linux-kernel - Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations balance at wake-up

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Message-ID: <20160524000424.GF18670@intel.com>
Date:	Tue, 24 May 2016 08:04:24 +0800
From:	Yuyang Du <yuyang.du@...el.com>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	peterz@...radead.org, mingo@...hat.com, dietmar.eggemann@....com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations
 balance at wake-up

On Mon, May 23, 2016 at 11:58:51AM +0100, Morten Rasmussen wrote:
> Currently, SD_WAKE_AFFINE always takes priority over wakeup balancing if
> SD_BALANCE_WAKE is set on the sched_domains. For asymmetric
> configurations SD_WAKE_AFFINE is only desirable if the waking task's
> compute demand (utilization) is suitable for the cpu capacities
> available within the SD_WAKE_AFFINE sched_domain. If not, let wakeup
> balancing take over (find_idlest_{group, cpu}()).
> 
> The assumption is that SD_WAKE_AFFINE is never set for a sched_domain
> containing cpus with different capacities. This is enforced by a
> previous patch based on the SD_ASYM_CPUCAPACITY flag.
> 
> Ideally, we shouldn't set 'want_affine' in the first place, but we don't
> know if SD_BALANCE_WAKE is enabled on the sched_domain(s) until we start
> traversing them.
> 
> cc: Ingo Molnar <mingo@...hat.com>
> cc: Peter Zijlstra <peterz@...radead.org>
> 
> Signed-off-by: Morten Rasmussen <morten.rasmussen@....com>
> ---
>  kernel/sched/fair.c | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 564215d..ce44fa7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -114,6 +114,12 @@ unsigned int __read_mostly sysctl_sched_shares_window = 10000000UL;
>  unsigned int sysctl_sched_cfs_bandwidth_slice = 5000UL;
>  #endif
>  
> +/*
> + * The margin used when comparing utilization with cpu capacity:
> + * util * 1024 < capacity * margin
> + */
> +unsigned int capacity_margin = 1280; /* ~20% */
> +
>  static inline void update_load_add(struct load_weight *lw, unsigned long inc)
>  {
>  	lw->weight += inc;
> @@ -5293,6 +5299,25 @@ static int cpu_util(int cpu)
>  	return (util >= capacity) ? capacity : util;
>  }
>  
> +static inline int task_util(struct task_struct *p)
> +{
> +	return p->se.avg.util_avg;
> +}
> +
> +static int wake_cap(struct task_struct *p, int cpu, int prev_cpu)
> +{
> +	long delta;
> +	long prev_cap = capacity_of(prev_cpu);
> +
> +	delta = cpu_rq(cpu)->rd->max_cpu_capacity - prev_cap;
> +
> +	/* prev_cpu is fairly close to max, no need to abort wake_affine */
> +	if (delta < prev_cap >> 3)
> +		return 0;

delta can be negative? still return 0?

> +
> +	return prev_cap * 1024 < task_util(p) * capacity_margin;
> +}