linux-kernel - Re: [PATCH] sche/fair: static cpumasks for load balance

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Date:   Wed, 25 May 2022 19:04:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Bing Huang <huangbing@...inos.cn>
Cc:     mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, brauner@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sche/fair: static cpumasks for load balance

On Mon, May 23, 2022 at 05:42:16PM +0800, Bing Huang wrote:
> The both cpu mask load_balance_mask and select_idle_mask just only used
> in fair.c, but allocation in core.c in CONFIG_CPUMASK_OFFSTACK=y case,
> and global via declare per cpu variations. More or less, it looks wired.
> 
> Signed-off-by: Bing Huang <huangbing@...inos.cn>
> ---
>  kernel/sched/core.c  | 13 +++----------
>  kernel/sched/fair.c  | 18 ++++++++++++++++--
>  kernel/sched/sched.h |  4 ++++
>  3 files changed, 23 insertions(+), 12 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index d58c0389eb23..64934c5e236d 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -9448,9 +9448,6 @@ LIST_HEAD(task_groups);
>  static struct kmem_cache *task_group_cache __read_mostly;
>  #endif
>  
> -DECLARE_PER_CPU(cpumask_var_t, load_balance_mask);
> -DECLARE_PER_CPU(cpumask_var_t, select_idle_mask);
> -
>  void __init sched_init(void)
>  {
>  	unsigned long ptr = 0;
> @@ -9494,14 +9491,10 @@ void __init sched_init(void)
>  
>  #endif /* CONFIG_RT_GROUP_SCHED */
>  	}
> +
>  #ifdef CONFIG_CPUMASK_OFFSTACK
> -	for_each_possible_cpu(i) {
> -		per_cpu(load_balance_mask, i) = (cpumask_var_t)kzalloc_node(
> -			cpumask_size(), GFP_KERNEL, cpu_to_node(i));
> -		per_cpu(select_idle_mask, i) = (cpumask_var_t)kzalloc_node(
> -			cpumask_size(), GFP_KERNEL, cpu_to_node(i));
> -	}
> -#endif /* CONFIG_CPUMASK_OFFSTACK */
> +	init_working_cpumask();
> +#endif

We already have init_sched_fair_class(), can't you use that?

>  
>  	init_rt_bandwidth(&def_rt_bandwidth, global_rt_period(), global_rt_runtime());
>  
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index a68482d66535..3a4b0dd1beb5 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5831,8 +5831,8 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags)
>  #ifdef CONFIG_SMP
>  
>  /* Working cpumask for: load_balance, load_balance_newidle. */
> -DEFINE_PER_CPU(cpumask_var_t, load_balance_mask);
> -DEFINE_PER_CPU(cpumask_var_t, select_idle_mask);
> +static DEFINE_PER_CPU(cpumask_var_t, load_balance_mask);
> +static DEFINE_PER_CPU(cpumask_var_t, select_idle_mask);
>  
>  #ifdef CONFIG_NO_HZ_COMMON
>  
> @@ -7052,6 +7052,20 @@ balance_fair(struct rq *rq, struct task_struct *prev, struct rq_flags *rf)
>  }
>  #endif /* CONFIG_SMP */
>  
> +#ifdef CONFIG_CPUMASK_OFFSTACK
> +void __init init_working_cpumask(void)
> +{
> +	int i;
> +
> +	for_each_possible_cpu(i) {
> +		per_cpu(load_balance_mask, i) = (cpumask_var_t)kzalloc_node(
> +				cpumask_size(), GFP_KERNEL, cpu_to_node(i));
> +		per_cpu(select_idle_mask, i) = (cpumask_var_t)kzalloc_node(
> +				cpumask_size(), GFP_KERNEL, cpu_to_node(i));
> +	}
> +}
> +#endif

If you stick that #ifdef inside the function, you can remove it
everwhere else. Less #ifdef is more better.