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

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Date:	Wed, 25 May 2016 14:57:00 +0800
From:	Wanpeng Li <kernellwp@...il.com>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Dietmar Eggemann <dietmar.eggemann@....com>,
	yuyang.du@...el.com, Vincent Guittot <vincent.guittot@...aro.org>,
	Mike Galbraith <mgalbraith@...e.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 09/16] sched/fair: Let asymmetric cpu configurations
 balance at wake-up

2016-05-23 18:58 GMT+08:00 Morten Rasmussen <morten.rasmussen@....com>:
> 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;
> +
> +       return prev_cap * 1024 < task_util(p) * capacity_margin;
> +}

If one task util_avg is SCHED_CAPACITY_SCALE and running on x86 box w/
SMT enabled, then each HT has capacity 589, wake_cap() will result in
always not wake affine, right?

Regards,
Wanpeng Li

> +
>  /*
>   * select_task_rq_fair: Select target runqueue for the waking task in domains
>   * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> @@ -5316,7 +5341,8 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
>
>         if (sd_flag & SD_BALANCE_WAKE) {
>                 record_wakee(p);
> -               want_affine = !wake_wide(p) && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
> +               want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> +                             && cpumask_test_cpu(cpu, tsk_cpus_allowed(p));
>         }
>
>         rcu_read_lock();
> --
> 1.9.1
>



-- 
Regards,
Wanpeng Li