg_policy, u64 time,
> >   *
> >   * next_freq = C * curr_freq * util_raw / max
> >   *
> > - * Take C = 1.25 for the frequency tipping point at (util / max) = 0.8.
> > + * Take C = capacity_margin / 1024 = 1.25, so it's for the frequency tipping
> > + * point at (util / max) = 0.8.
> >   *
> >   * The lowest driver-supported frequency which is equal or greater than the raw
> >   * next_freq (as calculated above) is returned, subject to policy min/max and
> > @@ -168,7 +169,7 @@ static unsigned int get_next_freq(struct sugov_policy *sg_policy,
> >  	unsigned int freq = arch_scale_freq_invariant() ?
> >  				policy->cpuinfo.max_freq : policy->cur;
> >  
> > -	freq = (freq + (freq >> 2)) * util / max;
> > +	freq = (freq * capacity_margin / 1024) * util / max;
> 
> The compiler should be smart enough but perhaps you can better use:
> 
>         freq *= (capacity_margin >> SCHED_CAPACITY_SHIFT));
>         freq *= util / max;

Thanks for the suggestion. The '*=' has low precedence than other
operators, will fix as below:

  freq = freq * capacity_margin >> SCHED_CAPACITY_SHIFT;
  freq = freq * util / max;

> >  	if (freq == sg_policy->cached_raw_freq && sg_policy->next_freq != UINT_MAX)
> >  		return sg_policy->next_freq;
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index 14db76c..cf75bdc 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -52,6 +52,7 @@ struct cpuidle_state;
> >  #define TASK_ON_RQ_MIGRATING	2
> >  
> >  extern __read_mostly int scheduler_running;
> > +extern unsigned int capacity_margin __read_mostly;
> >  
> >  extern unsigned long calc_load_update;
> >  extern atomic_long_t calc_load_tasks;
> > -- 
> > 2.7.4
> > 
> 
> -- 
> #include <best/regards.h>
> 
> Patrick Bellasi