linux-kernel - Re: [PATCH v2] sched/fair: Optimize select_idle

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Message-ID: <ac11014d-ffc9-f0b2-3d48-2456c1c5edfa@arm.com>
Date:   Fri, 13 Dec 2019 11:48:20 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Cheng Jian <cj.chengjian@...wei.com>, mingo@...nel.org,
        peterz@...radead.org, linux-kernel@...r.kernel.org
Cc:     chenwandun@...wei.com, xiexiuqi@...wei.com, liwei391@...wei.com,
        huawei.libin@...wei.com, bobo.shaobowang@...wei.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org
Subject: Re: [PATCH v2] sched/fair: Optimize select_idle_cpu

On 13/12/2019 02:45, Cheng Jian wrote:
> select_idle_cpu() will scan the LLC domain for idle CPUs,
> it's always expensive. so the next commit :
> 
> 	1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")
> 
> introduces a way to limit how many CPUs we scan.
> 
> But it consume some CPUs out of 'nr' that are not allowed
> for the task and thus waste our attempts. The function
> always return nr_cpumask_bits, and we can't find a CPU
> which our task is allowed to run.
> 
> Cpumask may be too big, similar to select_idle_core(), use
> per_cpu_ptr 'select_idle_mask' to prevent stack overflow.
> 
> Fixes: 1ad3aaf3fcd2 ("sched/core: Implement new approach to scale select_idle_cpu()")
> Signed-off-by: Cheng Jian <cj.chengjian@...wei.com>

Reviewed-by: Valentin Schneider <valentin.schneider@....com>

> ---
>  kernel/sched/fair.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 08a233e97a01..d48244388ce9 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5828,6 +5828,7 @@ static inline int select_idle_smt(struct task_struct *p, int target)
>   */
>  static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int target)
>  {
> +	struct cpumask *cpus = this_cpu_cpumask_var_ptr(select_idle_mask);
>  	struct sched_domain *this_sd;
>  	u64 avg_cost, avg_idle;
>  	u64 time, cost;
> @@ -5859,11 +5860,11 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, int t
>  
>  	time = cpu_clock(this);
>  
> -	for_each_cpu_wrap(cpu, sched_domain_span(sd), target) {
> +	cpumask_and(cpus, sched_domain_span(sd), p->cpus_ptr);
> +
> +	for_each_cpu_wrap(cpu, cpus, target) {
>  		if (!--nr)
>  			return si_cpu;
> -		if (!cpumask_test_cpu(cpu, p->cpus_ptr))
> -			continue;
>  		if (available_idle_cpu(cpu))
>  			break;
>  		if (si_cpu == -1 && sched_idle_cpu(cpu))
>