linux-kernel - Re: [PATCH] sched/fair: Fix load_above_capacity fixed point arithmetic width

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Date:	Wed, 10 Aug 2016 14:28:08 +0200
From:	Vincent Guittot <vincent.guittot@...aro.org>
To:	Dietmar Eggemann <dietmar.eggemann@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...nel.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Yuyang Du <yuyang.du@...el.com>
Subject: Re: [PATCH] sched/fair: Fix load_above_capacity fixed point
 arithmetic width

On 10 August 2016 at 12:27, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
> Since commit 2159197d6677 ("sched/core: Enable increased load resolution
> on 64-bit kernels") we now have two different fixed point units for
> load.
> load_above_capacity has to have 10 bit fixed point unit like PELT
> whereas NICE_0_LOAD has 20 bit fixed point unit on 64-bit kernels.
> Fix this by scaling down NICE_0_LOAD when multiplying
> load_above_capacity with it.
>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
> Acked-by: Morten Rasmussen <morten.rasmussen@....com>
> ---
>  kernel/sched/fair.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 4088eedea763..fe4093807852 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7147,7 +7147,7 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
>                 load_above_capacity = busiest->sum_nr_running * SCHED_CAPACITY_SCALE;
>                 if (load_above_capacity > busiest->group_capacity) {
>                         load_above_capacity -= busiest->group_capacity;
> -                       load_above_capacity *= NICE_0_LOAD;
> +                       load_above_capacity *= scale_load_down(NICE_0_LOAD);

FWIW, Acked-by: Vincent Guittot <vincent.guittot@...aro.org>

>                         load_above_capacity /= busiest->group_capacity;
>                 } else
>                         load_above_capacity = ~0UL;
> --
> 1.9.1
>