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

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Date:   Wed, 5 Jun 2019 10:16:46 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Viresh Kumar <viresh.kumar@...aro.org>
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()

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 :-)

>  				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.

Thanks,
Quentin