linux-kernel - Re: [PATCH] sched/fair: Introduce fits

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Date:   Thu, 6 Jun 2019 08:22:04 +0530
From:   Viresh Kumar <viresh.kumar@...aro.org>
To:     Quentin Perret <quentin.perret@....com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH] sched/fair: Introduce fits_capacity()

On 05-06-19, 10:16, Quentin Perret wrote:
> Hi Viresh,
> 
> On Tuesday 04 Jun 2019 at 12:31:52 (+0530), Viresh Kumar wrote:
> > The same formula to check utilization against capacity (after
> > considering capacity_margin) is already used at 5 different locations.
> > 
> > This patch creates a new macro, fits_capacity(), which can be used from
> > all these locations without exposing the details of it and hence
> > simplify code.
> > 
> > All the 5 code locations are updated as well to use it..
> > 
> > Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
> > ---
> >  kernel/sched/fair.c | 14 +++++++-------
> >  1 file changed, 7 insertions(+), 7 deletions(-)
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index 7f8d477f90fe..db3a218b7928 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -102,6 +102,8 @@ int __weak arch_asym_cpu_priority(int cpu)
> >   * (default: ~20%)
> >   */
> >  static unsigned int capacity_margin			= 1280;
> > +
> > +#define fits_capacity(cap, max)	((cap) * capacity_margin < (max) * 1024)
> >  #endif
> >  
> >  #ifdef CONFIG_CFS_BANDWIDTH
> > @@ -3727,7 +3729,7 @@ util_est_dequeue(struct cfs_rq *cfs_rq, struct task_struct *p, bool task_sleep)
> >  
> >  static inline int task_fits_capacity(struct task_struct *p, long capacity)
> >  {
> > -	return capacity * 1024 > task_util_est(p) * capacity_margin;
> > +	return fits_capacity(task_util_est(p), capacity);
> >  }
> >  
> >  static inline void update_misfit_status(struct task_struct *p, struct rq *rq)
> > @@ -5143,7 +5145,7 @@ static inline unsigned long cpu_util(int cpu);
> >  
> >  static inline bool cpu_overutilized(int cpu)
> >  {
> > -	return (capacity_of(cpu) * 1024) < (cpu_util(cpu) * capacity_margin);
> > +	return !fits_capacity(cpu_util(cpu), capacity_of(cpu));
> 
> This ...
> 
> >  }
> >  
> >  static inline void update_overutilized_status(struct rq *rq)
> > @@ -6304,7 +6306,7 @@ static int find_energy_efficient_cpu(struct task_struct *p, int prev_cpu)
> >  			/* Skip CPUs that will be overutilized. */
> >  			util = cpu_util_next(cpu, p, cpu);
> >  			cpu_cap = capacity_of(cpu);
> > -			if (cpu_cap * 1024 < util * capacity_margin)
> > +			if (!fits_capacity(util, cpu_cap))
> 
> ... and this isn't _strictly_ equivalent to the existing code but I
> guess we can live with the difference :-)

Yes, I missed the == part it seems. Good catch. Though as you said,
maybe we don't need to take that into account and can live with the
new macro :)

> 
> >  				continue;
> >  
> >  			/* Always use prev_cpu as a candidate. */
> > @@ -7853,8 +7855,7 @@ group_is_overloaded(struct lb_env *env, struct sg_lb_stats *sgs)
> >  static inline bool
> >  group_smaller_min_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
> >  {
> > -	return sg->sgc->min_capacity * capacity_margin <
> > -						ref->sgc->min_capacity * 1024;
> > +	return fits_capacity(sg->sgc->min_capacity, ref->sgc->min_capacity);
> >  }
> >  
> >  /*
> > @@ -7864,8 +7865,7 @@ group_smaller_min_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
> >  static inline bool
> >  group_smaller_max_cpu_capacity(struct sched_group *sg, struct sched_group *ref)
> >  {
> > -	return sg->sgc->max_capacity * capacity_margin <
> > -						ref->sgc->max_capacity * 1024;
> > +	return fits_capacity(sg->sgc->max_capacity, ref->sgc->max_capacity);
> >  }
> >  
> >  static inline enum
> > -- 
> > 2.21.0.rc0.269.g1a574e7a288b
> > 
> 
> Also, since we're talking about making the capacity_margin code more
> consistent, one small thing I had in mind: we have a capacity margin
> in sugov too, which happens to be 1.25 has well (see map_util_freq()).
> Conceptually, capacity_margin in fair.c and the sugov margin are both
> about answering: "do I have enough CPU capacity to serve X of util, or
> do I need more ?"
> 
> So perhaps we should factorize the capacity_margin code some more to use
> it in both places in a consistent way ? This could be done in a separate
> patch, though.

Hmm, even if the values are same currently I am not sure if we want
the same for ever. I will write a patch for it though, if Peter/Rafael
feel the same as you.

Thanks Quentin.

-- 
viresh