linux-kernel - Re: [PATCH v10 04/11] sched: Make sched entity usage tracking scale-invariant

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Date:	Tue, 24 Mar 2015 11:00:57 +0100
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Morten Rasmussen <Morten.Rasmussen@....com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Rik van Riel <riel@...hat.com>,
	Mike Galbraith <efault@....de>,
	Nicolas Pitre <nicolas.pitre@...aro.org>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	Paul Turner <pjt@...gle.com>, Ben Segall <bsegall@...gle.com>
Subject: Re: [PATCH v10 04/11] sched: Make sched entity usage tracking scale-invariant

On 23 March 2015 at 14:19, Peter Zijlstra <peterz@...radead.org> wrote:
> On Fri, Feb 27, 2015 at 04:54:07PM +0100, Vincent Guittot wrote:
>
>> +     unsigned long scale_freq = arch_scale_freq_capacity(NULL, cpu);
>
>> +                     sa->running_avg_sum += delta_w * scale_freq
>> +                             >> SCHED_CAPACITY_SHIFT;
>
> so the only thing that could be improved is somehow making this
> multiplication go away when the arch doesn't implement the function.
>
> But I'm not sure how to do that without #ifdef.
>
> Maybe a little something like so then... that should make the compiler
> get rid of those multiplications unless the arch needs them.

yes, it removes useless multiplication when not used by an arch.
It also adds a constraint on the arch side which have to define
arch_scale_freq_capacity like below:

#define arch_scale_freq_capacity xxx_arch_scale_freq_capacity
with xxx_arch_scale_freq_capacity an architecture specific function

If it sounds acceptable i can update the patch with your proposal ?

Vincent
>
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2484,8 +2484,6 @@ static u32 __compute_runnable_contrib(u6
>         return contrib + runnable_avg_yN_sum[n];
>  }
>
> -unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
> -
>  /*
>   * We can represent the historical contribution to runnable average as the
>   * coefficients of a geometric series.  To do this we sub-divide our runnable
> @@ -6015,11 +6013,6 @@ static unsigned long default_scale_capac
>         return SCHED_CAPACITY_SCALE;
>  }
>
> -unsigned long __weak arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
> -{
> -       return default_scale_capacity(sd, cpu);
> -}
> -
>  static unsigned long default_scale_cpu_capacity(struct sched_domain *sd, int cpu)
>  {
>         if ((sd->flags & SD_SHARE_CPUCAPACITY) && (sd->span_weight > 1))
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1387,7 +1387,14 @@ static inline int hrtick_enabled(struct
>
>  #ifdef CONFIG_SMP
>  extern void sched_avg_update(struct rq *rq);
> -extern unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu);
> +
> +#ifndef arch_scale_freq_capacity
> +static __always_inline
> +unsigned long arch_scale_freq_capacity(struct sched_domain *sd, int cpu)
> +{
> +       return SCHED_CAPACITY_SCALE;
> +}
> +#endif
>
>  static inline void sched_rt_avg_update(struct rq *rq, u64 rt_delta)
>  {
--
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